• 10-bit Resolution Analog to Digital Converter (ADC) with 8 Multiplexed Inputs
• On-chip Emulation Logic (Enhanced Hook System)
• Power Saving Modes:
– Idle Mode
– Power-down Mode
• Power Supply: 3V to 5.5V
• Temperature Range: Industrial (-40° to +85°C)
• Packages: VQFP44, PLCC44
Enhanced 8-bit
Microcontroller
with 32 KB Flash
Memory
AT89C51AC2
T89C51AC2
Description
The A/T89C51AC2 is a high performance Flash version of the 80C51 single chip 8-bit
microcontrollers. It contains a 32 KB Flash memory block for program and data.
The 32 KB Flash memory can be programmed either in parallel mode or in serial
mode with the ISP capability or with software. The programming voltage is internally
generated from the standard VCC pin.
The A/T89C51AC2 retains all features of the 80C51 with 256 bytes of internal RAM, a
7-source 4-level interrupt controller and three timer/cou nters. In addition, the
A/T89C51AC2 has a 10-bit A/D converter, a 2 KB Boot Flash memory, 2 KB EEPROM
for data, a Programmable Counter Array, an XRAM of 1024 bytes, a Hardware WatchDog Timer, and a more versatile se rial channel that facilitates multiprocessor
communication (EUART). The fully static design of the A/T89C51AC2 reduces system
power consumption by bringing the clock frequency down to any value, even DC,
without loss of data.
The A/T89C51AC2 has two software-selectable modes of reduced activity and an 8bit clock prescaler for further reduction in power consumption. In the idle mode the
CPU is frozen while the peripherals and the interrupt system are still operating. In the
Power-down mode the RAM is saved and all other functions are inoperative.
The added features of the A/T89C51AC2 make it more powerful for applications that
need A/D conversion, pulse width modulation, high speed I/O and counting capabilities such as industrial control, consumer goods, alarms, motor control, among others.
While remaining fully compatible with the 80C52, the T8C51AC2 offers a superset of
this standard microcontroller. In X2 mode, a maximum external clock rate of 20 MHz
reaches a 300 ns cycle time.
Is an 8-bit open drain bi-directional I/O port. Port 0 pins that have 1’s written to them float, and in this state can be used as
high-impedance inputs. Port 0 is also the multiplexed low-order address and data bus during accesses to external Program
and Data Memory. In this application it uses strong internal pull-ups when emitting 1’s.
Port 0 also outputs the code Bytes during program validation. External pull-ups are required during program verification.
Is an 8-bit bi-directional I/O port with internal pull-ups. Port 1 pins can be used for digital input/output or as analog inputs for
the Analog Digital Converter (ADC). Port 1 pins that have 1’s written to them are pulled high by the internal pull-up transistors
and can be used as inputs in this state. As inputs, Port 1 pins that are being pulled low externally will be the source of current
(IIL, see section "Electrical Characteristic") because of the internal pull-ups. Port 1 pins are assigned to be used as analog
inputs via the ADCCF register (in this case the internal pull-ups are disconnected).
As a secondary digital function, port 1 contains the Timer 2 external trigger and clock input; the PCA external clock input and
the PCA module I/O.
P1.0/AN0/T2
Analog input channel 0,
External clock input for Timer/counter2.
P1.1/AN1/T2EX
Analog input channel 1,
Trigger input for Timer/counter2.
P1.2/AN2/ECI
Analog input channel 2,
PCA external clock input.
P1.3/AN3/CEX0
Analog input channel 3,
PCA module 0 Entry of input/PWM output.
P1.4/AN4/CEX1
Analog input channel 4,
PCA module 1 Entry of input/PWM output.
P1.5/AN5/CEX2
Analog input channel 5,
PCA module 2 Entry of input/PWM output.
P1.6/AN6/CEX3
Analog input channel 6,
PCA module 3 Entry of input/PWM output.
P1.7/AN7/CEX4
Analog input channel 7,
PCA module 4 Entry ot input/PWM output.
Port 1 receives the low-order address byte during EPROM programming and program verification.
It can drive CMOS inputs without external pull-ups.
P2.0:7I/OPort 2:
4
Is an 8-bit bi-directional I/O port with internal pull-ups. Port 2 pins that have 1’s written to them are pulled high by the internal
pull-ups and can be used as inputs in this state. As inputs, Port 2 pins that are being pulled low externally will be a source of
current (IIL, see section "Electrical Characteristic") because of the internal pull-ups. Port 2 emits the high-order address byte
during accesses to the external Program Memory and during accesses to external Data Memory that uses 16-bit addresses
(MOVX @DPTR). In this application, it uses strong internal pull-ups when emitting 1’s. During accesses to external Data
Memory that use 8 bit addresses (MOVX @Ri), Port 2 transmits the contents of the P2 special function register.
It also receives high-order addresses and control signals during program validation.
It can drive CMOS inputs without external pull-ups.
4127H–8051–02/08
Table 1. Pin Description (Continued)
Pin NameTypeDescription
A/T89C51AC2
P3.0:7I/OPort 3:
P4.0:1I/OPort 4:
Is an 8-bit bi-directional I/O port with internal pull-ups. Port 3 pins that have 1’s written to them are pulled high by the internal
pull-up transistors and can be used as inputs in this state. As inputs, Port 3 pins that are being pulled low externally will be a
source of current (IIL, see section "Electrical Characteristic") because of the internal pull-ups.
The output latch corresponding to a secondary function must be programmed to one for that function to operate (except for
TxD and WR). The secondary functions are assigned to the pins of port 3 as follows:
P3.0/RxD:
Receiver data input (asynchronous) or data input/output (synchronous) of the serial interface
P3.1/TxD:
Transmitter data output (asynchronous) or clock output (synchronous) of the serial interface
P3.2/INT0:
External interrupt 0 input/timer 0 gate control input
P3.3/INT1:
External interrupt 1 input/timer 1 gate control input
P3.4/T0:
Timer 0 counter input
P3.5/T1:
Timer 1 counter input
P3.6/WR:
External Data Memory write strobe; latches the data byte from port 0 into the external data memory
P3.7/RD:
External Data Memory read strobe; Enables the external data memory.
It can drive CMOS inputs without external pull-ups.
Is an 2-bit bi-directional I/O port with internal pull-ups. Port 4 pins that have 1’s written to them are pulled high by the internal
pull-ups and can be used as inputs in this state. As inputs, Port 4 pins that are being pulled low externally will be a source of
current (IIL, on the datasheet) because of the internal pull-up transistor.
P4.0
P4.1:
It can drive CMOS inputs without external pull-ups.
RESETI/O
ALEO
PSENO
EAI
XTAL1I
XTAL2O
Reset:
A high level on this pin during two machine cycles while the oscillator is running resets the device. An internal pull-down
resistor to VSS permits power-on reset using only an external capacitor to VCC.
ALE:
An Address Latch Enable output for latching the low byte of the address during accesses to the external memory. The ALE is
activated every 1/6 oscillator periods (1/3 in X2 mode) except during an external data memory access. When instructions are
executed from an internal Flash (EA = 1), ALE generation can be disabled by the software.
PSEN:
The Program Store Enable output is a control signal that enables the external program memory of the bus during external
fetch operations. It is activated twice each machine cycle during fetches from the external program memory. However, when
executing from of the external program memory two activations of PSEN are skipped during each access to the external Data
memory. The PSEN is not activated for internal fetches.
EA:
When External Access is held at the high level, instructions are fetched from the internal Flash when the program counter is
less then 8000H. When held at the low level,A/T89C51AC2 fetches all instructions from the external program memory
XTAL1:
Input of the inverting oscillator amplifier and input of the internal clock generator circuits.
To drive the device from an external clock source, XTAL1 should be driven, while XTAL2 is left unconnected. To operate
above a frequency of 16 MHz, a duty cycle of 50% should be maintained.
XTAL2:
Output from the inverting oscillator amplifier.
.
4127H–8051–02/08
5
A/T89C51AC2
I/O Configurations
D
CL
QP1.X
LATCH
INTERNAL
WRITE
TO
LATCH
READ
PIN
READ
LATCH
P1.x
P3.X
P4.X
ALTERNATE
OUTPUT
FUNCTION
VCC
INTERNAL
PULL-UP (1)
ALTERNATE
INPUT
FUNCTION
P3.x
P4.x
BUS
Each Port SFR operates via type-D latches, as illustrated in Figure 1 for Ports 3 and 4. A
CPU "write to latch" signal initiates transfer of internal bus data into the type-D latch. A
CPU "read latch" signal transfers the latched Q output onto the internal bus. Similarly, a
"read pin" signal transfers the logical level of the Port pin. Some Port data instructions
activate the "read latch" signal while others activate the "read pin" signal. Latch instructi ons a re referred to as R e a d-Modify-Wri t e instructions. Ea c h I/O l in e may b e
independently programmed as input or output.
Port 1, Port 3 and Port 4
Figure 1 shows the structure of Ports 1 and 3, which have internal pull-ups. An external
source can pull the pin low. Each Port pin can be configured either for general-purpose
I/O or for its alternate input output function.
To use a pin for general-purpose output, set or clear the corresponding bit in the Px register (x = 1,3 or 4). To use a pin for general-purpose input, set the bit in the Px register.
This turns off the output FET drive.
To configure a pin for its alternate function, set the bit in the Px register. When the latch
is set, the "alternate output function" signal controls the output level (see Figure 1). The
operation of Ports 1, 3 and 4 is discussed further in the "quasi-Bidirectional Port Operation" section.
Figure 1. Port 1, Port 3 and Port 4 Structure
Port 0 and Port 2
6
Note:The internal pull-up can be disabled on P1 when analog function is selected.
Ports 0 and 2 are used for general-purpose I/O or as the external address/data bus. Port
0, shown in Figure 3, differs from the other Ports in not having internal pull-ups. Figure 3
shows the structure of Port 2. An external source can pull a Port 2 pin low.
To use a pin for general-purpose output, set or clear the corresponding bit in the Px register (x = 0 or 2). To use a pin for general-purpose input, set the bit in the Px register to
turn off the output driver FET.
4127H–8051–02/08
A/T89C51AC2
D
Q
P0.X
LATCH
INTERNAL
WRITE
TO
LATCH
READ
PIN
READ
LATCH
0
1
P0.x (1)
ADDRESS LOW/
DATA
CONTROL
VDD
BUS
(2)
D
Q
P2.X
LATCH
INTERNAL
WRITE
TO
LATCH
READ
PIN
READ
LATCH
0
1
P2.x (1)
ADDRESS HIGH/
CONTROL
BUS
VDD
INTERNAL
PULL-UP (2)
Figure 2. Port 0 Structure
Notes:1. Port 0 is precluded from use as general-purpose I/O Ports when used as
address/data bus drivers.
2. Port 0 internal strong pull-ups assist the logic-one output for memory bus cycles only.
Except for these bus cycles, the pull-up FET is off, Port 0 outputs are open-drain.
Figure 3. Port 2 Structure
Notes:1. Port 2 is precluded from use as general-purpose I/O Ports when as address/data bus
drivers.
2. Port 2 internal strong pull-ups FET (P1 in FiGURE) assist the logic-one output for
memory bus cycle.
When Port 0 and Port 2 are used for an external memory cycle, an internal control signal
switches the output-driver input from the latch output to the internal address/data line.
Read-Modify-Write
Instructions
4127H–8051–02/08
Some instructions read the latch data rather than the pin data. The latch based instructions read the data, modify the data and then rewrite the latch. These are called "ReadModify-Write" instructions. Below is a complete list of these special instructions (see
Table ). When the destination operand is a Port or a Port bit, these instructions read the
latch rather than the pin:
7
A/T89C51AC2
Table 2. Read-Modify-Write Instructions
InstructionDescriptionExample
ANLlogical ANDANL P1, A
ORLlogical ORORL P2, A
XRLlogical EX-ORXRL P3, A
JBCjump if bit = 1 and clear bitJBC P1.1, LABEL
CPLcomplement bitCPL P3.0
INCincrementINC P2
DECdecrementDEC P2
DJNZdecrement and jump if not zeroDJNZ P3, LABEL
MOV Px.y, Cmove carry bit to bit y of Port xMOV P1.5, C
CLR Px.yclear bit y of Port xCLR P2.4
SET Px.yset bit y of Port xSET P3.3
It is not obvious the last three instructions in this list are Read-Modify-Write instructions.
These instructions read the port (all 8 bits), modify the specifically addressed bit and
write the new byte back to the latch. These Read-Modify-Write instructions are directed
to the latch rather than the pin in order to avoid possible misinterpretation of voltage
(and therefore, logic) levels at the pin. For example, a Port bit used to drive the base of
an external bipolar transistor can not rise above the transistor’s base-emitter junction
voltage (a value lower than VIL). With a logic one written to the bit, attempts by the CPU
to read the Port at the pin are misinterpreted as logic zero. A read of the latch rather
than the pins returns the correct logic-one value.
Quasi-Bidirectional Port
Operation
Port 1, Port 2, Port 3 and Port 4 have fixed internal pull-ups and are referred to as
"quasi-bidirectional" Ports. When configured as an input, the pin impedance appears as
logic one and sources current in response to an external logic zero condition. Port 0 is a
"true bidirectional" pin. The pins float when configured as input. Resets write logic one to
all Port latches. If logical zero is subsequently written to a Port latch, it can be returned
to input conditions by a logical one written to the latch.
Note:Port latch values change near the end of Read-Modify-Write instruction cycles. Output
buffers (and therefore the pin state) update early in the instruction after Read-ModifyWrite instruction cycle.
Logical zero-to-one transitions in Port 1, Port 2, Port 3 and Port 4 use an additional pullup (p1) to aid this logic transition (see Figure 4.). This increases switch speed. This
extra pull-up sources 100 times normal internal circuit current during 2 oscillator clock
periods. The internal pull-ups are field-effect transistors rather than linear resistors. Pullups consist of three p-channel FET (pFET) devices. A pFET is on when the gate senses
logical zero and off when the gate senses logical one. pFET #1 is turned on for two
oscillator periods immediately after a zero-to-one transition in the Port latch. A logical
one at the Port pin turns on pFET #3 (a weak pull-up) through the inverter. This inverter
and pFET pair form a latch to drive logical one. pFET #2 is a very weak pull-up switched
on whenever the associated nFET is switched off. This is traditional CMOS switch convention. Current strengths are 1/10 that of pFET #3.
8
4127H–8051–02/08
A/T89C51AC2
READ PIN
INPUT DATA
P1.x
OUTPUT DATA
2 Osc. PERIODS
n
p1(1)
p2
p3
VCCVCCVCC
P2.x
P3.x
P4.x
Figure 4. Internal Pull-Up Configurations
Note:Port 2 p1 assists the logic-one output for memory bus cycles.
4127H–8051–02/08
9
A/T89C51AC2
SFR Mapping
The Special Function Registers (SFRs) of the A/T89C51AC2 fall into the following
categories:
Sixteen addresses in the SFR space are both byte–addressable and bit–addressable. The bit–addressable SFR’s are those
whose address ends in 0 and 8. The bit addresses, in this area, are 0x80 through to 0xFF.
TL1
0000 0000
DPH
0000 0000
TH0
0000 0000
TH1
0000 0000
WDTRST
1111 1111
AUXR
x00x 1100
WDTPRG
xxxx x000
CKCON
0000 0000
PCON
00x1 0000
A7h
9Fh
97h
8Fh
87h
4127H–8051–02/08
13
A/T89C51AC2
Clock
The A/T89C51AC2 core needs only 6 clock periods per machine cycle. This feature,
called ”X2”, provides the following advantages:
•Divides frequency crystals by 2 (cheaper crystals) while keeping the same CPU
power.
•Saves power consumption while keeping the same CPU power (oscillator power
saving).
•Saves power consumption by dividing dynamic operating frequency by 2 in
operating and idle modes.
•Increases CPU power by 2 while keeping the same crystal frequency.
In order to keep the original C51 compatibility, a divider-by-2 is inserted between the
XTAL1 signal and the main clock input of the core (phase generator). This divider may
be disabled by the software.
An extra feature is available to start after Reset in the X2 mode. This feature can be
enabled by a bit X2B in the Hardware Security Byte. This bit is described in the section
"In-System-Programming".
Description
The X2 bit in the CKCON register (see Table 12) allows switching from 12 clock cycles
per instruction to 6 clock cycles and vice versa. At reset, the standard speed is activated
(STD mode).
Setting this bit activates the X2 feature (X2 mode) for the CPU Clock only (see Figure
5.).
The Timers 0, 1 and 2, Uart, PCA, or Watchdog switch in X2 mode only if the corresponding bit is cleared in the CKCON register.
The clock for the whole circuit and peripheral is first divided by two before being used by
the CPU core and peripherals. This allows any cyclic ratio to be accepted on the XTAL1
input. In X2 mode, as this divider is bypassed, the signals on XTAL1 must have a cyclic
ratio between 40 to 60%. Figure 5. shows the clock generation block diagram. The X2
bit is validated on the XTAL1÷2 rising edge to avoid glitches when switching from the X2
to the STD mode. Figure 6 shows the mode switching waveforms.
14
4127H–8051–02/08
Figure 5. Clock CPU Generation Diagram
XTAL1
XTAL2
PD
PCON.1
CPU Core
1
0
÷
2
PERIPH
CLOCK
Clock
Peripheral Clock Symbol
CPU
CLOCK
CPU Core Clock Symbol
X2
CKCON.0
X2B
Hardware byte
WDX2
CKCON.6
PCAX2
CKCON.5
SIX2
CKCON.4
T2X2
CKCON.3
T1X2
CKCON.2
T0X2
CKCON.1
IDL
PCON.0
1
0
÷
2
1
0
÷
2
1
0
÷
2
1
0
÷
2
1
0
÷
2
1
0
÷
2
X2
CKCON.0
FWd Clock
FPca Clock
FUart Clock
FT2 Clock
FT1 Clock
FT0 Clock
and ADC
On RESET
A/T89C51AC2
4127H–8051–02/08
15
A/T89C51AC2
Figure 6. Mode Switching Waveforms
XTAL1/2
XTAL1
CPU clock
X2 bit
X2 ModeSTD ModeSTD Mode
Note:In order to prevent any incorrect operation while operating in the X2 mode, users must be aware that all peripherals using the
clock frequency as a time reference (UART, timers...) will have their time reference divided by two. For example a free running
timer generating an interrupt every 20 ms will then generate an interrupt every 10 ms. A UART with a 4800 baud rate will have
a 9600 baud rate.
16
4127H–8051–02/08
A/T89C51AC2
Register
Table 12. CKCON Register
CKCON (S:8Fh)
Clock Control Register
76543210
-WDX2PCAX2SIX2T2X2T1X2T0X2X2
Bit
Number
--
6WDX2
5PCAX2
4SIX2
3T2X2
2T1X2
1T0X2
Bit
Mnemonic Description
Reserved
Do not set this bit.
Watchdog clock
Clear to select 6 clock periods per peripheral clock cycle.
Set to select 12 clock periods per peripheral clock cycle.
Programmable Counter Array clock
Clear to select 6 clock periods per peripheral clock cycle.
Set to select 12 clock periods per peripheral clock cycle.
Enhanced UART clock (MODE 0 and 2)
Clear to select 6 clock periods per peripheral clock cycle.
Set to select 12 clock periods per peripheral clock cycle.
Timer 2 clock
Clear to select 6 clock periods per peripheral clock cycle.
Set to select 12 clock periods per peripheral clock cycle.
Timer 1 clock
Clear to select 6 clock periods per peripheral clock cycle.
Set to select 12 clock periods per peripheral clock cycle.
Timer 0 clock
Clear to select 6 clock periods per peripheral clock cycle.
Set to select 12 clock periods per peripheral clock cycle.
(1)
(1)
(1)
(1)
(1)
(1)
CPU clock
0X2
Clear to select 12 clock periods per machine cycle (STD mode) for CPU and all
the peripherals.
Set to select 6 clock periods per machine cycle (X2 mode) and to enable the
individual peripherals "X2"bits.
Note:1. This control bit is validated when the CPU clock bit X2 is set; when X2 is low, this bit
has no effect.
Reset Value = x000 0000b
4127H–8051–02/08
17
A/T89C51AC2
Power Management
0
VDD
Rrst
Crst
RST pin
Internal reset
Reset input circuitry
Two power reduction modes are implemented in the A/T89C51AC2: the Idle mode and
the Power-down mode. These modes are detailed in the following sections. In addition
to these power reduction modes, the clocks of the core and peripherals can be dynamically divided by 2 using the X2 Mode detailed in Section “Clock”.
Reset Pin
At Power-up (Cold Reset)
In order to start-up (cold reset) or to restart (warm reset) properly the microcontroller, a
high level has to be applied on the RST pin. A bad level leads to a wrong initialisation of
the internal registers like SFRs, PC, etc. and to unpredictable behavior of the microcontroller. A warm reset can be applied either directly on the RST pin or indirectly by an
internal reset source such as a watchdog, PCA, timer, etc.
Two conditions are required before enabling a CPU start-up:
•VDD must reach the specified VDD range,
•The level on xtal1 input must be outside the specification (VIH, VIL).
If one of these two conditions are not met, the microcontroller does not start correctly
and can execute an instruction fetch from anywhere in the program space. An active
level applied on the RST pin must be maintained until both of the above conditions are
met. A reset is active when the level VIH1 is reached and when the pulse width covers
the period of time where VDD and the oscillator are not stabilized. Two parameters have
to be taken into account to determine the reset pulse width:
•VDD rise time (vddrst),
•Oscillator startup time (oscrst).
To determine the capacitor the highest value of these two parameters has to be chosen.
The reset circuitry is shown in Figure 7.
Figure 7. Reset Circuitry
18
Table 13 and Table 15 give some typical examples for three values of VDD rise times,
two values of oscillator start-up time and two pull-down resistor values.
Table 13. Minimum Reset Capacitor for a 15k Pull-down Resistor
oscrst/vddrst1ms10ms100ms
5 ms2.7 µF4.7 µF47 µF
20 ms10 µF15 µF47 µF
Note:These values assume VDD starts from 0v to the nominal value. If the time between two
on/off sequences is too fast, the power-supply de coupling capacitors may not be fully
discharged, leading to a bad reset sequence.
4127H–8051–02/08
A/T89C51AC2
R
RST
RST
VSS
To CPU core
and peripherals
VDD
+
VSS
VDD
RST
1K
To other
on-board
circuitry
Warm ResetTo achieve a valid reset, the reset signal must be maintained for at least 2 machine
cycles (24 oscillator clock periods) while the oscillator is running. The number of clock
periods is mode independent (X2 or X1).
Watchdog ResetAs detailed in Section “PCA Watchdog Timer”, page 80, the WDT generates a 96-clock
period pulse on the RST pin. In order to properly propagate this pulse to the rest of the
application in case of external capacitor or power-supply supervisor circuit, a 1KΩ resistor must be added as shown Figure 8.
Figure 8. Reset Circuitry for WDT reset out usage
Reset Recommendation
to Prevent Flash
Corruption
An example of bad initialization situation may occur in an instance where the bit
ENBOOT in AUXR1 register is initialized from the hardware bit BLJB upon reset. Since
this bit allows mapping of the bootloader in the code area, a reset failure can be critical.
If one wants the ENBOOT cleared in order to unmap the boot from the code area (yet
due to a bad reset) the bit ENBOOT in SFRs may be set. If the value of Program
Counter is accidently in the range of the boot memory addresses then a flash access
(write or erase) may corrupt the Flash on-chip memory.
It is recommended to use an external reset circuitry featuring power supply monitoring to
prevent system malfunction during periods of insufficient power supply voltage (power
supply failure, power supply switched off).
Idle Mode
Idle mode is a power reduction mode that reduces the power consumption. In this mode,
program execution halts. Idle mode freezes the clock to the CPU at known states while
the peripherals continue to be clocked. The CPU status before entering Idle mode is
preserved, i.e., the program counter and program status word register retain their data
for the duration of Idle mode. The contents of the SFRs and RAM are also retained. The
status of the Port pins during Idle mode is detailed in Table 14.
Entering Idle ModeTo enter Idle mode, you must set the IDL bit in PCON register (see Table 15). The
T89C51CC02 enters Idle mode upon execution of the instruction that sets IDL bit. The
instruction that sets IDL bit is the last instruction executed.
Note:If IDL bit and PD bit are set simultaneously, the T89C51CC02 enters Power-down mode.
Then it does not go in Idle mode when exiting Power-down mode.
Exiting Idle ModeThere are two ways to exit Idle mode:
1.Generate an enabled interrupt.
4127H–8051–02/08
–Hardware clears IDL bit in PCON register which restores the clock to the
CPU. Execution resumes with the interrupt service routine. Upon completion
of the interrupt service routine, program execution resumes with the
instruction immediately following the instruction that activated Idle mode.
19
A/T89C51AC2
The general-purpose flags (GF1 and GF0 in PCON register) may be used to
indicate whether an interrupt occurred during normal operation or during Idle
mode. When Idle mode is exited by an interrupt, the interrupt service routine
may examine GF1 and GF0.
2.Generate a reset.
–A logic high on the RST pin clears IDL bit in PCON register directly and
asynchronously. This restores the clock to the CPU. Program execution
momentarily resumes with the instruction immediately following the
instruction that activated the Idle mode and may continue for a number of
clock cycles before the internal reset algorithm takes control. Reset
initializes the T89C51CC02 and vectors the CPU to address C:0000h.
Notes:1. During the time that execution resumes, the internal RAM cannot be accessed; how-
ever, it is possible for the Port pins to be accessed. To avoid unexpected outputs at
the Port pins, the instruction immediately following the instruction that activated Idle
mode should not write to a Port pin or to the external RAM.
2. If Idle mode is invoked by ADC Idle, the ADC conversion completion will exit Idle.
Power-down Mode
The Power-down mode places the T89C51CC02 in a very low power state. Power-down
mode stops the oscillator and freezes all clocks at known states. The CPU status prior to
entering Power-down mode is preserved, i.e., the program counter, program status
word register retain their data for the duration of Power-down mode. In addition, the
SFRs and RAM contents are preserved. The status of the Port pins during Power-down
mode is detailed in Table 14.
Entering Power-down ModeTo enter Power-down mode, set PD bit in PCON register. The T89C51CC02 enters the
Power-down mode upon execution of the instruction that sets PD bit. The instruction
that sets PD bit is the last instruction executed.
Exiting Power-down ModeIf VDD was reduced during the Power-down mode, do not exit Power-down mode until
VDD is restored to the normal operating level.
There are two ways to exit the Power-down mode:
1.Generate an enabled external interrupt.
–The T89C51CC02 provides capability to exit from Power-down using INT0#,
INT1#.
Hardware clears PD bit in PCON register which starts the oscillator and
restores the clocks to the CPU and peripherals. Using INTx# input,
execution resumes when the input is released (see Figure 9) while using
KINx input, execution resumes after counting 1024 clock ensuring the
oscillator is restarted properly (see Figure 8). Execution resumes with the
interrupt service routine. Upon completion of the interrupt service routine,
program execution resumes with the instruction immediately following the
instruction that activated Power-down mode.
20
Note:1. The external interrupt used to exit Power-down mode must be configured as level
sensitive (INT0# and INT1#) and must be assigned the highest priority. In addition,
the duration of the interrupt must be long enough to allow the oscillator to stabilize.
The execution will only resume when the interrupt is deasserted.
2. Exit from power-down by external interrupt does not affect the SFRs nor the internal
RAM content.
–A logic high on the RST pin clears PD bit in PCON register directly and
asynchronously. This starts the oscillator and restores the clock to the CPU
and peripherals. Program execution momentarily resumes with the
instruction immediately following the instruction that activated Power-down
mode and may continue for a number of clock cycles before the internal
reset algorithm takes control. Reset initializes the T89C51CC02 and vectors
the CPU to address 0000h.
Notes:1. During the time that execution resumes, the internal RAM cannot be accessed; how-
ever, it is possible for the Port pins to be accessed. To avoid unexpected outputs at
the Port pins, the instruction immediately following the instruction that activated the
Power-down mode should not write to a Port pin or to the external RAM.
2. Exit from power-down by reset redefines all the SFRs, but does not affect the internal
RAM content.
A/T89C51AC2
Table 14. Pin Conditions in Special Operating Modes
ModePort 0 Port 1Port 2Port 3Port 4ALEPSEN#
ResetFloatingHighHighHighHighHighHigh
Idle
(internal
code)
Idle
(external
code)
Power-
Down(inter
nal code)
Power-
Down
(external
code)
DataDataDataDataDataHighHigh
FloatingDataDataDataDataHighHigh
DataDataDataDataDataLowLow
FloatingDataDataDataDataLowLow
4127H–8051–02/08
21
A/T89C51AC2
Registers
Table 15. PCON Register
PCON (S:87h) – Power configuration Register
76543210
SMOD1SMOD0-POFGF1GF0PD IDL
Bit
Number
7SMOD1
6SMOD0
5-
4POF
3GF1
2GF0
1PD
Bit
Mnemonic Description
Serial port Mode bit 1
Set to select double baud rate in mode 1, 2 or 3
Serial port Mode bit 0
Clear to select SM0 bit in SCON register.
Set to select FE bit in SCON register.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Power-Off Flag
Clear to recognize next reset type.
Set by hardware when Vcc rises from 0 to its nominal voltage. Can also be set by
software.
General-purpose flag 1
One use is to indicate whether an interrupt occurred during normal operation or
during Idle mode.
General-purpose flag 0
One use is to indicate whether an interrupt occurred during normal operation or
during Idle mode.
Power-down Mode bit
Cleared by hardware when an interrupt or reset occurs.
Set to activate the Power-down mode.
If IDL and PD are both set, PD takes precedence.
Idle Mode bit
0IDL
Cleared by hardware when an interrupt or reset occurs.
Set to activate the Idle mode.
If IDL and PD are both set, PD takes precedence.
Reset Value = 00X1 0000b
22
4127H–8051–02/08
A/T89C51AC2
Upper
128 Bytes
Internal RAM
Lower
128 Bytes
Internal RAM
Special
Function
Registers
80h
80h
00h
FFh
FFh
direct addressing
addressing
7Fh
direct or indirect
indirect addressing
256 up to 1024 Bytes
00h
64 KB
External XRAM
0000h
FFFFh
Internal XRAM
EXTRAM = 0
EXTRAM = 1
FFh or 3FFh
Internal
External
Data Memory
The A/T89C51AC2 provides data memory access in two different spaces:
1.The internal space mapped in three separate segments:
•the lower 128 Bytes RAM segment.
•the upper 128 Bytes RAM segment.
•the expanded 1024 Bytes RAM segment (XRAM).
2.The external space.
A fourth internal segment is available but dedicated to Special Function Registers,
SFRs, (addresses 80h to FFh) accessible by direct addressing mode.
Figure 11 shows the internal and external data memory spaces organization.
Figure 10. Internal Memory - RAM
Figure 11. Internal and External Data Memory Organization XRAM-XRAM
4127H–8051–02/08
23
A/T89C51AC2
Internal Space
Bit-Addressable Space
4 Banks of
8 Registers
R0-R7
30h
7Fh
(Bit Addresses 0-7Fh)
20h
2Fh
18h
1Fh
10h
17h
08h
0Fh
00h
07h
Lower 128 Bytes RAMThe lower 128 Bytes of RAM (see Figure 11) are accessible from address 00h to 7Fh
using direct or indirect addressing modes. The lowest 32 Bytes are grouped into 4
banks of 8 registers (R0 to R7). Two bits RS0 and RS1 in PSW register (see Figure 18)
select which bank is in use according to Table 16. This allows more efficient use of code
space, since register instructions are shorter than instructions that use direct addressing, and can be used for context switching in interrupt service routines.
Table 16. Register Bank Selection
RS1RS0Description
00Register bank 0 from 00h to 07h
01Register bank 0 from 08h to 0Fh
10Register bank 0 from 10h to 17h
11Register bank 0 from 18h to 1Fh
The next 16 Bytes above the register banks form a block of bit-addressable memory
space. The C51 instruction set includes a wide selection of single-bit instructions, and
the 128 bits in this area can be directly addressed by these instructions. The bit
addresses in this area are 00h to 7Fh.
Upper 128 Bytes RAMThe upper 128 Bytes of RAM are accessible from address 80h to FFh using only indirect
Expanded RAMThe on-chip 1024 Bytes of expanded RAM (XRAM) are accessible from address 0000h
24
addressing mode.
to 03FFh using indirect addressing mode through MOVX instructions. In this address
range, the bit EXTRAM in AUXR register is used to select the XRAM (default) or the
XRAM. As shown in Figure 11 when EXTRAM = 0, the XRAM is selected and when
EXTRAM = 1, the XRAM is selected.
The size of XRAM can be configured by XRS1-0 bit in AUXR register (default size is
1024 Bytes).
Note:Lower 128 Bytes RAM, Upper 128 Bytes RAM, and expanded RAM are made of volatile
memory cells. This means that the RAM content is indeterminate after power-up and
must then be initialized properly.
4127H–8051–02/08
A/T89C51AC2
RAM
PERIPHERAL
A/T89C51AC2
P2
P0
AD7:0
A15:8
A7:0
A15:8
D7:0
A7:0
ALE
WR
OERD
WR
Latch
External Space
Memory InterfaceThe external memory interface comprises the external bus (port 0 and port 2) as well as
the bus control signals (RD, WR, and ALE).
Figure 13 shows the structure of the external address bus. P0 carries address A7:0
while P2 carries address A15:8. Data D7:0 is multiplexed with A7:0 on P0. Table 17
describes the external memory interface signals.
Figure 13. External Data Memory Interface Structure
Table 17. External Data Memory Interface Signals
Signal
NameTypeDescription
A15:8O
AD7:0I/O
ALEO
RDO
WRO
Address Lines
Upper address lines for the external bus.
Address/Data Lines
Multiplexed lower address lines and data for the external
memory.
Address Latch Enable
ALE signals indicates that valid address information are available
on lines AD7:0.
Read
Read signal output to external data memory.
Write
Write signal output to external memory.
Alternative
Function
P2.7:0
P0.7:0
-
P3.7
P3.6
External Bus CyclesThis section descri bes the bus cycles the A/T89C51AC2 executes to read (see
Figure 14), and write data (see Figure 15) in the external data memory.
External memory cycle takes 6 CPU clock periods. This is equivalent to 12 oscillator
clock period in standard mode or 6 oscillator clock periods in X2 mode. For further information on X2 mode.
Slow peripherals can be accessed by stretching the read and write cycles. This is done
using the M0 bit in AUXR register. Setting this bit changes the width of the RD and WR
signals from 3 to 15 CPU clock periods.
4127H–8051–02/08
For simplicity, the accompanying figures depict the bus cycle waveforms in idealized
form and do not provide precise timing information. For bus cycle timing parameters
refer to the Section “AC Characteristics”.
25
A/T89C51AC2
Figure 14. External Data Read Waveforms
ALE
P0
P2
RD 1
DPL or RiD7:0
DPH or P22
P2
CPU Clock
ALE
P0
P2
WR
1
DPL or RiD7:0
P2
CPU Clock
DPH or P22
Notes:1.
RD
signal may be stretched using M0 bit in AUXR register.
2. When executing MOVX @Ri instruction, P2 outputs SFR content.
Figure 15. External Data Write Waveforms
Notes:1.WR signal may be stretched using M0 bit in AUXR register.
2. When executing MOVX @Ri instruction, P2 outputs SFR content.
26
4127H–8051–02/08
A/T89C51AC2
0
1
DPH0
DPH1
DPL0
0
1
DPS
AUXR1.0
DPH
DPL
DPL1
DPTR
DPTR0
DPTR1
Dual Data Pointer
DescriptionThe A/T89C51AC2 implements a second data pointer for speeding up code execution
and reducing code size in case of intensive usage of external memory accesses.
DPTR 0 and DPTR 1 are seen by the CPU as DPTR and are accessed using the SFR
addresses 83h and 84h that are the DPH and DPL addresses. The DPS bit in AUXR1
register (see Figure 20) is used to select whether DPTR is the data pointer 0 or the data
pointer 1 (see Figure 16).
Figure 16. Dual Data Pointer Implementation
ApplicationSoftware can take advantage of the additional data pointers to both increase speed and
reduce code size, for example, block operations (copy, compare…) are well served by
using one data pointer as a “source” pointer and the other one as a “destination” pointer.
Hereafter is an example of block move implementation using the two pointers and coded
in assembler. The latest C compiler takes also advantage of this feature by providing
enhanced algorithm libraries.
The INC instruction is a short (2 Bytes) and fast (6 machine cycle) way to manipulate the
DPS bit in the AUXR1 register. However, note that the INC instruction does not directly
force the DPS bit to a particular state, but simply toggles it. In simple routines, such as
the block move example, only the fact that DPS is toggled in the proper sequence matters, not its actual value. In other words, the block move routine works the same whether
DPS is '0' or '1' on entry.
; ASCII block move using dual data pointers
; Modifies DPTR0, DPTR1, A and PSW
; Ends when encountering NULL character
; Note: DPS exits opposite to the entry state unless an extra INC AUXR1 is
added
AUXR1EQU0A2h
move:movDPTR,#SOURCE ; address of SOURCE
incAUXR1 ; switch data pointers
movDPTR,#DEST ; address of DEST
mv_loop:incAUXR1; switch data pointers
movxA,@DPTR; get a byte from SOURCE
incDPTR; increment SOURCE address
incAUXR1; switch data pointers
movx@DPTR,A; write the byte to DEST
incDPTR; increment DEST address
4127H–8051–02/08
jnzmv_loop; check for NULL terminator
end_move:
27
A/T89C51AC2
Registers
Table 18. PSW Register
PSW (S:D0h)
Program Status Word Register
76543210
CYACF0RS1RS0OVF1P
Bit
Number
7CY
6AC
5F0
4-3RS1:0
2OV
1F1
0P
Bit
Mnemonic Description
Carry Flag
Carry out from bit 1 of ALU operands.
Auxiliary Carry Flag
Carry out from bit 1 of addition operands.
User Definable Flag 0.
Register Bank Select Bits
Refer to Table 16 for bits description.
Overflow Flag
Overflow set by arithmetic operations.
User Definable Flag 1
Parity Bit
Set when ACC contains an odd number of 1’s.
Cleared when ACC contains an even number of 1’s.
Reset Value = 0000 0000b
Table 19. AUXR Register
AUXR (S:8Eh)
Auxiliary Register
28
76543210
--M0-XRS1XRS0EXTRAMA0
Bit
Number
7-6-
5M0
4-
3-2XRS1-0
Bit
Mnemonic Description
Reserved
The value read from these bits are indeterminate. Do not set this bit.
Stretch MOVX control:
the RD/ and the WR/ pulse length is increased according to the value of M0.
M0 Pulse length in clock period
06
1 30
Reserved
The value read from this bit is indeterminate. Do not set this bit.
access using MOVX @ Ri/@ DPTR
0 - Internal XRAM access using MOVX @ Ri/@ DPTR.
1 - External data memory access.
Disable/Enable ALE)
0 - ALE is emitted at a constant rate of 1/6 the oscillator frequency (or 1/3 if X2
mode is used)
1 - ALE is active only during a MOVX or MOVC instruction.
Reset Value = X00X 1100b
Not bit addressable
Table 20. AUXR1 Register
AUXR1 (S:A2h)
Auxiliary Control Register 1
76543210
--ENBOOT-GF30-DPS
Bit
Number
7-6-
Bit
Mnemonic Description
Reserved
The value read from these bits is indeterminate. Do not set these bits.
Enable Boot Flash
5ENBOOT
4-
3GF3
20
1-
0DPS
(1)
Set this bit for map the boot Flash between F800h -FFFFh
Clear this bit for disable boot Flash.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
General-purpose Flag 3
Always Zero
This bit is stuck to logic 0 to allow INC AUXR1 instruction without affecting GF3
flag.
Reserved for Data Pointer Extension.
Data Pointer Select Bit
Set to select second dual data pointer: DPTR1.
Clear to select first dual data pointer: DPTR0.
Reset Value = XXXX 00X0b
Note:1. ENBOOT is initialized with the invert BLJB at reset. See In-System Programming
section.
4127H–8051–02/08
29
A/T89C51AC2
EEPROM Data
Memory
The 2 KB on-chip EEPROM memory block is located at addresses 0000h to 07FFh of
the XRAM/XRAM memory space and is selected by setting control bits in the EECON
register. A read in the EEPROM memory is done with a MOVX instruction.
A physical write in the EEPROM memory is done in two steps: write data in the column
latches and transfer of all data latches into an EEPROM memory row (programming).
The number of data written on the page may vary from 1 up to 128 Bytes (the page
size). When programming, only the data written in the column latch is programmed and
a ninth bit is used to obtain this feature. This provides the capability to program the
whole memory by Bytes, by page or by a number of Bytes in a page. Indeed, each ninth
bit is set when the writing the corresponding byte in a row and all these ninth bits are
reset after the writing of the complete EEPROM row.
Write Data in the Column
Latches
Programming
Data is written by byte to the column latches as for an external RAM memory. Out of the
11 address bits of the data pointer, the 4 MSBs are used for page selection (row) and 7
are used for byte selection. Between two EEPROM programming sessions, all the
addresses in the column latches must stay on the same page, meaning that the 4 MSB
must no be changed.
The following procedure is used to write to the column latches:
•Save and disable interrupt.
•Set bit EEE of EECON register
•Load DPTR with the address to write
•Store A register with the data to be written
•Execute a MOVX @DPTR, A
•If needed loop the three last instructions until the end of a 128 Bytes page
•Restore interrupt.
Note:The last page address used when loading the column latch is the one used to select the
page programming address.
The EEPROM programming consists of the following actions:
•writing one or more Bytes of one page in the column latches. Normally, all Bytes
must belong to the same page; if not, the last page address will be latched and the
others discarded.
•launching programming by writing the control sequence (50h followed by A0h) to the
EECON register.
•EEBUSY flag in EECON is then set by hardware to indicate that programming is in
progress and that the EEPROM segment is not available for reading.
•The end of programming is indicated by a hardware clear of the EEBUSY flag.
Note:The sequence 5xh and Axh must be executed without instructions between then other-
wise the programming is aborted.
Read Data
30
The following procedure is used to read the data stored in the EEPROM memory:
Write 5Xh followed by AXh to EEPL to launch the programming.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Enable EEPROM Space bit
Set to map the EEPROM space during MOVX instructions (Write in the column
latches)
Clear to map the XRAM space during MOVX.
Programming Busy flag
Set by hardware when programming is in progress.
Cleared by hardware when programming is done.
Can not be set or cleared by software.
Reset Value = XXXX XX00b
Not bit addressable
32
4127H–8051–02/08
A/T89C51AC2
0000h
32 KB
7FFFh
internal
0000h
7FFFh
FFFFh
8000h
Flash
32 KB
external
memory
32 KB
external
memory
EA = 0
EA = 1
Program/Code
Memory
The A/T89C51AC2 implement 32 KB of on-chip program/code memory. Figure 17
shows the partitioning of internal and external program/code memory spaces depending
on the product.
The Flash memory increases EPROM and ROM functionality by in-circuit electrical erasure and programming. Thanks to the internal charge pump, the high voltage needed for
programming or erasing Flash cells is generated on-chip using the standard VDD voltage. Thus, the Flash Memory can be programmed using only one voltage and allows InSystem-Programming commonly known as ISP. Hardware programming mode is also
available using specific programming tool.
Figure 17. Program/Code Memory Organization
Notes:1. If the program executes exclusively from on-chip code memory (not from external
memory), beware of executing code from the upper byte of on-chip memory (7FFFh)
and thereby disrupt I/O Ports 0 and 2 due to external prefetch. Fetching code constant from this location does not affect Ports 0 and 2.
2. Default factory programmed parts come with maximum hardware protection. Execution from external memory is not possible unless the Hardware Security Byte is
reprogrammed. See Table 26.
4127H–8051–02/08
33
A/T89C51AC2
External Code Memory Access
Flash
EPROM
A/T89C51AC2
P2
P0
AD7:0
A15:8
A7:0
A15:8
D7:0
A7:0
ALE
Latch
OEPSEN#
Memory InterfaceThe external memory interface comprises the external bus (port 0 and port 2) as well as
the bus control signals (PSEN#, and ALE).
Figure 18 shows the structure of the external address bus. P0 carries address A7:0
while P2 carries address A15:8. Data D7:0 is multiplexed with A7:0 on P0. Table 18
describes the external memory interface signals.
Multiplexed lower address lines and data for the external memory.
Address Latch Enable
ALE signals indicates that valid address information are available on lines
AD7:0.
Program Store Enable Output
This signal is active low during external code fetch or external code read
(MOVC instruction).
P2.7:0
P0.7:0
-
-
External Bus CyclesThis section describes the bus cycles the A/T89C51AC2 executes to fetch code (see
Figure 19) in the external program/code memory.
External memory cycle takes 6 CPU clock periods. This is equivalent to 12 oscillator
clock period in standard mode or 6 oscillator clock periods in X2 mode. For further information on X2 mode see section “Clock “.
For simplicity, the accompanying figure depicts the bus cycle waveforms in idealized
form and do not provide precise timing information.
For bus cycling parameters refer to the ‘AC-DC parameters’ section.
34
4127H–8051–02/08
Figure 19. External Code Fetch Waveforms
ALE
P0
P2
PSEN#
PCL
PCHPCH
PCLD7:0D7:0
PCH
D7:0
CPU Clock
7FFFh
32 KB
Flash memory
FM0
0000h
Hardware Security (1 byte)
Column Latches (128 Bytes)
user space
Extra Row (128 Bytes)
2 KB
Flash memory
FM1
boot space
FFFFh
F800h
FM1 mapped between F800h and
FFFFh when bit ENBOOT is set in
AUXR1 register
A/T89C51AC2
Flash Memory
Architecture
A/T89C51AC2 features two on-chip Flash memories:
•Flash memory FM0:
•Flash memory FM1:
The FM0 can be program by both parallel programming and Serial In-System-Programming (ISP) whereas FM1 supports only parallel programming by programmers. The ISP
mode is detailed in the "In-System-Programming" section.
All Read/Write access operations on Flash Memory by user application are managed by
a set of API described in the "In-System-Programming" section.
Figure 20. Flash Memory Architecture
containing 32 KB of program memory (user space) organized into 128 byte pages,
2 KB for boot loader and Application Programming Interfaces (API).
4127H–8051–02/08
35
A/T89C51AC2
FM0 Memory ArchitectureThe Flash memory is made up of 4 blocks (see Figure 20):
•The memory array (user space) 32 KB
•The Extra Row
•The Hardware security bits
•The column latch registers
User SpaceThis space is composed of a 32 KB Flash memory organized in 256 pages of 128 Bytes.
It contains the user’s application code.
Extra Row (XRow)This row is a part of FM0 and has a size of 128 Bytes. The extra row may contain infor-
mation for boot loader usage.
Hardware Security ByteThe Hardware security Byte space is a part of FM0 and has a size of 1 byte.
The 4 MSB can be read/written by software, the 4 LSB can only be read by software and
written by hardware in parallel mode.
Column LatchesThe column latches, also part of FM0, have a size of full page (128 Bytes).
The column latches are the entrance buffers of the three previous memory locations
(user array, XROW and Hardware security byte).
Cross Flash Memory Access
Description
The FM0 memory can be program only from FM1. Programming FM0 from FM0 or from
external memory is impossible.
The FM1 memory can be program only by parallel programming.
The Table 23 show all software Flash access allowed.
Table 23. Cross Flash Memory Access
(user Flash)
(boot Flash)
Code executing from
FM0
FM1
External
memory
EA = 0
FM0
Action
Readok-
Load column latchok-
Write--
Readokok
Load column latchok-
Writeok-
Read--
Load column latch--
Write--
(user Flash)
FM1
(boot Flash)
36
4127H–8051–02/08
A/T89C51AC2
Overview of FM0
Operations
The CPU interfaces to the Flash memory through the FCON register and AUXR1
register.
These registers are used to:
•Map the memory spaces in the adressable space
•Launch the programming of the memory spaces
•Get the status of the Flash memory (busy/not busy)
Mapping of the Memory SpaceBy default, the user space is accessed by MOVC instruction for read only. The column
latches space is made accessible by setting the FPS bit in FCON register. Writing is
possible from 0000h to 7FFFh, address bits 6 to 0 are used to select an address within a
page while bits 14 to 7 are used to select the programming address of the page.
Setting FPS bit takes precedence on the EXTRAM bit in AUXR register.
The other memory spaces (user, extra row, hardware security) are made accessible in
the code segment by programming bits FMOD0 and FMOD1 in FCON register in accordance with Table 24. A MOVC instruction is then used for reading these spaces.
Table 24. FM0 Blocks Select Bits
FMOD1FMOD0FM0 Adressable space
00User (0000h-7FFFh)
01Extra Row(FF80h-FFFFh)
10Hardware Security Byte (0000h)
11Reserved
Launching ProgrammingFPL3:0 bits in FCON register are used to secure the launch of programming. A specific
sequence must be written in these bits to unlock the write protection and to launch the
programming. This sequence is 5xh followed by Axh. Table 25 summarizes the memory
spaces to program according to FMOD1:0 bits.
4127H–8051–02/08
37
A/T89C51AC2
Table 25. Programming Spaces
Write to FCON
5X00No action
OperationFPL3:0FPSFMOD1FMOD0
User
Extra Row
Hardware
Security
Byte
Reserved
Notes:1. The sequence 5xh and Axh must be executing without instructions between them
2. Interrupts that may occur during programming time must be disabled to avoid any
AX00
5X01No action
AX01
5X10No action
AX10Write the fuse bits space
5X11No action
AX11No action
otherwise the programming is aborted.
spurious exit of the programming mode.
Write the column latches in user
space
Write the column latches in extra row
space
Status of the Flash MemoryThe bit FBUSY in FCON register is used to indicate the status of programming.
FBUSY is set when programming is in progress.
Selecting FM1The bit ENBOOT in AUXR1 register is used to map FM1 from F800h to FFFFh.
Loading the Column LatchesAny number of data from 1 Byte to 128 Bytes can be loaded in the column latches. This
provides the capability to program the whole memory by byte, by page or by any number
of Bytes in a page.
When programming is launched, an automatic erase of the locations loaded in the column latches is first performed, then programming is effectively done. Thus no page or
block erase is needed and only the loaded data are programmed in the corresponding
page.
38
The following procedure is used to load the column latches and is summarized in
Figure 21:
•Save then disable interrupt and map the column latch space by setting FPS bit.
•Load the DPTR with the address to load.
•Load Accumulator register with the data to load.
•Execute the MOVX @DPTR, A instruction.
•If needed loop the three last instructions until the page is completely loaded.
•Unmap the column latch and Restore Interrupt
4127H–8051–02/08
Figure 21. Column Latches Loading Procedure
Column Latches
Loading
Data Load
DPTR = Address
ACC = Data
Exec: MOVX @DPTR, A
Last Byte
to load?
Column Latches Mapping
FCON = 08h (FPS=1)
Data memory Mapping
FCON = 00h (FPS = 0)
Save and Disable IT
EA = 0
Restore IT
A/T89C51AC2
Note:The last page address used when loading the column latch is the one used to select the
page programming address.
Programming the Flash Spaces
UserThe following procedure is used to program the User space and is summarized in
Figure 22:
•Load up to one page of data in the column latches from address 0000h to 7FFFh.
•Save then disable the interrupts.
•Launch the programming by writing the data sequence 50h followed by A0h in
FCON register (only from FM1).
The end of the programming indicated by the FBUSY flag cleared.
•Restore the interrupts.
Extra RowThe following procedure is used to program the Extra Row space and is summarized in
Figure 22:
•Load data in the column latches from address FF80h to FFFFh.
•Save then disable the interrupts.
•Launch the programming by writing the data sequence 52h followed by A2h in
FCON register. This step of the procedure must be executed from FM1. The end of
the programming indicated by the FBUSY flag cleared.
The end of the programming indicated by the FBUSY flag cleared.
•Restore the interrupts.
4127H–8051–02/08
39
A/T89C51AC2
Figure 22. Flash and Extra Row Programming Procedure
Flash Spaces
Programming
Save and Disable IT
EA = 0
Launch Programming
FCON = 5xh
FCON = Axh
End Programming
Restore IT
Column Latches Loading
see Figure 21
FBusy
Cleared?
Clear Mode
FCON = 00h
Hardware Security ByteThe following procedure is used to program the Hardware
summarized in Figure 23:
•Set FPS and map Hardware byte (FCON = 0x0C)
•Save and disable the interrupts.
•Load DPTR at address 0000h.
•Load Accumulator register with the data to load.
•Execute the MOVX @DPTR, A instruction.
•Launch the programming by writing the data sequence 54h followed by A4h in
FCON register. This step of the procedure must be executed from FM1. The end of
the programming indicated by the FBUSY flag cleared.
The end of the programming indicated by the FBusy flag cleared.
•Restore the interrupts.
Security
Byte space and is
40
4127H–8051–02/08
Figure 23. Hardware Programming Procedure
Flash Spaces
Programming
Save and Disable IT
EA = 0
Launch Programming
FCON = 54h
FCON = A4h
End Programming
RestoreIT
FBusy
Cleared?
Clear Mode
FCON = 00h
Data Load
DPTR = 00h
ACC = Data
Exec: MOVX @DPTR, A
FCON = 0Ch
Save and Disable IT
EA = 0
End Loading
Restore IT
A/T89C51AC2
Reading the Flash Spaces
UserThe following procedure is used to read the User space:
•Read one byte in Accumulator by executing MOVC A,@A+DPTR where A+DPTR is
the address of the code byte to read.
Note:FCON is supposed to be reset when not needed.
Extra RowThe following procedure is used to read the Extra Row space and is summarized in
Hardware Security ByteThe following procedure is used to read the Hardware
Figure 24:
•Map the Extra Row space by writing 02h in FCON register.
•Read one byte in Accumulator by executing MOVC A,@A+DPTR with A = 0 and
DPTR = FF80h to FFFFh.
•Clear FCON to unmap the Extra Row.
in Figure 24:
Security
space and is summarized
•Map the Hardware Security space by writing 04h in FCON register.
•Read the byte in Accumulator by executing MOVC A,@A+DPTR with A = 0 and
DPTR = 0000h.
•Clear FCON to unmap the Hardware Security Byte.
4127H–8051–02/08
41
A/T89C51AC2
Figure 24. Reading Procedure
Flash Spaces Reading
Flash Spaces Mapping
FCON = 0000aa0b
(1)
Data Read
DPTR = Address
ACC = 0
Exec: MOVC A, @A+DPTR
Clear Mode
FCON = 00h
Note:1. aa = 10 for the Hardware Security Byte.
Flash Protection from Parallel
Programming
The three lock bits in Hardware Security Byte (see "In-System-Programming" section)
are programmed according to Table 26 provide different level of protection for the onchip code and data located in FM0 and FM1.
The only way to write these bits are the parallel mode. They are set by default to level 4
Table 26. Program Lock bit
Program Lock Bits
Security
Level
1UUU
2PUU
3UPU
4UUP
LB0LB1LB2
Protection Description
No program lock features enabled. MOVC instruction executed from
external program memory returns non coded data.
MOVC instructions executed from external program memory are barred
to return code bytes from internal memory, EA is sampled and latched
on reset, and further parallel programming of the Flash is disabled.
Same as 2, also verify through parallel programming interface is
disabled.
Same as 3, also external execution is disabled if code roll over beyond
7FFFh
Program Lock bits
U: unprogrammed
Preventing Flash CorruptionSee the “Power Management” section.
42
P: programmed
WARNING: Security level 2 and 3 should only be programmed after Flash and Core
verification.
4127H–8051–02/08
A/T89C51AC2
Registers
FCON RegisterFCON (S:D1h)
Flash Control Register
76543210
FPL3FPL2FPL1FPL0FPSFMOD1FMOD0FBUSY
Bit
Number
7-4FPL3:0
3FPS
2-1FMOD1:0
0FBUSY
Bit
Mnemonic Description
Programming Launch Command Bits
Write 5Xh followed by AXh to launch the programming according to FMOD1:0
(see Table 25)
Flash Map Program Space
Set to map the column latch space in the data memory space.
Clear to re-map the data memory space.
Flash Mode
See Table 24 or Table 25.
Flash Busy
Set by hardware when programming is in progress.
Clear by hardware when programming is done.
Can not be changed by software.
Reset Value = 0000 0000b
4127H–8051–02/08
43
A/T89C51AC2
Operation Cross Memory Access
Space addressable in read and write are:
•RAM
•ERAM (Expanded RAM access by movx)
•XRAM (eXternal RAM)
•EEPROM DATA
•FM0 (user flash)
•Hardware byte
•XROW
•Boot Flash
•Flash Column latch
The table below provide the different kind of memory which can be accessed from different code location.
Table 27. Cross Memory Access
ActionRAM
boot FLASH
FM0
External
memory
EA = 0
or Code Roll
Over
ReadOKOKOKOK-
Write-OK
ReadOKOKOKOK-
Write-OK (idle)OK
Read--OK--
Write--OK
Note:1. RWW: Read While Write
XRAM
ERAMBoot FLASHFM0E² Data
(1)
OK
(1)
(1)
(1)
Hardware
ByteXROW
(1)
OK
-OK
--
OK
(1)
44
4127H–8051–02/08
A/T89C51AC2
Sharing Instructions
Table 28. Instructions shared
XRAM
ActionRAM
ReadMOVMOVXMOVXMOVCMOVCMOVCMOVC
WriteMOVMOVXMOVX-by clby clby cl
ERAM
Note:by cl: using Column Latch
EEPROM
DATA
Boot
FLASHFM0
Hardware
ByteXROW
Table 29. Read MOVX A, @DPTR
EEE bit in
EECON
Register
00XXOK
01XXOK
10XXOK
11XXOK
FPS in
FCON RegisterENBOOTEA
XRAM
ERAM
EEPROM
DATA
Table 30. Write MOVX @DPTR,A
EEE bit in
EECON
Register
FPS bit in
FCON RegisterENBOOTEA
XRAM
ERAM
EEPROM
Data
Flash
Column
Latch
Flash
Column
Latch
00XXOK
01X
10XXOK
11X
1OK
0OK
1OK
0OK
4127H–8051–02/08
45
A/T89C51AC2
Table 31. Read MOVC A, @DPTR
Code Execution
From FM0
From FM1
(ENBOOT =1
FCON Register
ENBOOTDPTRFM1FM0XROW
00000h to 7FFFhOK
00X
01XX
10XXXOK
11X
0
00
1
01X
1
0000h to 7FFFhOK
1
1
0XNA
1XOK
0XNA
1
0NA
0000h to 7FFFhOK
F800h to FFFFhDo not use this configuration
0000 to 007Fh
(1)
See
0000h to 7FFFhOK
F800h to FFFFhDo not use this configuration
0000h to 7FFFOK
F800h to FFFFhOK
0000h to 007h
(2)
See
OK
OK
Hardware
Byte
External
CodeFMOD1 FMOD0FPS
External code :
EA=0 or Code
Roll Over
10X
11X
X0XXXOK
1
0NA
1
0NA
X
OK
000h to 7FFFh
OK
1. For DPTR higher than 007Fh only lowest 7 bits are decoded, thus the behavior is the same as for addresses from 0000h to
007Fh
2. For DPTR higher than 007Fh only lowest 7 bits are decoded, thus the behavior is the same as for addresses from 0000h to
007Fh
46
4127H–8051–02/08
A/T89C51AC2
F800h
7FFFh
32 KB
Flash memory
2 KB IAP
bootloader
FM0
FM1
Custom
Boot Loader
[SBV]00h
FFFFh
FM1 mapped between F800h and FFFFh
when API called
0000h
In-System
Programming (ISP)
Flash Programming and
Erasure
With the implementation of the User Space (FM0) and the Boot Space (FM1) in Flash
technology the A/T89C51AC2 allows the system engineer the development of applications with a very high level of flexibility. This flexibility is based on the possibility to alter
the customer program at any stages of a product’s life:
•Before mounting the chip on the PCB, FM0 Flash can be programmed with the
application code. FM1 is always preprogrammed by Atmel with a bootloader (UART
bootloader).
•Once the chip is mounted on the PCB, it can be programmed by serial mode via the
UART.
Note:1. The user can also program his own bootloader in FM1.
This In-System-Programming (ISP) allows code modification over the total lifetime of the
product.
Besides the default Boot loader Atmel provide to the customer also all the needed Application-Programming-Interfaces (API) which are needed for the ISP. The API are located
also in the Boot memory.
This allow the customer to have a full use of the 32 KB user memory.
There are three methods of programming the Flash memory:
•The Atmel bootloader located in FM1 is activated by the application. Low level API
routines (located in FM1) will be used to program FM0. The interface used for serial
downloading to FM0 is the UART. API can be called also by the user’s bootloader
located in FM0 at [SBV]00h.
•A further method exists in activating the Atmel boot loader by hardware activation.
See Section “Hardware Security Byte”.
•The FM0 can be programmed also by the parallel mode using a programmer.
(1)
Figure 25. Flash Memory Mapping
4127H–8051–02/08
47
A/T89C51AC2
Boot Process
Software Boot Process
Example
Many algorithms can be used for the software boot process. Below are descriptions of
the different flags and Bytes.
Boot Loader Jump Bit (BLJB):
- This bit indicates if on RESET the user wants to jump to this application at address
@0000h on FM0 or execute the boot loader at address @F800h on FM1.
- BLJB = 0 (i.e. bootloader FM1 executed after a reset) is the default Atmel factory pro-
gramming.
- To read or modify this bit, the APIs are used.
Boot Vector Address (SBV):
- This byte contains the MSB of the user boot loader address in FM0.
- The default value of SBV is FCh (no user boot loader in FM0).
- To read or modify this byte, the APIs are used.
Extra Byte (EB) and Boot Status Byte (BSB):
- These Bytes are reserved for customer use.
- To read or modify these Bytes, the APIs are used.
Hardware Boot Process At the falling edge of RESET, the bit ENBOOT in AUXR1 register is initialized with the
value of Boot Loader Jump Bit (BLJB).
Further at the falling edge of RESET if the following conditions (called Hardware condition) are detected. The FCON register is initialized with the value 00h and the PC is
initialized with F800h (FM1 lower byte = Bootloader entry point).
Harware Conditions:
•PSEN low
(1)
•EA high,
•ALE high (or not connected).
The Hardware condition forces the bootloader to be executed, whatever BLJB value is.
Then BLBJ will be checked.
If no hardware condition is detected, the FCON register is initialized with the value F0h.
Then BLJB value will be checked.
Conditions are:
•If bit BLJB = 1:
User application in FM0 will be started at @0000h (standard reset).
•If bit BLJB = 0:
Boot loader will be started at @F800h in FM1.
Note:1. As PSEN is an output port in normal operating mode (running user applications or
bootloader applications) after reset it is recommended to release PSEN after the falling edge of Reset is signaled.
The hardware conditions are sampled at reset signal Falling Edge, thus they can be
released at any time when reset input is low.
2. To ensure correct microcontroller startup, the PSEN pin should not be tied to ground
during power-on.
48
4127H–8051–02/08
Figure 26. Hardware Boot Process Algorithm
RESET
Hardware
condition?
BLJB = = 0
?
bit ENBOOT in AUXR1 register
is initialized with BLJB inverted.
Hardware
Software
ENBOOT = 1
PC = F800h
ENBOOT = 1
PC = F800h
FCON = 00h
FCON = F0h
Boot Loader
in FM1
ENBOOT = 0
PC = 0000h
Yes
Yes
No
No
Application
in FM0
(Example, if BLJB=0, ENBOOT is set (=1) during reset,
thus the bootloader is executed after the reset)
A/T89C51AC2
Application
Programming Interface
XROW Bytes
Several Application Program Interface (API) calls are available for use by an application
program to permit selective erasing and programming of Flash pages. All calls are made
by functions.
All of these APIs are described in detail in the following documents on the Atmel web
site.
•Datasheet Bootloader UART A/T89C51AC2
Table 32. XROW Mapping
DescriptionDefault ValueAddress
Copy of the Manufacturer Code58h30h
Copy of the Device ID#1: Family codeD7h31h
Copy of the Device ID#2: Memories size and typeF7h60h
Copy of the Device ID#3: Name and RevisionFFh61h
4127H–8051–02/08
49
A/T89C51AC2
Hardware Security Byte
Table 33. Hardware Security Byte
76543210
X2BBLJB---LB2LB1LB0
Bit
Number
7X2B
6BLJB
5-3-
2-0LB2:0
Bit
Mnemonic Description
X2 Bit
Set this bit to start in standard mode.
Clear this bit to start in X2 mode.
Boot Loader JumpBit
- 1: To start the user’s application on next RESET (@0000h) located in FM0,
- 0: To start the boot loader(@F800h) located in FM1.
Reserved
The value read from these bits are indeterminate.
Lock Bits
After erasing the chip in parallel mode, the default value is : FFh
The erasing in ISP mode (from bootloader) does not modify this byte.
Notes:1. Only the 4 MSB bits can be accessed by software.
2. The 4 LSB bits can only be accessed by parallel mode.
50
4127H–8051–02/08
A/T89C51AC2
Write SBUF
RI
TI
SBUF
Transmitter
SBUF
Receiver
IB Bus
Mode 0 Transmit
Receive
Shift register
Load SBUF
Read SBUF
InterruptRequest
Serial Port
TXD
RXD
RITIRB8TB8RENSM2SM1SM0/FE
IDLPDGF0GF1POF-SMOD0SMOD1
To UART framing error control
SM0 to UART mode control
Set FE bit if stop bit is 0 (framing error)
Serial I/O Port
The A/T89C51AC2 I/O serial port is compatible with the I/O serial port in the 80C52.
It provides both synchronous and asynchronous communication modes. It operates as a
Universal Asynchronous Receiver and Transmitter (UART) in three full-duplex modes
(Modes 1, 2 and 3). Asynchronous transmission and reception can occur simultaneously
and at different baud rates
Serial I/O port includes the following enhancements:
•Framing error detection
•Automatic address recognition
Figure 27. Serial I/O Port Block Diagram
Framing Error Detection
Figure 28. Framing Error Block Diagram
4127H–8051–02/08
Framing bit error detection is provided for the three asynchronous modes. To enable the
framing bit error detection feature, set SMOD0 bit in PCON register.
When this feature is enabled, the receiver checks each incoming data frame for a valid
stop bit. An invalid stop bit may result from noise on the serial lines or from simultaneous
transmission by two CPUs. If a valid stop bit is not found, the Framing Error bit (FE) in
SCON register bit is set.
The software may examine the FE bit after each reception to check for data errors.
Once set, only software or a reset clears the FE bit. Subsequently received frames with
valid stop bits cannot clear the FE bit. When the FE feature is enabled, RI rises on the
stop bit instead of the last data bit (See Figure 29. and Figure 30.).
51
A/T89C51AC2
Figure 29. UART Timing in Mode 1
Data byte
RI
SMOD0=X
Stop
bit
Start
bit
RXD
D7D6D5D4D3D2D1D0
FE
SMOD0=1
RI
SMOD0=0
Data byteNinth
bit
Stop
bit
Start
bit
RXD
D8D7D6D5D4D3D2D1D0
RI
SMOD0=1
FE
SMOD0=1
Figure 30. UART Timing in Modes 2 and 3
Automatic Address
Recognition
The automatic address recognition feature is enabled when the multiprocessor communication feature is enabled (SM2 bit in SCON register is set).
Implemented in the hardware, automatic address recognition enhances the multiprocessor communication feature by allowing the serial port to examine the address of each
incoming command frame. Only when the serial port recognizes its own address will the
receiver set the RI bit in the SCON register to generate an interrupt. This ensures that
the CPU is not interrupted by command frames addressed to other devices.
If necessary, you can enable the automatic address recognition feature in mode 1. In
this configuration, the stop bit takes the place of the ninth data bit. Bit RI is set only when
the received command frame address matches the device’s address and is terminated
by a valid stop bit.
To support automatic address recognition, a device is identified by a given address and
a broadcast address.
Note:The multiprocessor communication and automatic address recognition features cannot
be enabled in mode 0 (i.e. setting SM2 bit in SCON register in mode 0 has no effect).
52
4127H–8051–02/08
A/T89C51AC2
Given Address
Each device has an individual address that is specified in the SADDR register; the
SADEN register is a mask byte that contains don’t-care bits (defined by zeros) to form
the device’s given address. The don’t-care bits provide the flexibility to address one or
more slaves at a time. The following example illustrates how a given address is formed.
To address a device by its individual address, the SADEN mask byte must be 11111111b.
For example:
SADDR0101 0110b
SADEN1111 1100b
Given0101 01XXb
Here is an example of how to use given addresses to address different slaves:
Slave A:SADDR1111 0001b
SADEN1111 1010b
Given1111 0X0Xb
Slave B:SADDR1111 0011b
SADEN1111 1001b
Given1111 0XX1b
Slave C:SADDR1111 0011b
SADEN1111 1101b
Given1111 00X1b
The SADEN byte is selected so that each slave may be addressed separately.
For slave A, bit 0 (the LSB) is a don’t-care bit; for slaves B and C, bit 0 is a 1. To communicate with slave A only, the master must send an address where bit 0 is clear (e.g.
1111 0000b).
For slave A, bit 1 is a 0; for slaves B and C, bit 1 is a don’t care bit. To communicate with
slaves A and B, but not slave C, the master must send an address with bits 0 and 1 both
set (e.g. 1111 0011b).
To communicate with slaves A, B and C, the master must send an address with bit 0 set,
bit 1 clear, and bit 2 clear (e.g. 1111 0001b).
Broadcast Address
4127H–8051–02/08
A broadcast address is formed from the logical OR of the SADDR and SADEN registers
with zeros defined as don’t-care bits, e.g.:
SADDR 0101 0110b
SADEN 1111 1100b
SADDR OR SADEN1111 111Xb
The use of don’t-care bits provides flexibility in defining the broadcast address, however
in most applications, a broadcast address is FFh. The following is an example of using
broadcast addresses:
Slave A:SADDR1111 0001b
SADEN1111 1010b
Given1111 1X11b,
Slave B:SADDR1111 0011b
SADEN1111 1001b
Given1111 1X11B,
Slave C:SADDR=1111 0010b
SADEN1111 1101b
Given1111 1111b
53
A/T89C51AC2
For slaves A and B, bit 2 is a don’t care bit; for slave C, bit 2 is set. To communicate with
all of the slaves, the master must send an address FFh. To communicate with slaves A
and B, but not slave C, the master can send and address FBh.
Registers
Table 34. SCON Register
SCON (S:98h)
Serial Control Register
76543210
FE/SM0SM1SM2RENTB8RB8TIRI
Bit
Number
7
6SM1
5SM2
Bit
Mnemonic Description
FE
SM0
Framing Error bit (SMOD0=1
Clear to reset the error state, not cleared by a valid stop bit.
Set by hardware when an invalid stop bit is detected.
Serial port Mode 2 bit/Multiprocessor Communication Enable bit
Clear to disable multiprocessor communication feature.
Set to enable multiprocessor communication feature in mode 2 and 3.
)
XTAL
XTAL
/12 (or F
/64 or F
/6 in mode X2)
XTAL
/32
XTAL
4REN
3TB8
2RB8
1TI
0RI
Reception Enable bit
Clear to disable serial reception.
Set to enable serial reception.
Transmitter Bit 8/Ninth bit to transmit in modes 2 and 3
Clear to transmit a logic 0 in the 9th bit.
Set to transmit a logic 1 in the 9th bit.
Receiver Bit 8/Ninth bit received in modes 2 and 3
Cleared by hardware if 9th bit received is a logic 0.
Set by hardware if 9th bit received is a logic 1.
Transmit Interrupt flag
Clear to acknowledge interrupt.
Set by hardware at the end of the 8th bit time in mode 0 or at the beginning of the
stop bit in the other modes.
Receive Interrupt flag
Clear to acknowledge interrupt.
Set by hardware at the end of the 8th bit time in mode 0, see Figure 29. and
Figure 30. in the other modes.
Reset Value = 0000 0000b
Bit addressable
54
4127H–8051–02/08
A/T89C51AC2
Table 35. SADEN Register
SADEN (S:B9h)
Slave Address Mask Register
76543210
––––––––
Bit
Number
7-0
Bit
Mnemonic Description
Mask Data for Slave Individual Address
Reset Value = 0000 0000b
Not bit addressable
Table 36. SADDR Register
SADDR (S:A9h)
Slave Address Register
76543210
––––––––
Bit
Number
7-0
Bit
Mnemonic Description
Slave Individual Address
Reset Value = 0000 0000b
Not bit addressable
Table 37. SBUF Register
4127H–8051–02/08
SBUF (S:99h)
Serial Data Buffer
76543210
––––––––
Bit
Number
7-0
Bit
Mnemonic Description
Data sent/received by Serial I/O Port
Reset Value = 0000 0000b
Not bit addressable
55
A/T89C51AC2
Table 38. PCON Register
PCON (S:87h)
Power Control Register
76543210
SMOD1SMOD0–POFGF1GF0PDIDL
Bit
Number
7SMOD1
6SMOD0
5-
4POF
3GF1
2GF0
1PD
0IDL
Bit
Mnemonic Description
Serial port Mode bit 1
Set to select double baud rate in mode 1, 2 or 3.
Serial port Mode bit 0
Clear to select SM0 bit in SCON register.
Set to select FE bit in SCON register.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Power-Off Flag
Clear to recognize next reset type.
Set by hardware when VCC rises from 0 to its nominal voltage. Can also be set
by software.
General-purpose Flag
Cleared by user for general-purpose usage.
Set by user for general-purpose usage.
General-purpose Flag
Cleared by user for general-purpose usage.
Set by user for general-purpose usage.
Power-Down mode bit
Cleared by hardware when reset occurs.
Set to enter power-down mode.
Idle mode bit
Clear by hardware when interrupt or reset occurs.
Set to enter idle mode.
56
Reset Value = 00X1 0000b
Not bit addressable
4127H–8051–02/08
A/T89C51AC2
Timers/Counters
Timer/Counter
Operations
The A/T89C51AC2 implements two general-purpose, 16-bit Timers/Counters. Such are
identified as Timer 0 and Timer 1, and can be independently configured to operate in a
variety of modes as a Timer or an event Counter. When operating as a Timer, the
Timer/Counter runs for a programmed length of time, then issues an interrupt request.
When operating as a Counter, the Timer/Counter counts negative transitions on an
external pin. After a preset number of counts, the Counter issues an interrupt request.
The various operating modes of each Timer/Counter are described in the following
sections.
A basic operation is Timer registers THx and TLx (x = 0, 1) connected in cascade to
form a 16-bit Timer. Setting the run control bit (TRx) in TCON register (see Figure 39)
turns the Timer on by allowing the selected input to increment TLx. When TLx overflows
it increments THx; when THx overflows it sets the Timer overflow flag (TFx) in TCON
register. Setting the TRx does not clear the THx and TLx Timer registers. Timer registers can be accessed to obtain the current count or to enter preset values. They can be
read at any time but TRx bit must be cleared to preset their values, otherwise the behavior of the Timer/Counter is unpredictable.
The C/Tx# control bit selects Timer operation or Counter operation by selecting the
divided-down peripheral clock or external pin Tx as the source for the counted signal.
TRx bit must be cleared when changing the mode of operation, otherwise the behavior
of the Timer/Counter is unpredictable.
For Timer operation (C/Tx# = 0), the Timer register counts the divided-down peripheral
clock. The Timer register is incremented once every peripheral cycle (6 peripheral clock
periods). The Timer clock rate is F
PER
/6, i.e. F
/12 in standard mode or F
OSC
OSC
/6 in X2
mode.
Timer 0
For Counter operation (C/Tx# = 1), the Timer register counts the negative transitions on
the Tx external input pin. The external input is sampled every peripheral cycles. When
the sample is high in one cycle and low in the next one, the Counter is incremented.
Since it takes 2 cycles (12 peripheral clock periods) to recognize a negative transition,
the maximum count rate is F
/12, i.e. F
PER
/24 in standard mode or F
OSC
/12 in X2
OSC
mode. There are no restrictions on the duty cycle of the external input signal, but to
ensure that a given level is sampled at least once before it changes, it should be held for
at least one full peripheral cycle.
Timer 0 functions as either a Timer or event Counter in four modes of operation.
Figure 31 to Figure 34 show the logical configuration of each mode.
Timer 0 is controlled by the four lower bits of TMOD register (see Figure 40) and bits 0,
1, 4 and 5 of TCON register (see Figure 39). TMOD register selects the method of Timer
gating (GATE0), Timer or Counter operation (T/C0#) and mode of operation (M10 and
M00). TCON register provides Timer 0 control functions: overflow flag (TF0), run control
bit (TR0), interrupt flag (IE0) and interrupt type control bit (IT0).
For normal Timer operation (GATE0 = 0), setting TR0 allows TL0 to be incremented by
the selected input. Setting GATE0 and TR0 allows external pin INT0# to control Timer
operation.
Timer 0 overflow (count rolls over from all 1s to all 0s) sets TF0 flag generating an interrupt request.
It is important to stop Timer/Counter before changing mode.
4127H–8051–02/08
57
A/T89C51AC2
Mode 0 (13-bit Timer)Mode 0 configures Timer 0 as an 13-bit Timer which is set up as an 8-bit Timer (TH0
FTx
CLOCK
TRx
TCON reg
TFx
TCON reg
0
1
GATEx
TMOD reg
÷
6
Overflow
Timer x
Interrupt
Request
C/Tx#
TMOD reg
TLx
(5 bits)
THx
(8 bits)
INTx#
Tx
See the “Clock” section
TRx
TCON reg
TFx
TCON reg
0
1
GATEx
TMOD reg
Overflow
Timer x
Interrupt
Request
C/Tx#
TMOD reg
TLx
(8 bits)
THx
(8 bits)
INTx#
Tx
FTx
CLOCK
÷
6
See the “Clock” section
register) with a modulo 32 prescaler implemented with the lower five bits of TL0 register
(see Figure 31). The upper three bits of TL0 register are indeterminate and should be
ignored. Prescaler overflow increments TH0 register.
Figure 31. Timer/Counter x (x = 0 or 1) in Mode 0
Mode 1 (16-bit Timer)Mode 1 configures Timer 0 as a 16-bit Timer with TH0 and TL0 registers connected in
cascade (see Figure 32). The selected input increments TL0 register.
Figure 32. Timer/Counter x (x = 0 or 1) in Mode 1
58
4127H–8051–02/08
A/T89C51AC2
TRx
TCON reg
TFx
TCON reg
0
1
GATEx
TMOD reg
Overflow
Timer x
Interrupt
Request
C/Tx#
TMOD reg
TLx
(8 bits)
THx
(8 bits)
INTx#
Tx
FTx
CLOCK
÷
6
See the “Clock” section
TR0
TCON.4
TF0
TCON.5
INT0#
0
1
GATE0
TMOD.3
Overflow
Timer 0
Interrupt
Request
C/T0#
TMOD.2
TL0
(8 bits)
TR1
TCON.6
TH0
(8 bits)
TF1
TCON.7
Overflow
Timer 1
Interrupt
Request
T0
FTx
CLOCK
÷
6
FTx
CLOCK
÷
6
See the “Clock” section
Mode 2 (8-bit Timer with AutoReload)
Mode 2 configures Timer 0 as an 8-bit Timer (TL0 register) that automatically reloads
from TH0 register (see Figure 33). TL0 overflow sets TF0 flag in TCON register and
reloads TL0 with the contents of TH0, which is preset by software. When the interrupt
request is serviced, hardware clears TF0. The reload leaves TH0 unchanged. The next
reload value may be changed at any time by writing it to TH0 register.
Figure 33. Timer/Counter x (x = 0 or 1) in Mode 2
Mode 3 (Two 8-bit Timers)Mode 3 configures Timer 0 such that registers TL0 and TH0 operate as separate 8-bit
Timers (see Figure 34). This mode is provided for applications requiring an additional 8bit Timer or Counter. TL0 uses the Timer 0 control bits C/T0# and GATE0 in TMOD register, and TR0 and TF0 in TCON register in the normal manner. TH0 is locked into a
Timer function (counting F
/6) and takes over use of the Timer 1 interrupt (TF1) and
PER
run control (TR1) bits. Thus, operation of Timer 1 is restricted when Timer 0 is in mode
3.
Figure 34. Timer/Counter 0 in Mode 3: Two 8-bit Counters
4127H–8051–02/08
59
A/T89C51AC2
Timer 1
Timer 1 is identical to Timer 0 excepted for Mode 3 which is a hold-count mode. The following comments help to understand the differences:
•Timer 1 functions as either a Timer or event Counter in three modes of operation.
Figure 31 to Figure 33 show the logical configuration for modes 0, 1, and 2. Timer
1’s mode 3 is a hold-count mode.
•Timer 1 is controlled by the four high-order bits of TMOD register (see Figure 40)
and bits 2, 3, 6 and 7 of TCON register (see Figure 39). TMOD register selects the
method of Timer gating (GATE1), Timer or Counter operation (C/T1#) and mode of
operation (M11 and M01). TCON register provides Timer 1 control functions:
overflow flag (TF1), run control bit (TR1), interrupt flag (IE1) and interrupt type
control bit (IT1).
•Timer 1 can serve as the Baud Rate Generator for the Serial Port. Mode 2 is best
suited for this purpose.
•For normal Timer operation (GATE1 = 0), setting TR1 allows TL1 to be incremented
by the selected input. Setting GATE1 and TR1 allows external pin INT1# to control
Timer operation.
•Timer 1 overflow (count rolls over from all 1s to all 0s) sets the TF1 flag generating
an interrupt request.
•When Timer 0 is in mode 3, it uses Timer 1’s overflow flag (TF1) and run control bit
(TR1). For this situation, use Timer 1 only for applications that do not require an
interrupt (such as a Baud Rate Generator for the Serial Port) and switch Timer 1 in
and out of mode 3 to turn it off and on.
•It is important to stop Timer/Counter before changing mode.
Mode 0 (13-bit Timer)Mode 0 configures Timer 1 as a 13-bit Timer, which is set up as an 8-bit Timer (TH1 reg-
ister) with a modulo-32 prescaler implemented with the lower 5 bits of the TL1 register
(see Figure 31). The upper 3 bits of TL1 register are ignored. Prescaler overflow increments TH1 register.
Mode 1 (16-bit Timer)Mode 1 configures Timer 1 as a 16-bit Timer with TH1 and TL1 registers connected in
cascade (see Figure 32). The selected input increments TL1 register.
Mode 2 (8-bit Timer with AutoReload)
Mode 3 (Halt)Placing Timer 1 in mode 3 causes it to halt and hold its count. This can be used to halt
Interrupt
Mode 2 configures Timer 1 as an 8-bit Timer (TL1 register) with automatic reload from
TH1 register on overflow (see Figure 33). TL1 overflow sets TF1 flag in TCON register
and reloads TL1 with the contents of TH1, which is preset by software. The reload
leaves TH1 unchanged.
Timer 1 when TR1 run control bit is not available i.e. when Timer 0 is in mode 3.
Each Timer handles one interrupt source that is the timer overflow flag TF0 or TF1. This
flag is set every time an overflow occurs. Flags are cleared when vectoring to the Timer
interrupt routine. Interrupts are enabled by setting
interrupts are globally enabled by setting EA bit in IEN0 register.
ETx
bit in IEN0 register. This assumes
60
4127H–8051–02/08
Figure 35. Timer Interrupt System
TF0
TCON.5
ET0
IEN0.1
Timer 0
Interrupt Request
TF1
TCON.7
ET1
IEN0.3
Timer 1
Interrupt Request
A/T89C51AC2
4127H–8051–02/08
61
A/T89C51AC2
Registers
Table 39. TCON Register
TCON (S:88h)
Timer/Counter Control Register
76543210
TF1TR1TF0TR0IE1IT1IE0 IT0
Bit
Number
7TF1
6TR1
5TF0
4TR0
3IE1
2IT1
1IE0
Bit
Mnemonic Description
Timer 1 Overflow Flag
Cleared by hardware when processor vectors to interrupt routine.
Set by hardware on Timer/Counter overflow, when Timer 1 register overflows.
Timer 1 Run Control Bit
Clear to turn off Timer/Counter 1.
Set to turn on Timer/Counter 1.
Timer 0 Overflow Flag
Cleared by hardware when processor vectors to interrupt routine.
Set by hardware on Timer/Counter overflow, when Timer 0 register overflows.
Timer 0 Run Control Bit
Clear to turn off Timer/Counter 0.
Set to turn on Timer/Counter 0.
Interrupt 1 Edge Flag
Cleared by hardware when interrupt is processed if edge-triggered (see IT1).
Set by hardware when external interrupt is detected on INT1# pin.
Interrupt 1 Type Control Bit
Clear to select low level active (level triggered) for external interrupt 1 (INT1#).
Set to select falling edge active (edge triggered) for external interrupt 1.
Interrupt 0 Edge Flag
Cleared by hardware when interrupt is processed if edge-triggered (see IT0).
Set by hardware when external interrupt is detected on INT0# pin.
62
0IT0
Interrupt 0 Type Control Bit
Clear to select low level active (level triggered) for external interrupt 0 (INT0#).
Set to select falling edge active (edge triggered) for external interrupt 0.
Reset Value = 0000 0000b
4127H–8051–02/08
A/T89C51AC2
Table 40. TMOD Register
TMOD (S:89h)
Timer/Counter Mode Control Register
76543210
GATE1C/T1#M11M01GATE0C/T0#M10 M00
Bit
Number
7GATE1
6C/T1#
5M11
4M01
3GATE0
2C/T0#
1M10
0
Bit
Mnemonic Description
Timer 1 Gating Control Bit
Clear to enable Timer 1 whenever TR1 bit is set.
Set to enable Timer 1 only while INT1# pin is high and TR1 bit is set.
Timer 1 Counter/Timer Select Bit
Clear for Timer operation: Timer 1 counts the divided-down system clock.
Set for Counter operation: Timer 1 counts negative transitions on external pin T1.
Clear to enable Timer 0 whenever TR0 bit is set.
Set to enable Timer/Counter 0 only while INT0# pin is high and TR0 bit is set.
Timer 0 Counter/Timer Select Bit
Clear for Timer operation: Timer 0 counts the divided-down system clock.
Set for Counter operation: Timer 0 counts negative transitions on external pin T0.
10Mode 2: 8-bit auto-reload Timer/Counter (TL0)
1 1Mode 3: TL0 is an 8-bit Timer/Counter
TH0 is an 8-bit Timer using Timer 1’s TR0 and TF0 bits.
Notes: 1. Reloaded from TH1 at overflow.
2. Reloaded from TH0 at overflow.
(1)
(2)
4127H–8051–02/08
Reset Value = 0000 0000b
63
A/T89C51AC2
Table 41. TH0 Register
TH0 (S:8Ch)
Timer 0 High Byte Register
76543210
––––––––
Bit
Number
7:0
Bit
Mnemonic Description
High Byte of Timer 0.
Reset Value = 0000 0000b
Table 42. TL0 Register
TL0 (S:8Ah)
Timer 0 Low Byte Register
76543210
––––––––
Bit
Number
7:0
Bit
Mnemonic Description
Low Byte of Timer 0.
Reset Value = 0000 0000b
Table 43. TH1 Register
TH1 (S:8Dh)
Timer 1 High Byte Register
64
76543210
––––––––
Bit
Number
7:0
Bit
Mnemonic Description
High Byte of Timer 1.
Reset Value = 0000 0000b
4127H–8051–02/08
A/T89C51AC2
Table 44. TL1 Register
TL1 (S:8Bh)
Timer 1 Low Byte Register
76543210
––––––––
Bit
Number
7:0
Bit
Mnemonic Description
Low Byte of Timer 1.
Reset Value = 0000 0000b
4127H–8051–02/08
65
A/T89C51AC2
Timer 2
(DOWN COUNTING RELOAD VALUE)
TF2
T2
EXF2
TH2
(8-bit)
TL2
(
8-bit)
RCAP2H
(8-bit)
RCAP2L
(8-bit)
FFh
(8-bit)
FFh
(
8-bit)
TOGGLE
(UP COUNTING RELOAD VALUE)
TIMER 2
INTERRUPT
:6
T2CONreg
T2CONreg
T2EX:
1=UP
2=DOWN
0
1
CT/2
T2CON.1
TR2
T2CON.2
FT2
CLOCK
see section “Clock”
The A/T89C51AC2 timer 2 is compatible with timer 2 in the 80C52.
It is a 16-bit timer/counter: the count is maintained by two eight-bit timer registers, TH2
and TL2 that are cascade- connected. It is controlled by T2CON register (See Table )
and T2MOD register (See Table 47). Timer 2 operation is similar to Timer 0 and Timer
1. C/T2 selects F
/6 (timer operation) or external pin T2 (counter operation) as
T2 clock
timer clock. Setting TR2 allows TL2 to be incremented by the selected input.
Timer 2 includes the following enhancements:
•Auto-reload mode (up or down counter)
•Programmable clock-output
Auto-Reload Mode
The auto-reload mode configures timer 2 as a 16-bit timer or event counter with automatic reload. This feature is controlled by the DCEN bit in T2MOD register (See
Table 47). Setting the DCEN bit enables timer 2 to count up or down as shown in
Figure 36. In this mode the T2EX pin controls the counting direction.
When T2EX is high, timer 2 counts up. Timer overflow occurs at FFFFh which sets the
TF2 flag and generates an interrupt request. The overflow also causes the 16-bit value
in RCAP2H and RCAP2L registers to be loaded into the timer registers TH2 and TL2.
When T2EX is low, timer 2 counts down. Timer underflow occurs when the count in the
timer registers TH2 and TL2 equals the value stored in RCAP2H and RCAP2L registers.
The underflow sets TF2 flag and reloads FFFFh into the timer registers.
The EXF2 bit toggles when timer 2 overflow or underflow, depending on the direction of
the count. EXF2 does not generate an interrupt. This bit can be used to provide 17-bit
resolution.
In clock-out mode, timer 2 operates as a 50%-duty-cycle, programmable clock generator (See Figure 37). The input clock increments TL2 at frequency F
/2. The timer
OSC
repeatedly counts to overflow from a loaded value. At overflow, the contents of RCAP2H
and RCAP2L registers are loaded into TH2 and TL2. In this mode, timer 2 overflows do
not generate interrupts. The formula gives the clock-out frequency depending on the
system oscillator frequency and the value in the RCAP2H and RCAP2L registers:
For a 16 MHz system clock in x1 mode, timer 2 has a programmable frequency range of
61 Hz (F
OSC
16)
/2
to 4 MHz (F
/4). The generated clock signal is brought out to T2 pin
OSC
(P1.0).
Timer 2 is programmed for the clock-out mode as follows:
•Set T2OE bit in T2MOD register.
•Clear C/T2 bit in T2CON register.
•Determine the 16-bit reload value from the formula and enter it in RCAP2H/RCAP2L
registers.
•Enter a 16-bit initial value in timer registers TH2/TL2. It can be the same as the
reload value or different depending on the application.
•To start the timer, set TR2 run control bit in T2CON register.
Figure 37. Clock-Out Mode
It is possible to use timer 2 as a baud rate generator and a clock generator simultane o usly. F o r thi s conf igu ration, t h e ba u d rat e s an d clo c k fr e que ncies a r e not
independent since both functions use the values in the RCAP2H and RCAP2L registers.
4127H–8051–02/08
67
A/T89C51AC2
Registers
Table 45. T2CON Register
T2CON (S:C8h)
Timer 2 Control Register
76543210
TF2EXF2RCLKTCLKEXEN2TR2C/T2#CP/RL2#
Bit
Number
7TF2
6EXF2
5RCLK
4TCLK
3EXEN2
2TR2
Bit
Mnemonic Description
Timer 2 Overflow Flag
TF2 is not set if RCLK=1 or TCLK = 1.
Must be cleared by software.
Set by hardware on timer 2 overflow.
Timer 2 External Flag
Set when a capture or a reload is caused by a negative transition on T2EX pin if
EXEN2=1.
Set to cause the CPU to vector to timer 2 interrupt routine when timer 2 interrupt
is enabled.
Must be cleared by software.
Receive Clock bit
Clear to use timer 1 overflow as receive clock for serial port in mode 1 or 3.
Set to use timer 2 overflow as receive clock for serial port in mode 1 or 3.
Transmit Clock bit
Clear to use timer 1 overflow as transmit clock for serial port in mode 1 or 3.
Set to use timer 2 overflow as transmit clock for serial port in mode 1 or 3.
Timer 2 External Enable bit
Clear to ignore events on T2EX pin for timer 2 operation.
Set to cause a capture or reload when a negative transition on T2EX pin is
detected, if timer 2 is not used to clock the serial port.
Timer 2 Run Control bit
Clear to turn off timer 2.
Set to turn on timer 2.
68
1C/T2#
0CP/RL2#
Timer/Counter 2 Select bit
Clear for timer operation (input from internal clock system: F
Set for counter operation (input from T2 input pin).
Timer 2 Capture/Reload bit
If RCLK=1 or TCLK=1, CP/RL2# is ignored and timer is forced to auto-reload on
timer 2 overflow.
Clear to auto-reload on timer 2 overflows or negative transitions on T2EX pin if
EXEN2=1.
Set to capture on negative transitions on T2EX pin if EXEN2=1.
Reset Value = 0000 0000b
Bit addressable
).
OSC
4127H–8051–02/08
A/T89C51AC2
Table 46. T2MOD Register
T2MOD (S:C9h)
Timer 2 Mode Control Register
76543210
------T2OEDCEN
Bit
Number
7-
6-
5-
4-
3-
2-
1T2OE
0DCEN
Bit
Mnemonic Description
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Timer 2 Output Enable bit
Clear to program P1.0/T2 as clock input or I/O port.
Set to program P1.0/T2 as clock output.
Down Counter Enable bit
Clear to disable timer 2 as up/down counter.
Set to enable timer 2 as up/down counter.
Reset Value = XXXX XX00b
Not bit addressable
4127H–8051–02/08
Table 47. TH2 Register
TH2 (S:CDh)
Timer 2 High Byte Register
76543210
--------
Bit
Number
7-0High Byte of Timer 2.
Bit
Mnemonic Description
Reset Value = 0000 0000b
Not bit addressable
69
A/T89C51AC2
Table 48. TL2 Register
TL2 (S:CCh)
Timer 2 Low Byte Register
76543210
--------
Bit
Number
7-0Low Byte of Timer 2.
Bit
Mnemonic Description
Reset Value = 0000 0000b
Not bit addressable
Table 49. RCAP2H Register
RCAP2H (S:CBh)
Timer 2 Reload/Capture High Byte Register
76543210
--------
Bit
Number
7-0High Byte of Timer 2 Reload/Capture.
Bit
Mnemonic Description
Reset Value = 0000 0000b
Not bit addressable
Table 50. RCAP2L Register
70
RCAP2L (S:CAH)
T
IMER
2 REload/Capture Low Byte Register
76543210
--------
Bit
Number
7-0Low Byte of Timer 2 Reload/Capture.
Bit
Mnemonic Description
Reset Value = 0000 0000b
Not bit addressable
4127H–8051–02/08
A/T89C51AC2
WDTPRG
Watchdog Timer
Figure 38. Watchdog Timer
A/T89C51AC2 contains a powerful programmable hardware Watchdog Timer (WDT)
that automatically resets the chip if it software fails to reset the WDT before the selected
time interval has elapsed. It permits large Time-Out ranking from 16ms to 2s @Fosc =
12MHz in X1 mode.
This WDT consists of a 14-bit counter plus a 7-bit programmable counter, a Watchdog
Timer reset register (WDTRST) and a Watchdog Timer programming (WDTPRG) register. When exiting reset, the WDT is -by default- disable.
To enable the WDT, the user has to write the sequence 1EH and E1H into WDTRST
register no instruction in between. When the Watchdog Timer is enabled, it will increment every machine cycle while the oscillator is running and there is no way to disable
the WDT except through reset (either hardware reset or WDT overflow reset). When
WDT overflows, it will generate an output RESET pulse at the RST pin. The RESET
pulse duration is 96xT
, where T
OSC
OSC
=1/F
. To make the best use of the WDT, it
OSC
should be serviced in those sections of code that will periodically be executed within the
time required to prevent a WDT reset
Note:When the Watchdog is enable it is impossible to change its period.
The three lower bits (S0, S1, S2) located into WDTPRG register permit to program the
WDT duration.
Table 51. Machine Cycle Count
S2S1S0Machine Cycle Count
000214 - 1
001215 - 1
0102
011217 - 1
100218 - 1
101219 - 1
110220 - 1
111221 - 1
16
- 1
To compute WD Time-Out, the following formula is applied:
Note:Svalue represents the decimal value of (S2 S1 S0)
The following table outlines the time-out value for Fosc
= 12 MHz in X1 mode
XTAL
Table 52. Time-Out Computation
S2S1S0 Fosc = 12 MHzFosc = 16 MHzFosc = 20 MHz
00016.38 ms12.28 ms9.82 ms
00132.77 ms24.57 ms19.66 ms
01065.54 ms49.14 ms39.32 ms
011131.07 ms98.28 ms78.64 ms
100262.14 ms196.56 ms157.28 ms
101524.29 ms393.12 ms314.56 ms
1101.05 s786.24 ms629.12 ms
1112.10 s1.57 s1.25 s
72
4127H–8051–02/08
A/T89C51AC2
Watchdog Timer During
Power-down Mode and
Idle
In Power-down mode the oscillator stops, which means the WDT also stops. While in
Power-down mode, the user does not need to service the WDT. There are 2 methods of
exiting Power-down mode: by a hardware reset or via a level activated external interrupt
which is enabled prior to entering Power-down mode. When Power-down is exited with
hardware reset, the Watchdog is disabled. Exiting Power-down with an interrupt is significantly different. The interrupt shall be held low long enough for the oscillator to
stabilize. When the interrupt is brought high, the interrupt is serviced. To prevent the
WDT from resetting the device while the interrupt pin is held low, the WDT is not started
until the interrupt is pulled high. It is suggested that the WDT be reset during the interrupt service for the interrupt used to exit Power-down.
To ensure that the WDT does not overflow within a few states of exiting powerdown, it is
best to reset the WDT just before entering powerdown.
In the Idle mode, the oscillator continues to run. To prevent the WDT from resetting
A/T89C51AC2 while in Idle mode, the user should always set up a timer that will periodically exit Idle, service the WDT, and re-enter Idle mode.
Note:The WDRST register is used to reset/enable the WDT by writing 1EH then E1H in
sequence without instruction between these two sequences.
74
4127H–8051–02/08
A/T89C51AC2
Programmable
Counter Array (PCA)
The PCA provides more timing capabilities with less CPU intervention than the standard
timer/counters. Its advantages include reduced software overhead and improved accuracy. The PCA consists of a dedicated timer/counter which serves as the time base for
an array of five compare/capture modules. Its clock input can be programmed to count
any of the following signals:
•PCA clock frequency/6 (see “clock” section)
•PCA clock frequency/2
•Timer 0 overflow
•External input on ECI (P1.2)
Each compare/capture modules can be programmed in any one of the following modes:
•rising and/or falling edge capture,
•software timer,
•high-speed output,
•pulse width modulator.
Module 4 can also be programmed as a Watchdog timer. see the "PCA Watchdog
Timer" section.
When the compare/capture modules are programmed in capture mode, software timer,
or high speed output mode, an interrupt can be generated when the module executes its
function. All five modules plus the PCA timer overflow share one interrupt vector.
The PCA timer/counter and compare/capture modules share Port 1 for external I/Os.
These pins are listed below. If the port is not used for the PCA, it can still be used for
standard I/O.
PCA Timer
PCA ComponentExternal I/O Pin
16-bit CounterP1.2/ECI
16-bit Module 0P1.3/CEX0
16-bit Module 1P1.4/CEX1
16-bit Module 2P1.5/CEX2
16-bit Module 3P1.6/CEX3
16-bit Module 4P1.7/CEX4
The PCA timer is a common time base for all five modules (see Figure 39). The timer
count source is determined from the CPS1 and CPS0 bits in the CMOD SFR (see Table
8) and can be programmed to run at:
•1/6 the PCA clock frequency.
•1/2 the PCA clock frequency.
•the Timer 0 overflow.
•the input on the ECI pin (P1.2).
4127H–8051–02/08
75
A/T89C51AC2
Figure 39. PCA Timer/Counter
CIDLCPS1 CPS0ECF
It
CHCL
16 bit up counter
To PCA
modules
FPca/6
FPca/2
T0 OVF
P1.2
Idle
CMOD
0xD9
WDTE
CFCR
CCON
0xD8
CCF4 CCF3 CCF2 CCF1 CCF0
overflow
The CMOD register includes three additional bits associated with the PCA.
•The CIDL bit which allows the PCA to stop during idle mode.
•The WDTE bit which enables or disables the Watchdog function on module 4.
•The ECF bit which when set causes an interrupt and the PCA overflow flag CF in
CCON register to be set when the PCA timer overflows.
PCA Modules
76
The CCON register contains the run control bit for the PCA and the flags for the PCA
timer and each module.
•The CR bit must be set to run the PCA. The PCA is shut off by clearing this bit.
•The CF bit is set when the PCA counter overflows and an interrupt will be generated
if the ECF bit in CMOD register is set. The CF bit can only be cleared by software.
•The CCF0:4 bits are the flags for the modules (CCF0 for module0...) and are set by
hardware when either a match or a capture occurs. These flags also can be cleared
by software.
Each one of the five compare/capture modules has six possible functions. It can
perform:
•16-bit Capture, positive-edge triggered
•16-bit Capture, negative-edge triggered
•16-bit Capture, both positive and negative-edge triggered
•16-bit Software Timer
•16-bit High Speed Output
•8-bit Pulse Width Modulator.
In addition module 4 can be used as a Watchdog Timer.
4127H–8051–02/08
PCA Interrupt
CFCR
CCON
CCF4 CCF3 CCF2 CCF1 CCF0
Module 4
Module 3
Module 2
Module 1
Module 0
PCA Timer/Counter
ECCFn
CCAPMn.0
To Interrupt
EA
IEN0.7
EC
IEN0.6
ECF
CMOD.0
A/T89C51AC2
Each module in the PCA has a special function register associated with it (CCAPM0 for
module 0...). The CCAPM0:4 registers contain the bits that control the mode that each
module will operate in.
•The ECCF bit enables the CCF flag in the CCON register to generate an interrupt
when a match or compare occurs in the associated module.
•The PWM bit enables the pulse width modulation mode.
•The TOG bit when set causes the CEX output associated with the module to toggle
when there is a match between the PCA counter and the module’s capture/compare
register.
•The match bit MAT when set will cause the CCFn bit in the CCON register to be set
when there is a match between the PCA counter and the module’s capture/compare
register.
•The two bits CAPN and CAPP in CCAPMn register determine the edge that a
capture input will be active on. The CAPN bit enables the negative edge, and the
CAPP bit enables the positive edge. If both bits are set both edges will be enabled.
•The bit ECOM in CCAPM register when set enables the comparator function.
Figure 40. PCA Interrupt System
PCA Capture Mode
To use one of the PCA modules in capture mode either one or both of the CCAPM bits
CAPN and CAPP for that module must be set. The external CEX input for the module
(on port 1) is sampled for a transition. When a valid transition occurs the PCA hardware
loads the value of the PCA counter registers (CH and CL) into the module’s capture registers (CCAPnL and CCAPnH). If the CCFn bit for the module in the CCON SFR and the
ECCFn bit in the CCAPMn SFR are set then an interrupt will be generated.
4127H–8051–02/08
77
A/T89C51AC2
Figure 41. PCA Capture Mode
CEXn
n = 0, 4
PCA Counter
CH
(8bits)
CL
(8bits)
CCAPnH
CCAPnL
CCFn
CCON
PCA
Interrupt
Request
-
0CAPPn CAPNn000ECCFn
7
CCAPMn Register (n = 0, 4)
0
CCAPnL
(8 bits)
CCAPnH
(8 bits)
-
ECOMn0 0 MATn TOGn0 ECCFn
70
CCAPMn Register
(n = 0, 4)
CH
(8 bits)CL(8 bits)
16-Bit Comparator
Match
Enable
CCFn
CCON reg
PCA
Interrupt
Request
CEXn
Compare/Capture Module
PCA Counter
“0”
“1”
Reset
Write to
CCAPnL
Write to CCAPnH
For software Timer mode, set ECOMn and MATn.
For high speed output mode, set ECOMn, MATn and TOGn.
Toggle
16-bit Software Timer
Mode
The PCA modules can be used as software timers by setting both the ECOM and MAT
bits in the modules CCAPMn register. The PCA timer will be compared to the module’s
capture registers and when a match occurs an interrupt will occur if the CCFn (CCON
SFR) and the ECCFn (CCAPMn SFR) bits for the module are both set.
Figure 42. PCA 16-bit Software Timer and High Speed Output Mode
78
4127H–8051–02/08
A/T89C51AC2
CHCL
CCAPnHCCAPnL
ECOMn
CCAPMn, n = 0 to 4
0xDA to 0xDE
CAPNn MATn TOGn PWMn ECCFnCAPPn
16-bit comparator
Match
CFCR
CCON
0xD8
CCF4 CCF3 CCF2 CCF1 CCF0
PCA IT
Enable
CEXn
PCA counter/timer
“1”“0”
Write to
CCAPnL
Reset
Write to
CCAPnH
High Speed Output Mode
In this mode the CEX output (on port 1) associated with the PCA module will toggle
each time a match occurs between the PCA counter and the module’s capture registers.
To activate this mode the TOG, MAT, and ECOM bits in the module’s CCAPMn SFR
must be set.
Figure 43. PCA High Speed Output Mode
Pulse Width Modulator
Mode
All the PCA modules can be used as PWM outputs. The output frequency depends on
the source for the PCA timer. All the modules will have the same output frequency
because they all share the PCA timer. The duty cycle of each module is independently
variable using the module’s capture register CCAPLn. When the value of the PCA CL
SFR is less than the value in the module’s CCAPLn SFR the output will be low, when it
is equal to or greater than it, the output will be high. When CL overflows from FF to 00,
CCAPLn is reloaded with the value in CCAPHn. the allows the PWM to be updated without glitches. The PWM and ECOM bits in the module’s CCAPMn register must be set to
enable the PWM mode.
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79
A/T89C51AC2
Figure 44. PCA PWM Mode
CL rolls over from FFh TO 00h loads
CCAPnH contents into CCAPnL
CCAPnL
CCAPnH
8-Bit
Comparator
CL (8 bits)
“0”
“1”
CL < CCAPnL
CL > = CCAPnL
CEX
PWMn
CCAPMn.1
ECOMn
CCAPMn.6
PCA Watchdog Timer
An on-board Watchdog timer is available with the PCA to improve system reliability without increasing chip count. Watchdog timers are useful for systems that are sensitive to
noise, power glitches, or electrostatic discharge. Module 4 is the only PCA module that
can be programmed as a Watchdog. However, this module can still be used for other
modes if the Watchdog is not needed. The user pre-loads a 16-bit value in the compare
registers. Just like the other compare modes, this 16-bit value is compared to the PCA
timer value. If a match is allowed to occur, an internal reset will be generated. This will
not cause the RST pin to be driven high.
To hold off the reset, the user has three options:
•periodically change the compare value so it will never match the PCA timer,
•periodically change the PCA timer value so it will never match the compare values,
or
•disable the Watchdog by clearing the WDTE bit before a match occurs and then reenable it.
The first two options are more reliable because the Watchdog timer is never disabled as
in the third option. If the program counter ever goes astray, a match will eventually occur
and cause an internal reset. If other PCA modules are being used the second option not
recommended either. Remember, the PCA timer is the time base for all modules;
changing the time base for other modules would not be a good idea. Thus, in most applications the first solution is the best option.
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4127H–8051–02/08
A/T89C51AC2
PCA Registers
Table 55. CMOD Register
CMOD (S:D9h)
PCA Counter Mode Register
76543210
CIDLWDTE---CPS1CPS0 ECF
Bit
Number
7CIDL
6WDTE
5-
4-
3-
2-1CPS1:0
Bit
Mnemonic Description
PCA Counter Idle Control bit
Clear to let the PCA run during Idle mode.
Set to stop the PCA when Idle mode is invoked.
Watchdog Timer Enable
Clear to disable Watchdog Timer function on PCA Module 4,
Set to enable it.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Clear to disable CF bit in CCON register to generate an interrupt.
Set to enable CF bit in CCON register to generate an interrupt.
Reset Value = 00XX X000b
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A/T89C51AC2
Table 56. CCON Register
CCON (S:D8h)
PCA Counter Control Register
76543210
CFCR-CCF4CCF3CCF2CCF1 CCF0
Bit
Number
7CF
6CR
5-
4CCF4
3CCF3
2CCF2
1CCF1
Bit
Mnemonic Description
PCA Timer/Counter Overflow flag
Set by hardware when the PCA Timer/Counter rolls over. This generates a PCA
interrupt request if the ECF bit in CMOD register is set.
Must be cleared by software.
PCA Timer/Counter Run Control bit
Clear to turn the PCA Timer/Counter off.
Set to turn the PCA Timer/Counter on.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
PCA Module 4 Compare/Capture flag
Set by hardware when a match or capture occurs. This generates a PCA
interrupt request if the ECCF 4 bit in CCAPM 4 register is set.
Must be cleared by software.
PCA Module 3 Compare/Capture flag
Set by hardware when a match or capture occurs. This generates a PCA
interrupt request if the ECCF 3 bit in CCAPM 3 register is set.
Must be cleared by software.
PCA Module 2 Compare/Capture flag
Set by hardware when a match or capture occurs. This generates a PCA
interrupt request if the ECCF 2 bit in CCAPM 2 register is set.
Must be cleared by software.
PCA Module 1 Compare/Capture flag
Set by hardware when a match or capture occurs. This generates a PCA
interrupt request if the ECCF 1 bit in CCAPM 1 register is set.
Must be cleared by software.
82
PCA Module 0 Compare/Capture flag
0CCF0
Set by hardware when a match or capture occurs. This generates a PCA
interrupt request if the ECCF 0 bit in CCAPM 0 register is set.
Must be cleared by software.
Reset Value = 00X0 0000b
4127H–8051–02/08
A/T89C51AC2
Table 57. CCAPnH Registers
CCAP0H (S:FAh)
CCAP1H (S:FBh)
CCAP2H (S:FCh)
CCAP3H (S:FDh)
CCAP4H (S:FEh)
PCA High Byte Compare/Capture Module n Register (n=0..4)
The Value read from this bit is indeterminate. Do not set this bit.
Enable Compare Mode Module x bit
Clear to disable the Compare function.
Set to enable the Compare function.
The Compare function is used to implement the software Timer, the high-speed
output, the Pulse Width Modulator (PWM) and the Watchdog Timer (WDT).
Capture Mode (Positive) Module x bit
Clear to disable the Capture function triggered by a positive edge on CEXx pin.
Set to enable the Capture function triggered by a positive edge on CEXx pin
Capture Mode (Negative) Module x bit
Clear to disable the Capture function triggered by a negative edge on CEXx pin.
Set to enable the Capture function triggered by a negative edge on CEXx pin.
Match Module x bit
Set when a match of the PCA Counter with the Compare/Capture register sets
CCFx bit in CCON register, flagging an interrupt.
Toggle Module x bit
The toggle mode is configured by setting ECOMx, MATx and TOGx bits.
Set when a match of the PCA Counter with the Compare/Capture register
toggles the CEXx pin.
Pulse Width Modulation Module x Mode bit
Set to configure the module x as an 8-bit Pulse Width Modulator with output
waveform on CEXx pin.
84
0ECCFn
Enable CCFx Interrupt bit
Clear to disable CCFx bit in CCON register to generate an interrupt request.
Set to enable CCFx bit in CCON register to generate an interrupt request.
Reset Value = X000 0000b
4127H–8051–02/08
A/T89C51AC2
Table 60. CH Register
CH (S:F9h)
PCA Counter Register High Value
76543210
CH 7CH 6CH 5CH 4CH 3CH 2CH 1 CH 0
Bit
Number
7:0CH 7:0High byte of Timer/Counter
Bit
Mnemonic Description
Reset Value = 0000 00000b
Table 61. CL Register
CL (S:E9h)
PCA counter Register Low Value
76543210
CL 7CL 6CL 5CL 4CL 3CL 2CL 1CL 0
Bit
Number
7:0CL0 7:0Low byte of Timer/Counter
Bit
Mnemonic Description
Reset Value = 0000 00000b
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85
A/T89C51AC2
Analog-to-Digital
Converter (ADC)
This sec t ion des c ri bes the on- c hip 10 bit anal o g-to - digit al c onver ter of t he
A/T89C51AC2. Eight ADC channels are available for sampling of the external sources
AN0 to AN7. An analog multiplexer allows the single ADC converter to select one from
the 8 ADC channels as ADC input voltage (ADCIN). ADCIN is converted by the 10-bit
cascaded potentiometric ADC.
Two modes of conversion are available:
- Standard conversion (8 bits).
- Precision conversion (10 bits).
For the precision conversion, set bit PSIDLE in ADCON register and start conversion.
The device is in a pseudo-idle mode, the CPU does not run but the peripherals are
always running. This mode allows digital noise to be as low as possible, to ensure high
precision conversion.
For this mode it is necessary to work with end of conversion interrupt, which is the only
way to wake the device up.
If another interrupt occurs during the precision conversion, it will be served only after
this conversion is completed.
Features
ADC Port 1 I/O Functions
VAREF
•8 channels with multiplexed inputs
•10-bit cascaded potentiometric ADC
•Conversion time 16 micro-seconds (typ.)
•Zero Error (offset) ± 2 LSB max
•Positive External Reference Voltage Range (VAREF) 2.4 to 3.0 Volt (typ.)
•Conversion Complete Flag or Conversion Complete Interrupt
•Selectable ADC Clock
Port 1 pins are general I/O that are shared with the ADC channels. The channel select
bit in ADCF register define which ADC channel/port1 pin will be used as ADCIN. The
remaining ADC channels/port1 pins can be used as general-purpose I/O or as the alternate function that is available.
A conversion launched on a channel which are not selected on ADCF register will not
have any effect.
VAREF should be connected to a low impedance point and must remain in the range
specified in Table 77. If the ADC is not used, it is recommended to connect VAREF to
VAGND.
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4127H–8051–02/08
Figure 45. ADC Description
Rai
AN0/P1.0
AN1/P1.1
AN2/P1.2
AN3/P1.3
AN4/P1.4
AN5/P1.5
AN6/P1.6
AN7/P1.7
000
001
010
011
100
101
110
111
SCH2
ADCON.2
SCH0
ADCON.0
SCH1
ADCON.1
ADC
CLOCK
ADEN
ADCON.5
ADSST
ADCON.3
ADEOC
ADCON.4
ADC
Interrupt
Request
EADC
IEN1.1
CONTROL
AVSS
Sample and Hold
ADDH
VAREF
R/2R DAC
VAGND
8
10
+
-
ADDL
2
SAR
ADCIN
Cai
ADEN
ADSST
ADEOC
T
SETUP
(1)
T
CONV
(2)
CLK
A/T89C51AC2
Figure 46 shows the timing diagram of a complete conversion. For simplicity, the figure
depicts the waveforms in idealized form and do not provide precise timing information.
For ADC characteristics and timing parameters refer to the Section “AC Characteristics”
of the A/T89C51AC2 datasheet.
Figure 46. Timing Diagram
Notes:1. Tsetup min, see the AC Parameter for A/D conversion.
2. Tconv = 11 clock ADC = 1sample and hold + 10 bit conversion
The user must ensure that Tsetup time between setting ADEN and the start of the first conversion.
4127H–8051–02/08
87
A/T89C51AC2
ADC Converter
Operation
A start of single A/D conversion is triggered by setting bit ADSST (ADCON.3).
After completion of the A/D conversion, the ADSST bit is cleared by hardware.
The end-of-conversion flag ADEOC (ADCON.4) is set when the value of conversion is
available in ADDH and ADDL, it must be cleared by software. If the bit EADC (IEN1.1) is
set, an interrupt occur when flag ADEOC is set (see Figure 48). Clear this flag for rearming the interrupt.
Voltage Conversion
The bits SCH0 to SCH2 in ADCON register are used for the analog input channel
selection.
Note:1. Always leave Tsetup time before starting a conversion unless ADEN is permanently
(1)
high. In this case one should wait Tsetup only before the first conversion.
Table 62. Selected Analog input
SCH2SCH1SCH0Selected Analog input
000AN0
001AN1
010AN2
011AN3
100AN4
101AN5
110AN6
111AN7
When the ADCIN is equals to VAREF the ADC converts the signal to 3FFh (full scale). If
the input voltage equals VAGND, the ADC converts it to 000h. Input voltage between
VAREF and VAGND are a straight-line linear conversion. All other voltages will result in
3FFh if greater than VAREF and 000h if less than VAGND.
Note:ADCIN should not exceed VAREF absolute maximum range (see “Absolute Maximum
Ratings” on page 141)
Clock Selection
88
The ADC clock is the same as CPU.
The maximum clock frequency is defined in the DC parmeter for A/D converter. A prescaler is featured (ADCCLK) to generate the ADC clock from the oscillator frequency.
if PRS > 0 then f
if PRS = 0 then f
ADC
ADC
= F
= F
periph
periph
/ 2 x PRS
/ 64
4127H–8051–02/08
Figure 47. A/D Converter clock
Prescaler ADCLK
A/D
Converter
ADC Clock
CPU
CLOCK
CPU Core Clock Symbol
÷
2
ADEOC
ADCON.2
EADC
IEN1.1
ADCI
A/T89C51AC2
ADC Standby Mode
IT ADC Management
Routines examples
When the ADC is not used, it is possible to set it in standby mode by clearing bit ADEN
in ADCON register. In this mode its power dissipation is reduced.
An interrupt end-of-conversion will occurs when the bit ADEOC is activated and the bit
EADC is set. For re-arming the interrupt the bit ADEOC must be cleared by software.
Figure 48. ADC Interrupt Structure
1. Configure P1.2 and P1.3 in ADC channels
// configure channel P1.2 and P1.3 for ADC
ADCF = 0Ch
// Enable the ADC
ADCON = 20h
2. Start a standard conversion
// The variable "channel" contains the channel to convert
// The variable "value_converted" is an unsigned int
// Clear the field SCH[2:0]
ADCON and = F8h
// Select channel
ADCON | = channel
// Start conversion in standard mode
ADCON | = 08h
// Wait flag End of conversion
while((ADCON and 01h)! = 01h)
// Clear the End of conversion flag
ADCON and = EFh
// read the value
value_converted = (ADDH << 2)+(ADDL)
4127H–8051–02/08
3. Start a precision conversion (need interrupt ADC)
// The variable "channel" contains the channel to convert
// Enable ADC
EADC = 1
89
A/T89C51AC2
// clear the field SCH[2:0]
ADCON and = F8h
// Select the channel
ADCON | = channel
// Start conversion in precision mode
ADCON | = 48h
Note:to enable the ADC interrupt:
EA = 1
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4127H–8051–02/08
A/T89C51AC2
Registers
Table 63. ADCF Register
ADCF (S:F6h)
ADC Configuration
76543210
CH 7CH 6CH 5CH 4CH 3CH 2CH 1CH 0
Bit
Number
7-0CH 0:7
Bit
Mnemonic Description
Channel Configuration
Set to use P1.x as ADC input.
Clear to use P1.x as standart I/O port.
Reset Value = 0000 0000b
Table 64. ADCON Register
ADCON (S:F3h)
ADC Control Register
76543210
-PSIDLEADENADEOCADSSTSCH2SCH1SCH0
Bit
Number
Bit
Mnemonic Description
7-
6PSIDLE
5ADEN
4ADEOC
3ADSST
2-0SCH2:0
Pseudo Idle Mode (Best Precision)
Set to put in idle mode during conversion
Clear to convert without idle mode.
Enable/Standby Mode
Set to enable ADC
Clear for Standby mode (power dissipation 1 uW).
End Of Conversion
Set by hardware when ADC result is ready to be read. This flag can generate an
interrupt.
Must be cleared by software.
Start and Status
Set to start an A/D conversion.
Cleared by hardware after completion of the conversion
Selection of Channel to Convert
see Table 62
Reset Value = X000 0000b
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91
A/T89C51AC2
Table 65. ADCLK Register
ADCLK (S:F2h)
ADC Clock Prescaler
76543210
---PRS 4PRS 3PRS 2PRS 1PRS 0
Bit
Number
7-5-
4-0PRS4:0
Bit
Mnemonic Description
Reserved
The value read from these bits are indeterminate. Do not set these bits.
Clock Prescaler
f
=
fcpu clock/ (4 (or 2 in X2 mode)* PRS )
ADC
Reset Value = XXX0 0000b
Table 66. ADDH Register
ADDH (S:F5h Read Only)
ADC Data High Byte Register
76543210
ADAT 9ADAT 8ADAT 7ADAT 6ADAT 5ADAT 4ADAT 3ADAT 2
Bit
Number
7-0ADAT9:2
Bit
Mnemonic Description
ADC result
bits 9-2
Reset Value = 00h
92
Table 67. ADDL Register
ADDL (S:F4h Read Only)
ADC Data Low Byte Register
76543210
------ADAT 1ADAT 0
Bit
Number
7-2-
1-0ADAT1:0
Bit
Mnemonic Description
Reserved
The value read from these bits are indeterminate. Do not set these bits.
ADC result
bits 1-0
Reset Value = 00h
4127H–8051–02/08
Interrupt System
EX0
IEN0.0
00
01
10
11
External
Interrupt 0
INT0#
EX1
IEN0.2
External
Interrupt 1
INT1#
ET0
IEN0.1
Timer 0
EC
IEN0.6
PCA
ET1
IEN0.3
Timer 1
ES
IEN0.4
UART
EADC
IEN1.1
A to D
Converter
Interrupt EnableLowest Priority Interrupts
Highest
Priority Enable
00
01
10
11
00
01
10
11
00
01
10
11
00
01
10
11
00
01
10
11
00
01
10
11
Priority
Interrupts
AIN1:0
Timer 2
00
01
10
11
ET2
IEN0.5
TxD
RxD
CEX0:5
A/T89C51AC2
Introduction
The controller has a total of 8 interrupt vectors: two external interrupts (INT0 and INT1),
three timer interrupts (Timers 0, 1 and 2), a serial port interrupt, a PCA, a timer overrun
interrupt and an ADC. These interrupts are shown below.
Figure 49. Interrupt Control System
4127H–8051–02/08
93
A/T89C51AC2
Each of the interrupt sources can be individually enabled or disabled by setting or clearing a bit in the Interrupt Enable register. This register also contains a global disable bit
which must be cleared to disable all the interrupts at the same time.
Each interrupt source can also be individually programmed to one of four priority levels
by setting or clearing a bit in the Interrupt Priority registers. The Table below shows the
bit values and priority levels associated with each combination.
Table 68. Priority Level Bit Values
IPH.xIPL.xInterrupt Level Priority
000 (Lowest)
011
102
113 (Highest)
A low-priority interrupt can be interrupted by a high priority interrupt but not by another
low-priority interrupt. A high-priority interrupt cannot be interrupted by any other interrupt
source.
If two interrupt requests of different priority levels are received simultaneously, the
request of the higher priority level is serviced. If interrupt requests of the same priority
level are received simultaneously, an internal polling sequence determines which
request is serviced. Thus within each priority level there is a second priority structure
determined by the polling sequence, see Table 69.
Table 69. Interrupt Priority Within level
Interrupt NameInterrupt Address VectorPriority Number
external interrupt (INT0)0003h1
Timer 0 (TF0)000Bh2
external interrupt (INT1)0013h3
Timer 1 (TF1)001Bh4
PCA (CF or CCFn)0033h5
UART (RI or TI)0023h6
Timer 2 (TF2)002Bh7
ADC (ADCI)0043h9
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4127H–8051–02/08
A/T89C51AC2
Registers
Table 70. IEN0 Register
IEN0 (S:A8h)
Interrupt Enable Register
76543210
EAECET2ESET1EX1ET0EX0
Bit
Number
7EA
6EC
5ET2
4ES
Bit
Mnemonic Description
Enable All Interrupt bit
Clear to disable all interrupts.
Set to enable all interrupts.
If EA=1, each interrupt source is individually enabled or disabled by setting or
clearing its interrupt enable bit.
PCA Interrupt Enable
Clear to disable the PCA interrupt.
Set to enable the PCA interrupt.
Timer 2 Overflow Interrupt Enable bit
Clear to disable Timer 2 overflow interrupt.
Set to enable Timer 2 overflow interrupt.
Serial Port Enable bit
Clear to disable serial port interrupt.
Set to enable serial port interrupt.
Timer 1 Overflow Interrupt Enable bit
3ET1
2EX1
1ET0
0EX0
Clear to disable timer 1 overflow interrupt.
Set to enable timer 1 overflow interrupt.
External Interrupt 1 Enable bit
Clear to disable external interrupt 1.
Set to enable external interrupt 1.
Timer 0 Overflow Interrupt Enable bit
Clear to disable timer 0 overflow interrupt.
Set to enable timer 0 overflow interrupt.
External Interrupt 0 Enable bit
Clear to disable external interrupt 0.
Set to enable external interrupt 0.
Reset Value = 0000 0000b
bit addressable
4127H–8051–02/08
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A/T89C51AC2
Table 71. IEN1 Register
IEN1 (S:E8h)
Interrupt Enable Register
76543210
------EADC-
Bit
Number
7-
6-
5-
4-
3-
2-
1EADC
0-
Bit
Mnemonic Description
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
ADC Interrupt Enable bit
Clear to disable the ADC interrupt.
Set to enable the ADC interrupt.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reset Value = xxxx xx00b
bit addressable
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A/T89C51AC2
Table 72. IPL0 Register
IPL0 (S:B8h)
Interrupt Enable Register
76543210
-PPCPT2PSPT1PX1PT0PX0
Bit
Number
7-
6PPC
5PT2
4PS
3PT1
2PX1
1PT0
0PX0
Bit
Mnemonic Description
Reserved
The value read from this bit is indeterminate. Do not set this bit.
PCA Interrupt Priority bit
Refer to PPCH for priority level
Timer 2 Overflow Interrupt Priority bit
Refer to PT2H for priority level.
Serial Port Priority bit
Refer to PSH for priority level.
Timer 1 Overflow Interrupt Priority bit
Refer to PT1H for priority level.
External Interrupt 1 Priority bit
Refer to PX1H for priority level.
Timer 0 Overflow Interrupt Priority bit
Refer to PT0H for priority level.
External Interrupt 0 Priority bit
Refer to PX0H for priority level.
Reset Value = X000 0000b
bit addressable
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A/T89C51AC2
Table 73. IPL1 Register
IPL1 (S:F8h)
Interrupt Priority Low Register 1
76543210
------PADCL-
Bit
Number
7-
6-
5-
4-
3-
2-
1PADCL
0-
Bit
Mnemonic Description
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
ADC Interrupt Priority Level Less Significant Bit
Refer to PSPIH for priority level.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reset Value = XXXX XX0Xb
bit addressable
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4127H–8051–02/08
A/T89C51AC2
Table 74. IPH0 Register
IPH0 (B7h)
Interrupt High Priority Register
76543210
-PPCHPT2HPSHPT1HPX1HPT0HPX0H
Bit
Number
7-
6PPCH
5PT2H
4PSH
3PT1H
Bit
Mnemonic Description
Reserved
The value read from this bit is indeterminate. Do not set this bit.