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LV8572AV
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Datasheet LV8572AV Datasheet (NSC)

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TL/F/11417
LV8572A Real Time Clock (RTC)
PRELIMINARY
December 1993
LV8572A Low Voltage Real Time Clock (RTC)
General Description
The LV8572A is intended for use in microprocessor based systems where information is required for multi-tasking, data logging or general time of day/date information. This device is implemented in low voltage silicon gate microCMOS tech­nology to provide low standby power in battery back-up en­vironments. The circuit’s architecture is such that it looks like a contiguous block of memory or I/O ports. The address space is organized as 2 software selectable pages of 32 bytes. This includes the Control Registers, the Clock Coun­ters, the Alarm Compare RAM, and the Time Save RAM. Any of the RAM locations that are not being used for their intended purpose may be used as general purpose CMOS RAM.
Time and date are maintained from 1/100 of a second to year and leap year in a BCD format, 12 or 24 hour modes. Day of week, day of month and day of year counters are provided. Time is controlled by an on-chip crystal oscillator requiring only the addition of the crystal and two capacitors. The choice of crystal frequency is program selectable.
Power failure logic and control functions have been integrat­ed on chip. This logic is used by the RTC to issue a power fail interrupt, and lock out the mp interface. The time power
fails may be logged into RAM automatically when V
BB
l
VCC. Additionally, two supply pins are provided. When V
BB
l
VCC, internal circuitry will automatically switch from the main supply to the battery supply. Status bits are provid­ed to indicate initial application of battery power, system power, and low battery detect. (Continued)
Features
Y
3.3Vg10% supply
Y
Full function real time clock/calendar Ð 12/24 hour mode timekeeping Ð Day of week and day of years counters Ð Four selectable oscillator frequencies Ð Parallel resonant oscillator
Y
Power fail features Ð Internal power supply switch to external battery Ð Power Supply Bus glitch protection Ð Automatic log of time into RAM at power failure
Y
On-chip interrupt structure Ð Periodic, alarm, and power fail interrupts
Y
Up to 44 bytes of CMOS RAM
Block Diagram
TL/F/11417– 1
FIGURE 1
TRI-STATEÉis a registered trademark of National Semiconductor Corporation.
C
1995 National Semiconductor Corporation RRD-B30M105/Printed in U. S. A.
Absolute Maximum Ratings (Notes1&2)
Specifications for the 883 version of this product are listed separately.
Supply Voltage (V
CC
)
b
0.5V toa7.0V
DC Input Voltage (V
IN
)
b
0.5V to V
CC
a
0.5V
DC Output Voltage (V
OUT
)
b
0.5V to V
CC
a
0.5V
Storage Temperature Range
b
65§Ctoa150§C
Power Dissipation (PD) 500 mW
Lead Temperature (Soldering, 10 sec.) 260
§
C
Operation Conditions
Min Max Unit
Supply Voltage (V
CC
) (Note 3) 3.0 3.6 V
Supply Voltage (VBB) (Note 3) 2.2 V
CC
b
0.4 V
DC Input or Output Voltage
0.0 V
CC
V
(V
IN,VOUT
)
Operation Temperature (T
A
)
b40a
85§C
Electr-Static Discharge Rating 1 kV
Typical Values
i
JA
DIP Board 61§C/W
Socket 67
§
C/W
i
JA
PLCC Board 80§C/W
Socket 88
§
C/W
DC Electrical Characteristics
V
CC
e
3.3Vg10%, V
BB
e
2.5V, V
PFAIL
l
VIH,C
L
e
100 pF (unless otherwise specified)
Symbol Parameter Conditions Min Max Units
V
IH
High Level Input Voltage Any Inputs Except OSC IN, 2.0 V
CC
a
0.3 V
(Note 4) OSC IN with External Clock V
BB
b
0.2 V
V
IL
Low Level Input Voltage All Inputs Except OSC IN
b
0.3 0.8 V
OSC IN with External Clock
b
0.3 0.2 V
V
OH
High Level Output Voltage I
OUT
eb
20 mAV
CC
b
0,2 V
(Excluding OSC OUT, INTR
)I
OUT
eb
2.0 mA 2.4 V
V
OL
Low Level Output Voltage I
OUT
e
20 mA 0.2 V
(Excluding OSC OUT) I
OUT
e
2.0 mA 0.3 V
I
IN
Input Current (Except OSC IN) V
IN
e
VCCor GND
g
0.7 mA
I
OZ
Output TRI-STATEÉCurrent V
OUT
e
VCCor GND
g
1 mA
I
LKG
Output High Leakage Current V
OUT
e
VCCor GND
g
1 mA
MFO, INTR Pins Outputs Open Drain
I
CC
Quiescent Supply Current F
OSC
e
32.768 kHz
(Note 7) V
IN
e
VCCor GND (Note 5) 220 mA
V
IN
e
VCCor GND (Note 6) 700 mA
V
IN
e
VIHor VIL(Note 6) 5 mA
F
OSC
e
4.194304 MHz or
4.9152 MHz
V
IN
e
VCCor GND (Note 6) 4 mA
V
IN
e
VIHor VIL(Note 6) 6 mA
I
CC
Quiescent Supply Current V
BB
e
GND
(Single Supply Mode) V
IN
e
VCCor GND
(Note 7) F
OSC
e
32.768 kHz 30 mA
F
OSC
e
4.9152 MHz or 3 mA
4.194304 MHz
I
BB
Standby Mode Battery V
CC
e
GND
Supply Current OSC OUT
e
open circuit,
(Note 7) other pins
e
GND
F
OSC
e
32.768 kHzmA8mA
F
OSC
e
4.9152 MHz or 400 mA
4.194304 MHz
I
BLK
Battery Leakage 2.2VsV
BB
s
2.6V other pins at GND V
CC
e
GND, V
BB
e
2.6V 0.8 m A
V
CC
e
3.6V, V
BB
e
2.2V
b
0.8 mA
Note 1: Absolute Maximum Ratings are those values beyond which damage to the device may occur.
Note 2: Unless otherwise specified all voltages are referenced to ground.
Note 3: For F
OSC
e
4.194304 or 4.9152 MHz, VBBminimume2.8V. In battery backed mode, V
BB
s
V
CC
b
0.4V. Single Supply Mode: Data retention voltage is 2.2V min. In single Supply Mode (Power connected to V
CC
pin) 3.0VsV
CC
s
3.6.
Note 4: This parameter (V
IH
) is not tested on all pins at the same time.
Note 5: This specification tests I
CC
with all power fail circuitry disabled, by setting D7 of Interrupt Control Register 1 to 0.
Note 6: This specification tests I
CC
with all power fail circuitry enabled, by setting D7 of Interrupt Control Register 1 to 1.
Note 7: OSC IN is driven by a signal generator. Contents of the Test Register
e
00(H) and the MFO pin is not configured as buffered oscillator out.
2
AC Electrical Characteristics
V
CC
e
3.3Vg10%, V
BB
e
2.5V, V
PFAIL
l
VIH,C
L
e
100 pF (unless otherwise specified)
Symbol Parameter Min Max Units
READ TIMING
t
AR
Address Valid Prior to Read Strobe 10 ns
t
RW
Read Strobe Width (Note 8) 100 ns
t
CD
Chip Select to Data Valid Time 100 ns
t
RAH
Address Hold after Read (Note 9) 2 ns
t
RD
Read Strobe to Valid Data 90 ns
t
DZ
Read or Chip Select to TRI-STATE 80 ns
t
RCH
Chip Select Hold after Read Strobe (Note 9) 0 ns
t
DS
Minimum Inactive Time between Read or Write Accesses 70 ns
WRITE TIMING
t
AW
Address Valid before Write Strobe 10 ns
t
WAH
Address Hold after Write Strobe (Note 9) 2 ns
t
CW
Chip Select to End of Write Strobe 110 ns
t
WW
Write Strobe Width (Note 10) 100 ns
t
DW
Data Valid to End of Write Strobe 70 ns
t
WDH
Data Hold after Write Strobe (Note 9) 2 ns
t
WCH
Chip Select Hold after Write Strobe (Note 9) 0 ns
INTERRUPT TIMING
t
ROLL
Clock Rollover to INTR Out is Typically 20 ms
Note 8: Read Strobe width as used in the read timing table is defined as the period when both chip select and read inputs are low. Hence read commences when both signals are low and terminates when either signal returns high.
Note 9: Hold time is guaranteed by design but not production tested. This limit is not used to calculate outgoing quality levels.
Note 10: Write Strobe width as used in the write timing table is defined as the period when both chip select and write inputs are low. Hence write commences when
both signals are low and terminates when either signal returns high.
AC Test Conditions
Input Pulse Levels GND to 3.0V Input Rise and Fall Times 6 ns (10% –90%) Input and Output
1.3V
Reference Levels TRI-STATE Reference Active High
a
0.5V
Levels (Note 12) Active Low
b
0.5V
Note 11: C
L
e
100 pF, includes jig and scope capacitance.
Note 12: S1
e
VCCfor active low to high impedance measurements.
S1
e
GND for active high to high impedance measurements.
S1
e
open for all other timing measurements.
Capacitance (T
A
e
25§C, fe1 MHz)
Symbol
Parameter
Typ Units
(Note 13)
C
IN
Input Capacitance 5 pF
C
OUT
Output Capacitance 7 pF
Note 13: This parameter is not 100% tested.
Note 14: Output rise and fall times 25 ns max (10%– 90%) with 100 pF load.
TL/F/11417– 2
3
Timing Waveforms
Read Timing Diagram
TL/F/11417– 3
Write Timing Diagram
TL/F/11417– 4
4
General Description (Continued)
The LV8572A’s interrupt structure provides three basic types of interrupts: Periodic, Alarm/Compare, and Power Fail. Interrupt mask and status registers enable the masking and easy determination of each interrupt.
Pin Description
CS,RD,WR(Inputs): These pins interface to mP control
lines. The CS
pin is an active low enable for the read and write operations. Read and Write pins are also active low and enable reading or writing to the RTC. All three pins are disabled when power failure is detected. However, if a read or write is in progress at this time, it will be allowed to com­plete its cycle.
A0–A4 (Inputs): These 5 pins are for register selection. They individually control which location is to be accessed. These inputs are disabled when power failure is detected.
OSC IN (Input): OSC OUT (Output): These two pins are used to connect the crystal to the internal parallel resonant oscillator. The oscillator is always running when power is applied to V
BB
and VCC, and the correct crystal select bits in
the Real Time Mode Register have been set.
MFO (Output): The multi-function output can be used as a second interrupt output for interrupting the mP. This pin can also provide an output for the oscillator. The MFO output is configured as push-pull, active high for normal or single power supply operation and as an open drain during stand­by mode (V
BB
l
VCC). If in battery backed mode and a pull­up resistor is attached, it should be connected to a voltage no greater than V
BB
.
INTR (Output): The interrupt output is used to interrupt the processor when a timing event or power fail has occurred and the respective interrupt has been enabled. The INTR output is permanently configured active low, open drain. If in battery backed mode and a pull-up resistor is attached, it should be connected to a voltage no greater than V
BB
.
D0–D7 (Input/Output): These 8 bidirectional pins connect to the host mP’s data bus and are used to read from and write to the RTC. When the PFAIL
pin goes low and a write
is not in progress, these pins are at TRI-STATE.
PFAIL
(Input): In battery backed mode, this pin can have a
digital signal applied to it via some external power detection logic. When PFAIL
e
logic 0 the RTC goes into a lockout mode, in a minimum of 30 ms or a maximum of 63 ms unless lockout delay is programmed. In the single power supply mode, this pin is not useable as an input and should be tied to V
CC
. Refer to section on Power Fail Functional Descrip-
tion.
V
BB
(Battery Power Pin): This pin is connected to a back-
CC
becomes lower than VBB. Utiliz­ing this pin eliminates the need for external logic to switch in and out the back-up power supply. If this feature is not to be used then this pin must be tied to ground, the RTC pro­grammed for single power supply only, and power applied to the V
CC
pin.
V
CC
: This is the main system power pin.
GND: This is the common ground power pin for both V
BB
and VCC.
Connection Diagrams
In-Line Packages
TL/F/11417– 5
Top View
DIP: Order Number LV8572AN
See NS Package Number N24C
SOIC: Order Number LV8572AM
See NS Package Number M24B
Plastic Chip Carrier
TL/F/11417– 6
Top View
Order Number LV8572AV
See NS Package Number V28A
5
Functional Description
The LV8572A contains a fast access real time clock, inter­rupt control logic, power fail detect logic, and CMOS RAM. All functions of the RTC are controlled by a set of seven registers. A simplified block diagram that shows the major functional blocks is given in
Figure 1
.
The blocks are described in the following sections:
1. Real Time Clock
2. Oscillator Prescaler
3. Interrupt Logic
4. Power Failure Logic
5. Additional Supply Management
The memory map of the RTC is shown in the memory ad­dressing table. The memory map consists of two 31 byte pages with a main status register that is common to both pages. A control bit in the Main Status Register is used to select either page.
Figure 2
shows the basic concept. Page 0 contains all the clock timer functions, while page 1 has scratch pad RAM. The control registers are split into two separate blocks to allow page 1 to be used entirely as scratch pad RAM. Again a control bit in the Main Status Register is used to select either control register block.
TL/F/11417– 7
FIGURE 2. LV8572A Internal Memory Map
6
Functional Description (Continued)
INITIAL POWER-ON of BOTH V
BB
and V
CC
VBBand VCCmay be applied in any sequence. In order for the power fail circuitry to function correctly, whenever power is off, the V
CC
pin must see a path to ground through a maximum of 1 MX. The user should be aware that the con­trol registers will contain random data. The first task to be carried out in an initialization routine is to start the oscillator by writing to the crystal select bits in the Real Time Mode Register. If the LV8572A is configured for single supply mode, an extra 50 mA may be consumed until the crystal select bits are programmed. The user should also ensure that the RTC is not in test mode (see register descriptions).
REAL TIME CLOCK FUNCTIONAL DESCRIPTION
As shown in
Figure 2
, the clock has 10 bytes of counters, which count from 1/100 of a second to years. Each counter counts in BCD and is synchronously clocked. The count se­quence of the individual byte counters within the clock is shown later in Table VII. Note that the day of week, day of month, day of year, and month counters all roll over to 1. The hours counter in 12 hour mode rolls over to 1 and the AM/PM bit toggles when the hours rolls over to 12 (AM
e
0, PMe1). The AM/PM bit is bit D7 in the hours
counter.
All other counters roll over to 0. Also note that the day of year counter is 12 bits long and occupies two addresses. Upon initial application of power the counters will contain random information.
READING THE CLOCK: VALIDATED READ
1. Initialize program for reading clock.
2. Dummy read of periodic status bit to clear it.
3. Read counter bytes and store.
4. Read rollover bit, and test it.
5. If rollover occured go to 3.
6. If no rollover, done.
To detect the rollover, individual periodic status bits can be polled. The periodic bit chosen should be equal to the high­est frequency counter register to be read. That is if only SECONDS through HOURS counters are read, then the SECONDS periodic bit should be used.
READING THE CLOCK: INTERRUPT DRIVEN
Enabling the periodic interrupt mask bits cause interrupts just as the clock rolls over. Enabling the desired update rate and providing an interrupt service routine that executes in less than 10 ms enables clock reading without checking for a rollover.
READING THE CLOCK: LATCHED READ
Another method to read the clock that does not require checking the rollover bit is to write a one into the Time
INITIALIZING AND WRITING TO THE CALENDAR-CLOCK
Upon initial application of power to the RTC or when making time corrections, the time must be written into the clock. To correctly write the time to the counters, the clock would normally be stopped by writing the Start/Stop
bit in the Real Time Mode Register to a zero. This stops the clock from counting and disables the carry circuitry. When initializing the clock’s Real Time Mode Register, it is recommended that first the various mode bits be written while maintaining the Start/Stop
bit reset, and then writing to the register a
second time with the Start/Stop
bit set.
PRESCALER/OSCILLATOR FUNCTIONAL DESCRIPTION
Feeding the counter chain is a programmable prescaler which divides the crystal oscillator frequency to 32 kHz and further to 100 Hz for the counter chain (see
Figure 3
). The crystal frequency that can be selected are: 32 kHz, 32.768 kHz, 4.9152 MHz, and 4.194304 MHz.
TL/F/11417– 8
FIGURE 3. Programmable Clock Prescaler Block
7
Functional Description (Continued)
The oscillator is programmed via the Real Time Mode Reg­ister to operate at various frequencies. The crystal oscillator is designed to offer optimum performance at each frequen­cy. Thus, at 32.768 kHz the oscillator is configured as a low frequency and low power oscillator. At the higher frequen­cies the oscillator inverter is reconfigured. In addition to the inverter, the oscillator feedback bias resistor is included on chip, as shown in
Figure 4
. The oscillator input may be driv­en from an external source if desired. Refer to test mode application note for details. The oscillator stability is en­hanced through the use of an on chip regulated power sup­ply.
The typical range of trimmer capacitor (as shown in Oscilla­tor Circuit Diagram
Figure 4
, and in the typical application) at the oscillator input pin is suggested only to allow accurate tuning of the oscillator. This range is based on a typical printed circuit board layout and may have to be changed depending on the parasitic capacitance of the printed circuit board or fixture being used. In all cases, the load capaci- tance specified by the crystal manufacturer (nominal value 11 pF for the 32.768 crystal) is what determines proper os­cillation. This load capcitance is the series combination of capacitance on each side of the crystal (with respect to ground).
TL/F/11417– 9
FIGURE 4. Oscillator Circuit Diagram
R
OUT
XTAL C
o
C
t
(Switched Internally)
32/32.768 kHz 47 pF 2 pF–22 pF 150 kX to 350 kX
4.194304 MHz 68 pF 0 pF –80 pF 500X to 900X
4.9152 MHz 68 pF 29 pF – 49 pF 500X to 900X
INTERRUPT LOGIC FUNCTIONAL DESCRIPTION
The RTC has the ability to coordinate processor timing ac­tivities. To enhance this, an interrupt structure has been im­plemented which enables several types of events to cause interrupts. Interrupts are controlled via two Control Regis­ters in block 1 and two Status Registers in block 0. (See Register Description for notes on paging and also
Figure 5
and Table I.)
The interrupts are enabled by writing a one to the appropri­ate bits in Interrupt Control Register 0 and/or 1.
TABLE I. Registers that are
Applicable to Interrupt Control
Register Name
Register Page
Address
Select Select
Main Status Register X X 00H Periodic Flag Register 0 0 03H Interrupt Control
1 0 03H
Register 0
Interrupt Control
1 0 04H
Register 1
Output Mode
1 0 02H
Register
The Interrupt Status Flag D0, in the Main Status Register, indicates the state of INTR and MFO outputs. It is set when either output becomes active and is cleared when all RTC interrupts have been cleared and no further interrupts are pending (i.e., both INTR and MFO are returned to their inac­tive state). This flag enables the RTC to be rapidly polled by the mP to determine the source of an interrupt in a wiredÐ OR interrupt system. (The Interrupt Status Flag provides a true reflection of all conditions routed to the external pins.)
Status for the interrupts are provided by the Main Status Register and the Periodic Flag Register. Bits D1 – D5 of the Main Status Register are the main interrupt bits.
These register bits will be set when their associated timing events occur. Enabled Alarm comparisons that occur will set its Main Status Register bit to a one. However, an exter­nal interrupt will only be generated if the Alarm interrupt enable bit is set (see
Figure 5
).
Disabling the periodic interrupts will mask the Main Status Register periodic bit, but not the Periodic Flag Register bits. The Power Fail Interrupt bit is set when the interrupt is en­abled and a power fail event has occurred, and is not reset until the power is restored. If all interrupt enable bits are 0 no interrupt will be asserted. However, status still can be read from the Main Status Register in a polled fashion (see
Figure 5
).
To clear a flag in bits D2 and D3 of the Main Status Register a 1 must be written back into the bit location that is to be cleared. For the Periodic Flag Register reading the status will reset all the periodic flags.
8
Functional Description (Continued)
Interrupts Fall Into Three Categories:
1. The Alarm Compare Interrupt: Issued when the value in the time compared RAM equals the counter.
3. The Power Fail Interrupt: Issued upon recognition of a power fail condition by the internal sensing logic. The power failed condition is determined by the signal on the PFAIL
pin. The internal power fail signal is gated with the chip select signal to ensure that the power fail interrupt does not lock the chip out during a read or write.
ALARM COMPARE INTERRUPT DESCRIPTON
The alarm/time comparison interrupt is a special interrupt similar to an alarm clock wake up buzzer. This interrupt is generated when the clock time is equal to a value pro­grammed into the alarm compare registers. Up to six bytes can be enabled to perform alarm time comparisons on the counter chain. These six bytes, or some subset thereof, would be loaded with the future time at which the interrupt will occur. Next, the appropriate bits in the Interrupt Control Register 1 are enabled or disabled (refer to detailed descrip­tion of Interrupt Control Register 1). The RTC then com­pares these bytes with the clock time. When all the enabled compare registers equal the clock time an alarm interrupt is issued, but only if the alarm compare interrupt is enabled can the interrupt be generated externally. Each alarm com­pare bit in the Control Register will enable a specific byte for comparison to the clock. Disabling a compare byte is the same as setting its associated counter comparator to an ‘‘always equal’’ state. For example, to generate an interrupt at 3:15 AM of every day, load the hours compare with 0 3 (BCD), the minutes compare with 1 5 (BCD) and the faster counters with 0 0 (BCD), and then disable all other compare registers. So every day when the time rolls over from 3:14:59.99, an interrupt is issued. This bit may be reset by writing a one to bit D3 in the Main Status Register at any time after the alarm has been generated.
If time comparison for an individual byte counter is disabled, that corresponding RAM location can then be used as gen­eral purpose storage.
PERIODIC INTERRUPTS DESCRIPTION
The Periodic Flag Register contains six flags which are set by real-time generated ‘‘ticks’’ at various time intervals, see
Figure 5
. These flags constantly sense the periodic signals and may be used whether or not interrupts are enabled. These flags are cleared by any read or write operation per­formed on this register.
To generate periodic interrupts at the desired rate, the asso­ciated Periodic Interrupt Enable bit in Interrupt Control Reg­ister 0 must be set. Any combination of periodic interrupts may be enabled to operate simultaneously. Enabled period­ic interrupts will now affect the Periodic Interrupt Flag in the Main Status Register.
When a periodic event occurs, the Periodic Interrupt Flag in the Main Status Register is set, causing an interrupt to be generated. The mP clears both flag and interrupt by writing a ‘‘1’’ to the Periodic Interrupt Flag. The individual flags in the periodic Interrupt Flag Register do not require clearing to cancel the interrupt.
If all periodic interrupts are disabled and a periodic interrupt is left pending (i.e., the Periodic Interrupt Flag is still set), the Periodic Interrupt Flag will still be required to be cleared to cancel the pending interrupt.
POWER FAIL INTERRUPTS DESCRIPTION
The Power Fail Status Flag in the Main Status Register monitors the state of the internal power fail signal. This flag may be interrogated by the mP, but it cannot be cleared; it is cleared automatically by the RTC when system power is restored. To generate an interrupt when the power fails, the Power Fail Interrupt Enable bit in Interrupt Control Register 1 is set. Although this interrupt may not be cleared, it may be masked by clearing the Power Fail Interrupt Enable bit.
POWER FAILURE CIRCUITRY FUNCTIONAL DESCRIPTION
Since the clock must be operated from a battery when the main system supply has been turned off, the LV8572A pro­vides circuitry to simplify design in battery backed systems. This switches over to the back up supply, and isolates itself from the host system.
Figure 6
shows a simplified block diagram of this circuitry, which consists of three major sec­tions; 1) power loss logic: 2) battery switch over logic: and 3) isolation logic.
Detection of power loss occurs when PFAIL
is low. De­bounce logic provides a 30 ms–63 ms debounce time, which will prevent noise on the PFAIL
pin from being interpreted as a system failure. After 30 ms–63 ms the debounce logic times out and a signal is generated indicating that system power is marginal and is failing. The Power Fail Interrupt will then be generated.
9
Functional Description (Continued)
TL/F/11417– 10
FIGURE 5. Interrupt Control Logic Overview
10
Functional Description (Continued)
TL/F/11417– 11
FIGURE 6. System-Battery Switchover (Upper Left), Power Fail
and Lock-Out Circuits (Lower Right)
If chip select is low when a power failure is detected, a safety circuit will ensure that if a read or write is held active continuously for greater than 30 ms after the power fail sig­nal is asserted, the lock-out will be forced. If a lock-out delay is enabled, the LV8572A will remain active for 480 ms after power fail is detected. This will enable the mP to perform last minute bookkeeping before total system collapse. When the host CPU is finished accessing the RTC it may force the bus lock-out before 480 ms has elapsed by reset­ting the delay enable bit.
The battery switch over circuitry is completely independent of the PFAIL
pin. A separate circuit compares VCCto the
V
BB
voltage. As the main supply fails, the RTC will continue
to operate from the V
CC
pin until VCCfalls below the V
BB
voltage. At this time, the battery supply is switched in, VCCis disconnected, and the device is now in the standby mode. If indeterminate operation of the battery switch over circuit is to be avoided, then the voltage at the V
CC
pin must not be
allowed to equal the voltage at the V
BB
pin.
After the generation of a lock-out signal, and eventual switch in of the battery supply, the pins of the RTC will be configured as shown in Table II. Outputs that have a pull-up
resistor should be connected to a voltage no greater than V
BB
.
TABLE II. Pin Isolation during a Power Failure
Pin
PFAIL
e
Standby Mode
Logic 0 V
BB
l
V
CC
CS,RD,WR Locked Out Locked Out
A0–A4 Locked Out Locked Out D0–D7 Locked Out Locked Out
Oscillator Not Isolated Not Isolated
PFAIL
Not Isolated Not Isolated
INTR, MFO Not Isolated Open Drain
The Interrupt Power Fail Operation bit in the Real-Time Mode Register determine whether or not the interrupts will continue to function after a power fail event.
As power returns to the system, the battery switch over cir­cuit will switch back to V
CC
power as soon as it becomes greater than the battery voltage. The chip will remain in the locked out state as long as PFAIL
e
0. When PFAILe1
11
Functional Description (Continued)
the chip is unlocked, but only after another 30 ms min
x
63 ms max debounce time. The system designer must en­sure that his system is stable when power has returned.
CC
. In some cases this may be undesirable, so this circuit can be disabled by masking the power fail interrupt. The power fail input can perform all lock-out functions previously mentioned, except that no ex­ternal interrupt will be issued. Note that the linear power fail circuitry is switched off automatically when using V
BB
in
standby mode.
LOW BATTERY, INITIAL POWER ON DETECT, AND POWER FAIL TIME SAVE
There are three other functions provided on the LV8572A to ease power supply control. These are an initial Power On detect circuit, which also can be used as a time keeping failure detect, a low battery detect circuit, and a time save on power failure.
On initial power up the Oscillator Fail Flag will be set to a one and the real time clock start bit reset to a zero. This indicates that an oscillator fail event has occurred, and time keeping has failed.
The Oscillator Fail flag will not be reset until the real-time clock is started. This allows the system to discriminate be­tween an initial power-up and recovery from a power failure. If the battery backed mode is selected, then bit D6 of the Periodic Flag Register must be written low. This will not af­fect the contents of the Oscillator Fail Flag.
Another status bit is the low battery detect. This bit is set only when the clock is operating under the V
CC
pin, and when the battery voltage is determined to be less than 2.1V (typical). When the power fail interrupt enable bit is low, it disables the power fail circuit and will also shut off the low battery voltage detection circuit as well.
To relieve CPU overhead for saving time upon power failure, the Time Save Enable bit is provided to do this automatical­ly. (See also Reading the Clock: Latched Read.) The Time Save Enable bit, when set, causes the Time Save RAM to follow the contents of the clock. This bit can be reset by software, but if set before a power failure occurs, it will auto­matically be reset when the clock switches to the battery supply (not when a power failure is detected by the PFAIL pin). Thus, writing a one to the Time Save bit enables both a software write or power fail write.
SINGLE POWER SUPPLY APPLICATIONS
The LV8572A can be used in a single power supply applica­tion. To achieve this, the V
BB
pin must be connected to
ground, and the power connected to V
CC
and PFAIL pins. The Oscillator Failed/Single Supply bit in the Periodic Flag Register should be set to a logic 1, which will disable the oscillator battery reference circuit. The power fail interrupt should also be disabled. This will turn off the linear power fail detection circuits, and will eliminate any quiescent power drawn through these circuits. Until the crystal select bits are initialized, the LV8572A may consume about 50 mA due to arbitrary oscillator selection at power on.
(This extra 50 mA is not consumed if the battery backed mode is selected).
DETAILED REGISTER DESCRIPTION
There are 5 external address bits: Thus, the host microproc­essor has access to 28 locations at one time. An internal switching scheme provides a total of 61 locations.
This complete address space is organized into two pages. Page 0 contains two blocks of control registers, timers, real time clock counters, and special purpose RAM, while page 1 contains general purpose RAM. Using two blocks enables the 9 control registers to be mapped into 5 locations. The only register that does not get switched is the Main Status Register. It contains the page select bit and the register select bit as well as status information.
A memory map is shown in
Figure 2
and register addressing in Table III. They show the name, address and page loca­tions for the LV8572A.
TABLE III. Register/Counter/RAM
Addressing for LV8572A
A0-4
PS RS
Description
(Note 1) (Note 2)
CONTROL REGISTERS
00 X X Main Status Register 03 0 0 Periodic Flag Register 04 0 0 Time Save Control Register 01 0 1 Real Time Mode Register 02 0 1 Output Mode Register 03 0 1 Interrupt Control Register 0 04 0 1 Interrupt Control Register 1
COUNTERS (CLOCK CALENDAR)
05 0 X 1/100, 1/10 Seconds (0 –99) 06 0 X Seconds (0–59) 07 0 X Minutes (0 –59) 08 0 X Hours (1– 12, 0– 23) 09 0 X Days of
Month (1 – 28/29/30/31) 0A 0 X Months (1– 12) 0B 0 X Years (0– 99) 0C 0 X Julian Date (LSB) (1– 99) 0D 0 X Julian Date (0 –3) 0E 0 X Day of Week (1 –7)
TIME COMPARE RAM
13 0 X Sec Compare RAM (0 –59) 14 0 X Min Compare RAM (0– 59) 15 0 X Hours Compare
RAM (1 –12, 0 –23) 16 0 X DOM Compare
RAM (1– 28/29/30/31) 17 0 X Months Compare
RAM (1 –12) 18 0 X DOW Compare RAM (1 –7)
TIME SAVE RAM
19 0 X Seconds Time Save RAM 1A 0 X Minutes Time Save RAM 1B 0 X Hours Time Save RAM 1C 0 X Day of Month Time Save RAM 1D 0 X Months Time Save RAM
1E 0 1 RAM 1F 0 X RAM/Test Mode Register
01– 1F 1 X 2nd Page General Purpose RAM
1 PSÐPage Select (Bit D7 of Main Status Register) 2 RSÐRegister Select (Bit D6 of Main Status Register)
12
Functional Description (Continued)
MAIN STATUS REGISTER
TL/F/11417– 12
The Main Status Register is always located at address 0 regardless of the register block or the page selected.
D0: This read only bit is a general interrupt status bit that is taken directly from the interrupt pins. The bit is a one when an interrupt is pending on either the INTR pin or the MFO pin (when configured as an interrupt). This is unlike D3 which can be set by an internal event but may not cause an interrupt. This bit is reset when the interrupt status bits in the Main Status Register are cleared.
D4–D5: General purpose RAM bits.
which register block or RAM page is to be selected. Bit D6 controls the register block to be accessed (see memory map). The memory map of the clock is further divided into two memory pages. One page is the registers, clock and timers, and the second page contains 31 bytes of general purpose RAM. The page selection is determined by bit D7.
PERIODIC FLAG REGISTER
TL/F/11417– 13
The Periodic Flag Register has the same bit for bit corre­spondence as Interrupt Control Register 0 except for D6 and D7. For normal operation (i.e., not a single supply appli­cation) this register must be written to on initial power up or after an oscillator fail event. D0– D5 are read only bits, D6 and D7 are read/write.
e
1). The bits are reset when the register is
read and can be used as selective data change flags.
D6: This bit performs a dual function. When this bit is read, a one indicates that an oscillator failure has occurred and the time information may have been lost. Some of the ways an oscillator failure might be caused are: failure of the crystal, shorting OSC IN or OSC OUT to GND or V
CC
, removal of crystal, removal of battery when in the battery backed mode (when a ‘‘0’’ is written to D6), lowering the voltage at the V
BB
pin to a value less than 2.2V when in the battery backed mode. Bit D6 is automatically set to 1 on initial pow­er-up or an oscillator fail event. The oscillator fail flag is reset by writing a one to the clock start/stop bit in the Real Time Mode Register, with the crystal oscillating.
When D6 is written to, it defines whether the TCP is being used in battery backed (normal) or in a single supply mode application. When set to a one this bit configures the TCP for single power supply applications. This bit is automatically set on initial power-up or an oscillator fail event. When set, D6 disables the oscillator reference circuit. The result is that the oscillator is referenced to V
CC
. When a zero is written to D6 the oscillator reference is enabled, thus the oscillator is referenced to V
BB
. This allows operation in standard battery
standby applications.
At initial power on, if the LV8572A is going to be pro­grammed for battery backed mode, the V
BB
pin should be
connected to a potential in the range of 2.2V to V
CC
b
0.4V.
For single supply mode operation, the V
BB
pin should be
connected to GND and the PFAIL
pin connected to VCC.
D7: Writing a one to this bit enables the test mode register at location 1F (see Table III). This bit should be forced to zero during initialization for normal operation. If the test mode has been entered, clear the test mode register before leaving test mode. (See separate test mode application note for further details.)
TIME SAVE CONTROL REGISTER
TL/F/11417– 14
D0–D4: General purpose RAM bits.
13
Functional Description (Continued)
D5: The Delay Enable bit is used when a power fail occurs.
If this bit is set, a 480 ms delay is generated internally before the mP interface is locked out. This will enable the mPto access the registers for up to 480 ms after it receives a power fail interrupt. After a power failure is detected but prior to the 480 ms delay timing out, the host mP may force immediate lock out by resetting the Delay Enable bit. Note if this bit is a 0 when power fails then after a delay of 30 ms min/63 ms max the mP cannot read the chip.
D6: This read only bit is set and reset by the voltage at the V
BB
pin. It can be used by the mP to determine whether the
battery voltage at the V
BB
pin is getting too low. A compara-
tor monitors the battery and when the voltage is lower than
2.1V (typical) this bit is set. The power fail interrupt must be enabled to check for a low battery voltage.
This bit must be set to a one prior to power failing to enable the Time Save feature. When the power fails this bit is auto­matically reset and the time is saved in the Time Save RAM.
REAL TIME MODE REGISTER
TL/F/11417– 15
D0–D1: These are the leap year counter bits. These bits are written to set the number of years from the previous leap year. The leap year counter increments on December 31st and it internally enables the February 29th counter state. This method of setting the leap year allows leap year to occur whenever the user wishes to, thus providing flexibility in implementing Japanese leap year function.
LY1 LY0
Leap Year
Counter
0 0 Leap Year Current Year 0 1 Leap Year Last Year 1 0 Leap Year 2 Years Ago 1 1 Leap Year 3 Years Ago
D2: The count mode for the hours counter can be set to either 24 hour mode or 12 hour mode with AM/PM indicator. A one will place the clock in 12 hour mode.
D3: This bit is the master Start/Stop
bit for the clock. When a one is written to this bit the real time counter’s prescaler and counter chain are enabled. When this bit is reset to zero the contents of the real time counter is stopped and the prescaler is cleared. When the RTC is initially powered up this bit will be held at a logic 0 until the oscillator starts functioning correctly after which this bit may be modified. If an oscillator fail event occurs, this bit will be reset to logic 0.
D4: This bit controls the operation of the interrupt output in standby mode. If set to a one it allows Alarm, Periodic, and Power Fail interrupts to be functional in standby mode. Note that the MFO pin is configured as open drain in standby mode.
If bit D4 is set to a zero then interrupt control register and the periodic interrupt flag will be reset when the RTC enters the standby mode (V
BB
l
VCC). They will have to be re-
configured when system (V
CC
) power is restored.
D5: General purpose RAM.
D6 and D7: These two bits select the crystal clock frequen-
cy as per the following table:
XT1 XT0
Crystal
Frequency
0 0 32.768 kHz 0 1 4.194304 MHz 1 0 4.9152 MHz 1 1 32.000 kHz
All bits are Read/Write, and any mode written into this regis­ter can be determined by reading the register. On initial power up these bits are random.
OUTPUT MODE REGISTER
TL/F/11417– 16
D0–D6: General Purpose RAM
14
Functional Description (Continued)
D7: This bit is used to program the signal appearing at the
MFO output, as follows:
D7 MFO Output Signal
0 Power Fail Interrupt 1 Buffered Crystal Oscillator
INTERRUPT CONTROL REGISTER 0
TL/F/11417– 17
D0–D5: These bits are used to enable one of the selected periodic interrupts by writing a one into the appropriate bit. These interrupts are issued at the rollover of the clock. For example, the minutes interrupt will be issued whenever the minutes counter increments. In all likelihood the interrupt will be enabled asynchronously with the real time change. Therefore, the very first interrupt will occur in less than the periodic time chosen, but after the first interrupt all subse­quent interrupts will be spaced correctly. These interrupts are useful when minute, second, real time reading, or task switching is required. When all six bits are written to a 0 this disables periodic interrupts from the Main Status Register and the interrupt pin.
D6 and D7: General Purpose RAM.
INTERRUPT CONTROL REGISTER 1
TL/F/11417– 18
D6: In order to generate an external alarm compare inter­rupt to the mP from bit D3 of the Main Status Register, this bit must be written to a logic 1. If battery backed mode is selected then this bit is controlled by D4 of the Real Time Mode Register.
D7: The MSB of this register is the enable bit for the Power Fail Interrupt. When this bit is set to a one an interrupt will be generated to the mP when V
BB
l
VCC. If battery backed mode is selected then this bit is controlled by D4 of the Real Time Mode Register.
This bit also enables the low battery detection analog cir­cuitry.
15
Control and Status Register Address Bit Map
D7 D6 D5 D4 D3 D2 D1 D0
1. Reset by
Main Status Register PS
e
XRSeX ADDRESSe00H
writing
R/W R/W R/W R/W R/W
1
R/W
1
R
2
R
3
1 to bit.
Page Register
RAM RAM
Alarm Periodic Power Fail Interrupt
2. Set/reset by
Select Select Interrupt Interrupt Interrupt Status
voltage at PFAIL pin.
3. Reset when all pending interrupts are removed.
Periodic Flag Register PS
e
0RS
e
0 Addresse03H
4. Read Osc fail
R/W R/W
4
R
5
R
5
R
5
R
5
R
5
R
5
Write 0 Batt-
Test Osc. Fail/ 1 ms 10 ms 100 ms Seconds 10 Second Minute
Backed Mode
Mode Single Supply Flag Flag Flag Flag Flag Flag
Write 1 Single Supply Mode
5. Reset by positive edge of read.
Time Save Control Register PS
e
0RS
e
0 Addresse04H
R/W R
6
R/W R/W R/W R/W R/W R/W
Time Save Low Battery
Power Fail 6. Set and reset
by V
BB
Enable Flag
Delay RAM RAM RAM RAM RAM
voltage.
Enable
Real Time Mode Register PSe0RS
e
1 Addresse01H
Crystal Crystal
RAM
Interrupt EN Clock 12/24 Hr. Leap Year Leap Year
All Bits R/W
Freq. XT1 Freq. XT0 on Back-Up Start/Stop
Mode MSB LSB
Output Mode Register PSe0RS
e
1 Addresse02H
MFO as
RAM RAM RAM RAM RAM RAM RAM All Bits R/W
Crystal
Interrupt Control Register 0 PSe0RS
e
1 Addresse03H
1 ms 10 ms 100 ms Seconds 10 Second Minute
RAM RAM Interrupt Interrupt Interrupt Interrupt Interrupt Interrupt All Bits R/W
Enable Enable Enable Enable Enable Enable
Interrupt Control Register 1 PSe0RS
e
1 Addresse04H
Power Fail Alarm DOW Month DOM Hours Minute Second
Interrupt Interrupt Interrupt Interrupt Interrupt Interrupt Interrupt Interrupt All Bits R/W
Enable Enable Enable Enable Enable Enable Enable Enable
16
Application Hints
Suggested Initialization Procedure for LV8572A in Battery Backed Applications that use the V
BB
Pin.
1. Enter the test mode by writinga1tobitD7inthePeriod­ic Flag Register.
2. Write zero to the RAM/TEST mode Register located in page 0, address HEX 1F.
3. Leave the test mode by writinga0tobitD7inthePeri­odic Flag Register. Steps 1,2,3 guarantee that if the test mode had been entered during power on (due to ran­dom pulses from the system), all test mode conditions are cleared. Most important is that the OSC Fail Disable bit is cleared. Refer to AN-589 for more information on test mode operation.
4. After power on (V
CC
and VBBpowered), select the cor­rect crystal frequency bits (D7, D6 in the Real Time Mode Register) as shown in Table IV.
TABLE IV
Frequency D7 D6
32.768 kHz 0 0
4.194304 MHz 0 1
4.9152 MHz 1 0
32.0 kHz 1 1
5. Enter a software loop that does the following:
Set a 3 second(approx) software counter. The crystal oscillator may take 1 second to start.
5.1 Writea1tobitD3intheReal Time Mode Register (try to start the clock). Make sure the crystal select bits re-
main the same as in step 1. Under normal operation, this bit can be set only if the oscillator is running. During the software loop, RAM, real time counters, output configu­ration, interrupt control and timer functions may be ini­tialized.
6. Test bit D6 in the Periodic Flag Register:
IFa1,go to 5.1. If this bit remains a 1 after 3 seconds, then abort and check hardware. The crystal may be de­fective or not installed. There may be a short at OSC IN or OSC OUT to V
CC
or GND, or to some impedance that
is less than 10 MX.
IFa0,then the oscillator is running, go to step 7.
7. Writea0tobitD6inthePeriodic Flag Register. This action puts the clock chip in the battery backed mode. This mode can be entered only if the OSC fail flag (bit D6 of the Periodic Flag Register) is a 0. Reminder, Bit D6 is a dual function bit. When read, D6 returns oscilla­tor status. When written, D6 causes either the Battery Backed Mode, or the Single Supply Mode of operation.
The only method to ensure the chip is in the battery backed mode is to measure the waveform at the OSC OUT pin. If the battery backed mode was selected suc­cessfully, then the peak to peak waveform at OSC OUT is referenced to the battery voltage. If not in battery backed mode, the waveform is referenced to V
CC
. The measurement should be made with a high impedance low capacitance probe (10 MX, 10 pF oscilloscope probe or better). Typical peak to peak swings are within
0.6V of V
CC
and ground respectively.
8. Writea1tobitD7ofInterrupt Control Register 1. This action enables the PFAIL pin and associated circuitry.
9. Initialize the rest of the chip as needed.
Typical Application
TL/F/11417– 19
*These components may be necessary to meet UL requirements
for lithium batteries. Consult battery manufacturer.
17
Typical Performance Characteristics
Operating Current vs Supply Voltage (Single Supply Mode F
OSC
e
32.768 kHz)
TL/F/11417– 20
Operating Current vs Supply Voltage (Battery Backed Mode F
OSC
e
32.768 kHz,
V
BB
e
2.5V)
TL/F/11417– 21
Standby Current vs Power Supply Voltage (F
OSC
e
32.768 kHz)
TL/F/11417– 22
Standby Current vs Power Supply Voltage F
OSC
e
4.194304 MHz
TL/F/11417– 23
18
Physical Dimensions inches (millimeters)
Molded Small Outline Package (M)
Order Number LV8572AM
NS Package Number M24B
Molded Dual-In-Line Package (N)
Order Number LV8572AN
NS Package Number N24C
19
LV8572A Real Time Clock (RTC)
Physical Dimensions inches (millimeters) (Continued)
Plastic Chip Carrier Package (V)
Order Number LV8572AV
NS Package Number V28A
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