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MM58167B Datasheet(PDF) 5 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. MM58167B
Description  Microprocessor Real Time Clock
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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MM58167B Datasheet(HTML) 5 Page - National Semiconductor (TI)

 
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Functional Description (Continued)
logical zero) When switching VDD to the standby or power
down mode the POWER DOWN input should go to a logical
zero at least 1 ms before VDD is switched When switching
VDD all other inputs must remain between VSS b 03V and
VDD a 03V When restoring VDD to the normal operating
mode it is necessary to insure that all other inputs are at
valid levels before switching the POWER DOWN input back
to a logical one These precautions are necessary to insure
that no data is lost or altered when changing to or from the
power down mode
Counter and RAM Resets GO Command
The counters and RAM can be reset by writing all 1’s (FF) at
address 12H or 13H respectively
A write pulse at address 15H will reset the thousandths
hundredths tenths units and tens of seconds counters
This GO command is used for precise starting of the clock
The data on the data bus is ignored during the write If the
seconds counter is at a value greater than 39 when the GO
is issued the minute counter will increment otherwise the
minute counter is unaffected This command is not neces-
sary to start the clock but merely a convenient way to start
precisely at a given minute
Status Bit
The status bit is provided to inform the user that the clock is
in the process of rolling over when a counter is read The
status bit is set if this 1 kHz clock occurs during or after any
counter read This tells the user that the clock is rippling
through the real time counter Because the clock is rippling
invalid data may be read from the counter If the status bit is
set following a counter read the counter should be reread
The status bit appears on D0 when address 14H is read All
the other data lines will zero The bit is set when a logical
one appears This bit should be read every time a counter
read or after a series of counter reads are done The trailing
edge of the read at address 14H will reset the status bit
Using the Rollover Status Bit
If a single read of any clock counter is made it should be
followed by reading the rollover status bit
Example Read months then read rollover status
If a sequential read of the clock counters is made then the
rollover status bit should be read after the last counter is
read
Example Read hours minutes seconds then read the roll-
over status
Oscillator
The oscillator used is the standard Pierce parallel resonant
oscillator Externally 2 capacitors a 20 MX resistor and the
crystal are required The 20 MX resistor is connected be-
tween OSC IN and OSC OUT to bias the internal inverter in
the linear region For micropower crystals a resistor in series
with the oscillator output may be necessary to insure the
crystal is not overdriven This resistor should be approxi-
mately 200 kX The capacitor values should be typically
20 pF – 25 pF The crystal frequency is 32768 Hz
The oscillator input can be externally driven if desired In
this case the oscillator output should be left floating and the
oscillator input levels should be within 03V of the supplies
A ground line or ground plane between pins 9 and 10 may
be necessary to reduce interference of the oscillator by the
A4 address
Control Lines
The READ WRITE AND CHIP SELECT signals are active
low inputs The READY signal is an open drain output At
the start of each read or write cycle the READY line (open
drain) will pull low and will remain low until valid data from a
chip read appears on the bus or data on the bus is latched
in during a write READ and WRITE must be accompanied
by a CHIP SELECT (see
Figures 3 and 4 for read and write
cycle timing)
During a read or write address bits must not change while
chip select and control strobes are low
Test Mode
The test mode is for production testing It allows the coun-
ters to count at a higher than normal rate In this mode the
32768 kHz oscillator input is connected directly to the ten
thousandths of seconds counter The chip select and write
lines must be low and the address must be held at 1FH
TLF11070 – 3
FIGURE 1 Interrupt Register Format
5


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