Datasheet MM58274CN, MM58274CN-12, MM58274CV, MM58274CVX Datasheet (NSC)

TL/F/11219

MM58274C Microprocessor Compatible Real Time Clock

April 1991

MM58274C
Microprocessor Compatible Real Time Clock

General Description

The MM58274C is fabricated using low threshold metal gate
CMOS technology and is designed to operate in bus orient­ed microprocessor systems where a real time clock and cal­endar function are required. The on-chip 32.768 kHz crystal
controlled oscillator will maintain timekeeping down to 2.2V
to allow low power standby battery operation. This device is
pin compatible with the MM58174A but continues timekeep­ing up to tens of years. The MM58274C is a direct replace­ment for the MM58274 offering improved Bus access cycle
times.

Applications

Y

Point of sale terminals

Y

Teller terminals

Y

Word processors

Y

Data logging

Y

Industrial process control

Features

Y

Same pin-out as MM58174A, MM58274B, and
MM58274

Y

Timekeeping from tenths of seconds to tens of years in
independently accessible registers

Y

Leap year register

Y

Hours counter programmable for 12 or 24-hour
operation

Y

Buffered crystal frequency output in test mode for easy
oscillator setting

Y

Data-changed flag allows simple testing for time
rollover

Y

Independent interrupting time with open drain output

Y

Fully TTL compatible

Y

Low power standby operation (10 mA at 2.2V)

Y

Low cost 16-pin DIP and 20-pin PCC

Block Diagram

TL/F/11219– 1

FIGURE 1

TRI-STATEÉis a registered trademark of National Semiconductor Corp.
Microbus

TM

is a trademark of National Semiconductor Corp.

C

1995 National Semiconductor Corporation RRD-B30M105/Printed in U. S. A.

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.

DC Input or Output Voltage

b

0.3V to V

DD

a

0.3V

DC Input or Output Diode Current

g

5.0 mA

Storage Temperature, T

STG

b

65§Ctoa150§C

Supply Voltage, V

DD

6.5V

Power Dissipation, P

D

500 mW

Lead Temperature

(Soldering, 10 seconds) 260

§

Operating Conditions

Min Max Units

Operating Supply Voltage 4.5 5.5 V
Standby Mode Supply Voltage 2.2 5.5 V
DC Input or Output Voltage 0 V

DD

V

Operating Temperature Range

b

40 85

§

C

Electrical Characteristics V

DD

e

5Vg10%, Teb40§Ctoa85§C unless otherwise stated.

Symbol Parameter Conditions Min Typ Max Units

V

IH

High Level Input 2.0 V
Voltage (except
XTAL IN)

V

IL

Low Level Input 0.8 V
Voltage (except
XTAL IN)

V

OH

High Level Output I

OH

eb

20 mAV

DD

b

0.1 V

Voltage (DB0–DB3) I

OH

eb

1.6 mA 3.7 V

V

OH

High Level Output I

OH

eb

20 mAV

DD

b

0.1 V

Voltage (INT) (In Test Mode)

V

OL

Low Level Output I

OL

e

20 mA 0.1 V

Voltage (DB0–DB3, i

OL

e

1.6 mA 0.4 V

INT)

I

IL

Low Level Input Current V

IN

e

VSS(Note 2)

b

5

b

80 mA

(AD0–AD3, DB0 –DB3)

I

IL

Low Level Input Current V

IN

e

VSS(Note 2)

b

5

b

190 mA

(WR

,RD)

I

IL

Low Level Input Current V

IN

e

VSS(Note 2)

b

5

b

550 mA

(CS)

I

OZH

Ouput High Level V

OUT

e

V

DD

2.0 mA

Leakage Current (INT

)

I

DD

Average Supply Current All V

IN

e

VCCor Open Circuit

V

DD

e

2.2V (Standby Mode) 4 10 mA

V

DD

e

5.0V (Active Mode) 1 mA

C

IN

Input Capacitance 5 10 pF

C

OUT

Output Capacitance (Outputs Disabled) 10 pF

Note 1: Absolute Maximum Ratings are those values beyond which damage to the device may occur. All voltages referenced to ground unless otherwise noted.

Note 2: The DB0 –DB3 and AD0 –AD3 lines all have active P-channel pull-up transistors which will source current. The CS

,RD, and WR lines have internal pull-up

resistors to V

DD

.

2

AC Switching Characteristics

READ TIMING: DATA FROM PERIPHERAL TO MICROPROCESSOR V

DD

e

5Vg0.5V, C

L

e

100 pF

Commercial

Symbol Parameter

Specification

Units

T

A

eb

40§Ctoa85§C

Min Typ Max

t

AD

Address Bus Valid to Data Valid 390 650 ns

t

CSD

Chip Select On to Data Valid 140 300 ns

t

RD

Read Strobe On to Data Valid 140 300 ns

t

RW

Read Strobe Width (Note 3, Note 7) DC

t

RA

Address Bus Hold Time from Trailing Edge 0 ns
of Read Strobe

t

CSH

Chip Select Hold Time from Trailing Edge 0 ns
of Read Strobe

t

RH

Data Hold Time from Trailing Edge 70 160 ns
of Read Strobe

t

HZ

Time from Trailing Edge of Read Strobe 250 ns
Until O/P Drivers are TRI-STATE

É

WRITE TIMING: DATA FROM MICROPROCESSOR TO PERIPHERAL V

DD

e

5Vg0.5V

Commercial

Symbol Parameter

Specification

Units

T

A

eb

40§Ctoa85§C

Min Typ Max

t

AW

Address Bus Valid to Write Strobe O 400 125 ns
(Note 4, Note 6)

t

CSW

Chip Select On to Write Strobe O 250 100 ns

t

DW

Data Bus Valid to Write Strobe O 400 220 ns

t

WW

Write Strobe Width (Note 6) 250 95 ns

t

WCS

Chip Select Hold Time Following 0 ns
Write Strobe O

t

WA

Address Bus Hold Time Following 0 ns
Write Strobe O

t

WD

Data Bus Hold Time Following 100 35 ns
Write Strobe O

t

AWS

Address Bus Valid Before 70 20 ns
Start of Write Strobe

Note 3: Except for special case restriction: with interrupts programmed, max read strobe width of control register (ADDR 0) is 30 ms. See section on Interrupt
Programming.

Note 4: All timings measured to the trailing edge of write strobe (data latched by the trailing edge of WR

).

Note 5: Input test waveform peak voltages are 2.4V and 0.4V. Output signals are measured to their 2.4V and 0.4V levels.

Note 6: Write strobe as used in the Write Timing Table is defined as the period when both chip select and write inputs are low, ie., WS

,eCSaWR. Hence write

strobe commences when both signals are low, and terminates when the first signal returns high.

Note 7: Read strobe as used in the Read Timing Table is defined as the period when both chip select and read inputs are low, ie., RS

eCSa

RD.

Note 8: Typical numbers are at V

CC

e

5.0V and T

A

e

25§C.

3

Switching Time Waveforms

Read Cycle Timing (Notes 5 and 7)

TL/F/11219– 2

Write Cycle Timing (Notes 5 and 6)

TL/F/11219– 3

Connection Diagrams

Dual-In-Line Package

TL/F/11219– 4

Top View

PCC Package

TL/F/11219– 5

Top View

FIGURE 2

Order Number MM58274CJ, MM58274CN or MM58274CV

See NS Package J16A, N16A, or V20A

4

Functional Description

The MM58274C is a bus oriented microprocessor real time
clock. It has the same pin-out as the MM58174A while offer­ing extended timekeeping up to units and tens of years. To
enhance the device further, a number of other features have
been added including: 12 or 24 hours counting, a testable
data-changed flag giving easy error-free time reading and
simplified interrupt control.

A buffered oscillator signal appears on the interrupt output
when the device is in test mode. This allows for easy oscilla­tor setting when the device is initially powered up in a sys­tem.

The counters are arranged as 4-bit words and can be ran­domly accessed for time reading and setting. The counters
output in BCD (binary coded decimal) 4-bit numbers. Any
register which has less than 4 bits (e.g., days of week uses
only 3 bits) will return a logic 0 on any unused bits. When
written to, the unused inputs will be ignored.

Writing a logic 1 to the clock start/stop control bit resets the
internal oscillator divider chain and the tenths of seconds
counter. Writing a logic 0 will start the clock timing from the
nearest second. The time then updates every 100 ms with
all counters changing synchronously. Time changing during
a read is detected by testing the data-changed bit of the
control register after completing a string of clock register
reads.

Interrupt delay times of 0.1s, 0.5s, 1s, 5s, 10s, 30s or 60s
can be selected with single or repeated interrupt outputs.
The open drain output is pulled low whenever the interrupt
timer times out and is cleared by reading the control regis­ter.

CIRCUIT DESCRIPTION

The block diagram in

Figure 1

shows the internal structure

of the chip. The 16-pin package outline is shown in

Figure 2

.

Crystal Oscillator

This consists of a CMOS inverter/amplifier with an on-chip
bias resistor. Externally a 20 pF capacitor,a6pF–36pF
trimmer capacitor and a crystal are suggested to complete
the 32.768 kHz timekeeping oscillator circuit.

The 6 pF –36 pF trimmer fine tunes the crystal load imped­ance, optimizing the oscillator stability. When properly ad­justed (i.e., to the crystal frequency of 32.768 kHz), the cir­cuit will display a frequency variation with voltage of less
than 3 ppm/V. When an external oscillator is used, connect
to oscillator input and float (no connection) the oscillator
output.

When the chip is enabled into test mode, the oscillator is
gated onto the interrupt output pin giving a buffered oscilla­tor output that can be used to set the crystal frequency
when the device is installed in a system. For further informa­tion see the section on Test Mode.

Divider Chain

The crystal oscillator is divided down in three stages to pro­duce a 10 Hz frequency setting pulse. The first stage is a
non-integer divider which reduces the 32.768 kHz input to

30.720 kHz. This is further divided by a 9-stage binary ripple
counter giving an output frequency of 60 Hz. A 3-stage
Johnson counter divides this by six, generating a 10 Hz out­put. The 10 Hz clock is gated with the 32.768 kHz crystal
frequency to provide clock setting pulses of 15.26 ms dura­tion. The setting pulse drives all the time registers on the

*Use resistor with Ni-cad cells only

TL/F/11219– 6

FIGURE 3. Typical System Connection Diagram

5

Functional Description (Continued)

device which are synchronously clocked by this signal. All
time data and data-changed flag change on the falling edge
of the clock setting pulse.

Data-Changed Flag

The data-changed flag is set by the clock setting pulse to
indicate that the time data has been altered since the clock
was last read. This flag occupies bit 3 of the control register
where it can be tested by the processor to sense data­changed. It will be reset by a read of the control register.
See the section, ‘‘Methods of Device Operation’’, for sug­gested clock reading techniques using this flag.

Seconds Counters

There are three counters for seconds:

a) tenths of seconds

b) units of seconds

c) tens of seconds.

The registers are accessed at the addresses shown in Ta­ble I. The tenths of seconds register is reset to 0 when the
clock start/stop bit (bit 2 of the control register) is set to
logic 1. The units and tens of seconds are set up by the
processor, giving time setting to the nearest second. All
three registers can be read by the processor for time output.

Minutes Counters

There are two minutes counters:

a) units of minutes

b) tens of minutes.

Both registers may be read to or written from as required.

Hours Counters

There are two hours counters:

a) units of hours

b) tens of hours.

Both counters may be accessed for read or write operations
as desired.

In 12-hour mode, the tens of hours register has only one
active bit and the top three bits are set to logic 0. Data bit 1
of the clock setting register is the AM/PM indicator; logic 0
indicating AM, logic 1 for PM.

When 24-hour mode is programmed, the tens of hours reg­ister reads out two bits of data and the two most significant
bits are set to logic 0. There is no AM/PM indication and bit
1 of the clock setting register will read out a logic 0.

In both 12/24-hour modes, the units of hours will read out
four active data bits. 12 or 24-hour mode is selected by bit 0
of the clock setting register, logic 0 for 12-hour mode, logic
1 for the 24-hour mode.

Days Counters

There are two days counters:

a) units of days

b) tens of days.

The days counters will count up to 28, 29, 30 or 31 depend­ing on the state of the months counters and the leap year
counter. The microprocessor has full read/write access to
these registers.

Months Counters

There are two months counters:

a) units of months

b) tens of months.

Both these counters have full read/write access.

Years Counters

There are two years counters:

a) units of years

b) tens of years.

Both these counters have full read/write access. The years
will count up to 99 and roll over to 00.

TABLE I. Address Decoding of Real-Time Clock Internal Registers

Register Selected

Address (Binary)

(Hex) Access

AD3 AD2 AD1 AD0

0 Control Register 0 0 0 0 0 Split Read and Write
1 Tenths of Seconds 0 0 0 1 1 Read Only
2 Units Seconds 0 0 1 0 2 R/W
3 Tens Seconds 0 0 1 1 3 R/W
4 Units Minutes 0 1 0 0 4 R/W
5 Tens Minutes 0 1 0 1 5 R/W
6 Unit Hours 0 1 1 0 6 R/W
7 Tens Hours 0 1 1 1 7 R/W
8 Units Days 1 0 0 0 8 R/W

9 Tens Days 1 0 0 1 9 R/W
10 Units Months 1 0 1 0 A R/W
11 Tens Months 1 0 1 1 B R/W
12 Units Years 1 1 0 0 C R/W
13 Tens Years 1 1 0 1 D R/W
14 Day of Week 1 1 1 0 E R/W
15 Clock Setting/ 1 1 1 1 F R/W

Interrupt Registers

6

Functional Description (Continued)

Day of Week Counter

The day of week counter increments as the time rolls from
23:59 to 00:00 (11:59 PM to 12:00 AM in 12-hour mode). It
counts from 1 to 7 and rolls back to 1. Any day of the week
may be specified as day 1.

Clock Setting Register/Interrupt Register

The interrupt select bit in the control register determines
which of these two registers is accessible to the processor
at address 15. Normal clock and interrupt timing operations
will always continue regardless of which register is selected
onto the bus. The layout of these registers is shown in
Table II.

The clock setting register is comprised of three separate
functions:

a) leap year counter: bits 2 and 3

b) AM/PM indicator: bit 1

c) 12-hour mode set: bit 0 (see Table IIA).

The leap year counter is a 2-stage binary counter which
is clocked by the months counter. It changes state as the
time rolls over from 11:59 on December 31 to 00:00 on
January 1.

The counter should be loaded with the ‘number of years
since last leap year’ e.g., if 1980 was the last leap year, a
clock programmed in 1983 should have 3 stored in the leap
year counter. If the clock is programmed during a leap year,
then the leap year counter should be set to 0. The contents
of the leap year counter can be read by the mP.

The AM/PM indicator returns a logic 0 for AM and a logic 1
for PM. It is clocked when the hours counter rolls from 11:59
to 12:00 in 12-hour mode. In 24-hour mode this bit is set to
logic 0.

The 12/24-hour mode set determines whether the hours
counter counts from 1 to 12 or from 0 to 23. It also controls
the AM/PM indicator, enabling it for 12-hour mode and forc­ing it to logic 0 for the 24-hour mode. The 12/24-hour mode
bit is set to logic 0 for 12-hour mode and it is set to logic 1
for 24-hour mode.

IMPORTANT NOTE:

Hours mode and AM/PM bits cannot

be set in the same write operation. See the section on

Ini-

tialization (Methods of Device Operation)

for a suggested

setting routine.

All bits in the clock setting register may be read by the proc­essor.

The interrupt register controls the operation of the timer for
interrupt output. The processor programs this register for
single or repeated interrupts at the selected time intervals.

The lower three bits of this register set the time delay period
that will occur between interrupts. The time delays that can
be programmed and the data words that select these are
outlined in Table IIB.

Data bit 3 of the interrupt register sets for either single or
repeated interrupts; logic 0 gives single mode, logic 1 sets
for repeated mode.

Using the interrupt is described in the Device Operation sec­tion.

TABLE IIA. Clock Setting Register Layout

Function

Data Bits Used

Comments Access

DB3 DB2 DB1 DB0

Leap Year Counter X X 0 Indicates a Leap Year R/W
AM/PM Indicator (12-Hour Mode) X 0

e

AM 1ePM R/W

0 in 24-Hour Mode

12/24-Hour Select Bit X 0

e

12-Hour Mode R/W

1e24-Hour Mode

TABLE IIB. Interrupt Control Register

Function Comments

Control Word

DB3 DB2 DB1 DB0

No Interrupt Interrupt output cleared, X 0 0 0

start/stop bit set to 1.

0.1 Second 0/1 0 0 1

0.5 Second 0/1 0 1 0
1 Second

DB3

e

0 for single interrupt

0/1 0 1 1

5 Seconds

DB3

e

1 for repeated interrupt

0/1 1 0 0
10 Seconds 0/1 1 0 1
30 Seconds 0/1 1 1 0
60 Seconds 0/1 1 1 1

Timing Accuracy: single interrupt mode (all time delays):g1ms
Repeated Mode:

g

1 ms on initial timeout, thereafter synchronous

with first interrupt (i.e., timing errors do not accumulate).

7

Functional Description (Continued)

Control Register

There are three registers which control different operations
of the clock:

a) the clock setting register

b) the interrupt register

c) the control register.

The clock setting and interrupt registers both reside at ad­dress 15, access to one or the other being controlled by the
interrupt select bit; data bit 1 of the control register.

The clock setting register programs the timekeeping of the
clock. The 12/24-hour mode select and the AM/PM indica­tor for 12-hour mode occupy bits 0 and 1, respectively. Data
bits 2 and 3 set the leap year counter.

The interrupt register controls the operation of the interrupt
timer, selecting the required delay period and either single
or repeated interrupt.

The control register is responsible for controlling the opera­tions of the clock and supplying status information to the
processor. It appears as two different registers; one with
write only access and one with read only access.

The write only register consists of a bank of four latches
which control the internal processes of the clock.

The read only register contains two output data latches
which will supply status information for the processor. Table
III shows the mapping of the various control latches and
status flags in the control register. The control register is
located at address 0.

The write only portion of the control register contains four
latches:

A logic 1 written into the test bit puts the device into test
mode. This allows setting of the oscillator frequency as well
as rapid testing of the device registers, if required. A more
complete description is given in the Test Mode section. For
normal operation the test bit is loaded with logic 0.

The clock start/stop bit stops the timekeeping of the clock
and resets to 0 the tenths of seconds counter. The time of
day may then be written into the various clock registers and
the clock restarted synchronously with an external time
source. Timekeeping is maintained thereafter.

A logic 1 written to the start/stop bit halts clock timing. Tim­ing is restarted when the start/stop bit is written with a logic

0.

The interrupt select bit determines which of the two regis­ters mapped onto address 15 will be accessed when this
address is selected.

A logic 0 in the interrupt select bit makes the clock setting
register available to the processor. A logic 1 selects the
interrupt register.

The interrupt start/stop bit controls the running of the inter­rupt timer. It is programmed in the same way as the clock
start/stop bit; logic 1 to halt the interrupt and reset the tim­er, logic 0 to start interrupt timing.

When no interrupt is programmed (interrupt control register
set to 0), the interrupt start/stop bit is automatically set to a
logic 1. When any new interrupt is subsequently pro­grammed, timing will not commence until the start/stop bit
is loaded with 0.

In the single interrupt mode, interrupt timing stops when a
timeout occurs. The processor restarts timing by writing log­ic 0 into the start/stop bit.

In repeated interrupt mode the interrupt timer continues to
count with no intervention by the processor necessary.

Interrupt timing may be stopped in either mode by writing a
logic 1 into the interrupt start/stop bit. The timer is reset and
can be restarted in the normal way, giving a full time delay
period before the next interrupt.

In general, the control register is set up such that writing 0’s
into it will start anything that is stopped, pull the clock out of
test mode and select the clock setting register onto the bus.
In other words, writing 0 will maintain normal clock operation
and restart interrupt timing, etc.

The read only portion of the control register has two status
outputs:

Since the MM58274C keeps real time, the time data
changes asynchronously with the processor and this may
occur while the processor is reading time data out of the
clock.

Some method of warning the processor when the time data
has changed must thus be included. This is provided for by
the data-changed flag located in bit 3 of the control register.
This flag is set by the clock setting pulse which also clocks
the time registers. Testing this bit can tell the processor
whether or not the time has changed. The flag is cleared by
a read of the control register but not by any write operations.
No other register read has any effect on the state of the
data-changed flag.

Data bit 0 is the interrupt flag. This flag is set whenever the
interrupt timer times out, pulling the interrupt output low. In a
polled interrupt routine the processor can test this flag to
determine if the MM58274C was the interrupting device.
This interrupt flag and the interrupt output are both cleared
by a read of the control register.

TABLE III. The Control Register Layout

Access (addr0) DB3 DB2 DB1 DB0

Read From: Data-Changed Flag 0 0 Interrupt Flag

Write To: Test Clock Start/Stop Interrupt Select Interrupt Start/Stop

0

e

Normal 0eClock Run 0eClock Setting Register 0eInterrupt Run

1

e

Test Mode 1eClock Stop 1eInterrupt Register 1eInterrupt Stop

8

Functional Description (Continued)

Both of the flags and the interrupt output are reset by the
trailing edge of the read strobe. The flag information is held
latched during a control register read, guaranteeing that sta­ble status information will always be read out by the proces­sor.

Interrupt timeout is detected and stored internally if it occurs
during a read of the control register, the interrupt output will
then go low only after the read has been completed.

A clock setting pulse occurring during a control register read
will

not

affect the data-changed flag since time data read
out before or after the control read will not be affected by
the time change.

METHODS OF DEVICE OPERATION

Test Mode

National Semiconductor uses test mode for functionally
testing the MM58274C after fabrication and again after
packaging. Test mode can also be used to set up the oscil­lator frequency when the part is first commissioned.

Figure 4

shows the internal clock connections when the de­vice is written into test mode. The 32.768 kHz oscillator is
gated onto the interrupt output to provide a buffered output
for initial frequency setting. This signal is driven from a
TRI-STATE output buffer, enabling easy oscillator setting in
systems where interrupt is not normally used and there is no
external resistor on the pin.

If an interrupt is programmed, the 32.768 kHz output is
switched off to allow high speed testing of the interrupt tim­er. The interrupt output will then function as normal.

The clock start/stop bit can be used to control the fast
clocking of the time registers as shown in

Figure 4

.

Initialization

When it is first installed and power is applied, the device will
need to be properly initialized. The following operation steps
are recommended when the device is set up (all numbers
are decimal):

1) Disable interrupt on the processor to allow oscillator set­ting. Write 15

10

into the control register:

The clock and inter­rupt start/stop bits are set to 1, ensuring that the clock and
interrupt timers are both halted. Test mode and the interrupt
register are selected.

2) Write 0 to the interrupt register:

Ensure that there are no
interrupts programmed and that the oscillator will be gated
onto the interrupt output.

3) Set oscillator frequency:

All timing has been halted and

the oscillator is buffered out onto the interrupt line

.

4) Write 5 to the control register:

The clock is now out of test
mode but is still halted. The clock setting register is now
selected by the interrupt select bit.

5) Write 0001 to all registers. This ensures starting with a
valid BCD value in each register.

6) Set 12/24 Hours Mode:

Write to the clock setting register

to select the hours counting mode required.

7) Load Real-Time Registers:

All time registers (including
Leap Years and AM/PM bit) may now be loaded in any
order. Note that when writing to the clock setting register to
set up Leap Years and AM/PM, the Hours Mode bit must
not be altered from the value programmed in step 5.

8) Write 0 to the control register:

This operation finishes the
clock initialization by starting the time. The final control reg­ister write should be synchronized with an external time
source.

In general, timekeeping should be halted before the time
data is altered in the clock. The data can, however, be al­tered at any time if so desired. Such may be the case if the
user wishes to keep the clock corrected without having to
stop and restart it; i.e., winter/summer time changing can be
accomplished without halting the clock. This can be done in
software by sensing the state of the data-changed flag and
only altering time data just after the time has rolled over
(data-changed flag set).

TL/F/11219– 7

FIGURE 4. Test Mode Organization

9

Functional Description (Continued)

Reading the Time Registers

Using the data-changed flag technique supports microproc­essors with block move facilities, as all the necessary time
data may be read sequentially and then tested for validity as
shown below.

1) Read the control register, address 0:

This is a dummy
read to reset the data-changed flag (DCF) prior to reading
the time registers.

2) Read time registers:

All desired time registers are read

out in a block.

3) Read the control register and test DCF:

If DCF is cleared
(logic 0), then no clock setting pulses have after occurred
since step 1. All time data is guaranteed good and time
reading is complete.

If DCF is set (logic 1), then a time change has occurred
since step 1 and time data may not be consistent. Repeat
steps 2 and 3 until DCF is clear. The control read of step 3
will have reset DCF, automatically repeating the step 1 ac­tion.

Interrupt Programming

The interrupt timer generates interrupts at time intervals
which are programmed into the interrupt register. A single
interrupt after delay or repeated interrupts may be pro­grammed. Table IIB lists the different time delays and the
data words that select them in the interrupt register.

Once the interrupt register has been used to set up the
delay time and to select for single or repeat, it takes no
further part in the workings of the interrupt system. All activi­ty by the processor then takes place in the control register.

Initializing:

1) Write 3 to the control register (AD0):

Clock timing contin-

ues, interrupt register selected and interrupt timing stopped.

2) Write interrupt control word to address 15:

The interrupt
register is loaded with the correct word (chosen from Table
IIB) for the time delay required and for single or repeated
interrupts.

3) Write 0 or 2 to the control register:

Interrupt timing com­mences. Writing 0 selects the clock setting register onto the
data bus; writing 2 leaves the interrupt register selected.
Normal timekeeping remains unaffected.

On Interrupt:

Read the control register and test for Interrupt Flag (bit 0).

If the flag is cleared (logic 0), then the device is not the
source of the interrupt.

If the flag is set (logic 1), then the clock did generate an
interrupt. The flag is reset and the interrupt output is cleared
by the control register read that was used to test for inter­rupt.

Single Interrupt Mode:

When appropriate, write 0 or 2 to the control register to
restart the interrupt timer.

Repeated Interrupt Mode:

Timing continues, synchronized with the control register
write which originally started interrupt timing. No further in­tervention is necessary from the processor to maintain tim­ing.

In either mode interrupt timing can be stopped by writing 1
into the control register (interrupt start/stop set to 1). Timing
for the full delay period recommences when the interrupt
start/stop bit is again loaded with 0 as normal.

IMPORTANT NOTE: Using the interrupt timer places a con­straint on the maximum Read Strobe width which may be
applied to the clock. Normally all registers may be read from
with a t

RW

down to DC (i.e., CS and RD held continuously
low). When the interrupt timer is active however, the maxi­mum read strobe width that can be applied to the control
register (Addr 0) is 30 ms.

This restriction is to allow the interrupt timer to properly re­set when it times out. Note that it only affects reading of the
control registerÐall other addresses in the clock may be
accessed with DC read strobes, regardless of the state of
the interrupt timer. Writes to any address are unaffected.

NOTES ON AC TIMING REQUIREMENTS

Although the Switching Time Waveforms show Microbus
control signals used for clock access, this does not pre­clude the use of the MM58274C in other non-Microbus sys­tems.

Figure 5

is a simplified logic diagram showing how the
control signals are gated internally to control access to the
clock registers. From this diagram it is clear that CS

could
be used to generate the internal data transfer strobes, with
RD

and WR inputs set up first. This situation is illustrated in

Figure 6

.

The internal data busses of the MM58274C are fully CMOS,
contributing to the flexibility of the control inputs. When de­termining the suitability of any given control signal pattern
for the MM58274C the timing specifications in AC Switching
Characteristics should be examined. As long as these tim­ings are met (or exceeded) the MM58274C will function cor­rectly.

When the MM58274C is connected to the system via a pe­ripheral port, the freedom from timing constraints allows for
very simple control signal generation, as in

Figure 7

. For

reading (

Figure 7a

), Address, CS and RD may be activated
simultaneously and the data will be available at the port
after t

AD

-max (650 ns). For writing (

Figure 7b

), the address

and data may be applied simultaneously; 70 ns later CS

and

WR

may be strobed together.

10

Functional Description (Continued)

TL/F/11219– 8

FIGURE 5. MM58274C Microprocessor Interface Diagram

TL/F/11219– 9

FIGURE 6. Valid MM58274C Control Signals Using Chip Select Generated Access Strobes

11

Functional Description (Continued)

TL/F/11219– 10

a. Port Generated Read AccessÐ2 Addresses Read Out

TL/F/11219– 11

b. Port Generated Write AccessÐ2 Addresses Written To

FIGURE 7. Simple Port Generated Control Signals

12

Functional Description (Continued)

APPLICATION HINTS

Time Reading Using Interrupt

In systems such as point of sale terminals and data loggers,
time reading is usually only required on a random demand
basis. Using the data-changed flag as outlined in the section
on methods of operation is ideal for this type of system.
Some systems, however, need to sense a change in real
time; e.g., industrial timers/process controllers, TV/VCR
clocks, any system where real time is displayed.

The interrupt timer on the MM58274C can generate inter­rupts synchronously with the time registers changing, using
software to provide the initial synchronization.

In single interrupt mode the processor is responsible for ini­tiating each timing cycle and the timed period is accurate to

g

1 ms.

In repeated interrupt mode the period from the initial proces­sor start to the first timeout is also only accurate to

g

1 ms.
The following interrupts maintain accurate delay periods rel­ative to the first timeout. Thus, to utilize interrupt to control
time reading, we will use repeated interrupt mode.

In repeated mode the time period between interrupts is ex­act, which means that timeouts will always occur at the
same point relative to the internal clock setting pulses. The
case for 0.1s interrupts is shown in

Figure A-1

. The same is
true for other delay periods, only there will be more clock
setting pulses between each interrupt timeout. If we set up
the interrupt timer so that interrupt always times out just
after the clock setting pulse occurs (

Figure A-2

), then there
is no need to test the data-changed flag as we know that
the time data has just changed and will not alter again for
another 100 ms.

This can be achieved as outlined below:

1) Follow steps 1 and 2 of the section on interrupt program­ming. In step 2 set up for repeated interrupt.

2) Read control register AD0:

This is a dummy read to reset

the data-changed flag.

3) Read control register AD0 until data-changed flag is set.

4) Write 0 or 2 to control register. Interrupt timing com­mences.

Time Reading with Very Slow Read Cycles

If a system takes longer than 100 ms to complete reading of
all the necessary time registers (e.g., when CMOS proces­sors are used) or where high level interpreted language rou­tines are used, then the data-changed flag will always be set
when tested and is of no value. In this case, the time regis­ters themselves must be tested to ensure data accuracy.

The technique below will detect both time changing

be-

tween

read strobes (i.e., between reading tens of minutes

and units of hours) and also time changing

during

read,

which can produce invalid data.

1) Read and store the value of the

lowest

order time register

required.

2) Read out all the time registers required. The registers
may be read out in any order, simplifying software require­ments.

3) Read the lowest order register and compare it with the
value stored previously in step 1. If it is still the same, then
all time data is good. If it has changed, then store the new
value and go back to step 2.

In general, the rule is that the first and last reads

must

both
be of the lowest order time register. These two values can
then be compared to ensure that no change has occurred.
This technique works because for any higher order time reg­ister to change, all the lower order registers must also
change. If the lowest order register does not change, then
no higher order register has changed either.

TL/F/11219– 12

FIGURE A-1. Time Delay from Clock Setting Pulses to Interrupt is Constant

TL/F/11219– 13

FIGURE A-2. Interrupt Timer Synchronized with Clock Setting Pulses

13

14

Physical Dimensions inches (millimeters)

Cavity Dual-In-Line Package (J)

Order Number MM58274CJ

NS Package Number J16A

Molded Dual-In-Line Package (N)

Order Number MM58274CN

NS Package Number N16A

15

MM58274C Microprocessor Compatible Real Time Clock

Physical Dimensions inches (millimeters) (Continued)

Plastic Chip Carrier (V)

Order Number MM58274CV

NS Package Number V20A

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