Functional Description

The MM58174 is a microprocessor bus-oriented real-time

clock The circuit includes addressable real-time counters

for tenths of seconds through months and a write only regis-

ter for leap year calculation The counters are arranged as

bytes of four bits each When addressed a byte will appear

on the data IO bus so that each word can be accessed

independently If any byte does not contain four bits (eg

days of the week uses only 3 bits) the unused bits will be

a read operation

The addressable reset latch causes the pre-scaler tenths of

seconds seconds and tens of seconds to be held in a reset

condition If a register is updated during a read operation the

IO data is prevented from updating and a subsequent read

will return the illegal bcd code ’1111’ The interrupt timer

may be programmed for intervals of 05 second 5 seconds

or 60 seconds and may be coded as a single or repeated

when the timer times out and reading the interrupt register

provides the internal selected information

Circuit Description

The block diagram shown in

Figure 1 shows the structure of

the CMOS clock chip A 16-pin DIL package is used

CRYSTAL OSCILLATOR

This consists of a CMOS inverteramplifier with on-chip bias

resistor and capacitors A single 6 pF – 36 pF trimmer is all

that is required to fine tune the crystal (see

Figure 2 ) How-

ever for improved stability some crystals may require a ca-

pacitor of typical value 20 pF to be added between pin 14

and ground The output of the oscillator is blocked by the

startstop FF

NON-INTEGER DIVIDER

This counter divides the incoming 32768 Hz frequency by

1516 down to 30720 Hz

FIXED DIVIDER (512)

This is a standard 9-stage binary ripple counter Output fre-

quency is 60 Hz This counter is reset to zero by startstop

FF

FIXED DIVIDER (6)

This is a 3-stage Johnson counter with a 10 Hz output sig-

nal This counter is reset to zero state by the startstop FF

SYNCHRONIZATION STAGE

Both 10 Hz and 32768 Hz clocks are fed into this section It

is used to generate a pulse of 1525 ms width on the rising

edge of each 10 Hz pulse

This pulse is used to increment all the seconds minutes

hours days months and year counter and also to set the

data changed FF

DATA CHANGED FF

This is set by the rising edge of each 10 Hz pulse to indicate

that the clock value has changed since the last read opera-

tion It is reset by any clock read command

The flip flop sets all data bus bits to a ‘‘1’’ during RD time

indicating that a register has been updated This transient

condition may occur at the end of the Read Data strobe

Hence invalid data may still be read from the clock if the

strobe width was less than 3 ms

Connection Diagram

Dual-In-Line Package

TLF6681 – 2

Top View

Order Number MM58174AN

See NS Package Number N16A

The possibility may be overcome by implementing a further

read of the tenths of seconds register at the end of every

series of reads (starting with a read at the tenths of seconds

register) and checking for unchanged data

SECONDS COUNTERS

There are three counters for Seconds

a) tenths of seconds

b) units of seconds

c) tens of seconds

The outputs of all three counters can be separately multi-

plexed on to the command 4-bit output bus Table I shows

the address decoding for each counter All three counters

are reset to zero by the startstop FF

MINUTES COUNTERS

There are two Minutes counters

a) units of minutes

b) tens of minutes

Both counters are parallel loaded with data from the 4-bit

input bus when addressed by the microprocessor and a

Write Data Strobe pulse given Similarly the output of both

counters can be read separately onto the common 4-bit out-

put bus (Table I)

HOURS COUNTERS

There are two Hours counters which will count in a 24-hour

mode

a) units of hours

b) tens of hours

Both counters have identical parallel load and read multi-

plex features to the Minutes counters

SEVEN DAY COUNTER

There is a 7-state counter which increments every 24 hours

It will have identical parallel load and read multiplex capabili-

ties to the Minutes and Hours counters The counter counts

cyclically from 1 – 7

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