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DP8573A Datasheet(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. DP8573A
Description  Real Time Clock (RTC)
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com

DP8573A Datasheet(HTML) 6 Page - National Semiconductor (TI)

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Functional Description (Continued)
Since clocking of the counter occurs asynchronously to
reading of the counter it is possible to read the counter
while it is being incremented (rollover) This may result in an
incorrect time reading Thus to ensure a correct reading of
the entire contents of the clock (or that part of interest) it
must be read without a clock rollover occurring In general
this can be done by checking a rollover bit On this chip the
periodic interrupt status bits can serve this function The
following program steps can be used to accomplish this
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 high-
est frequency counter register to be read That is if only
SECONDS through HOURS counters are read then the
SECONDS periodic bit should be used
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
Another method to read the clock that does not require
checking the rollover bit is to write a one into the Time Save
Enable bit (D7) of the Time Save Control Register and then
to write a zero Writing a one into this bit will enable the
clock contents to be duplicated in the Time Save RAM
Changing the bit from a one to a zero will freeze and store
the contents of the clock in Time Save RAM The time then
can be read without concern for clock rollover since inter-
nal logic takes care of synchronization of the clock Be-
cause only the bits used by the clock counters will be
latched the Time Save RAM should be cleared prior to use
to ensure that random data stored in the unused bits do not
confuse the host microprocessor This bit can also provide
time save at power failure see the Additional Supply Man-
agement Functions section With the Time Save Enable bit
at a logical 0 the Time Save RAM may be used as RAM if
the latched read function is not necessary
Upon initial application of power to the TCP 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 StartStop 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 StartStop bit reset and then writing to the register a
second time with the StartStop bit set
The above method is useful when the entire clock is being
corrected If one location is being updated the clock need
not be stopped since this will reset the prescaler and time
will be lost An ideal example of this is correcting the hours
for daylight savings time To write to the clock ‘‘on the fly’’
the best method is to wait for the 1100 of a second period-
ic interrupt Then wait an additional 16 ms and then write
the data to the clock
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 )
TLF9981 – 8
FIGURE 3 Programmable Clock Prescaler Block
In addition to the inverter the oscillator feedback bias resis-
tor is included on chip as shown in
Figure 4 The oscillator
input may be driven from an external source if desired Re-
fer 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 Oscilla-
tor 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-
specified by the crystal manufacturer (nominal value
11 pF for the 32768 crystal) is what determines proper os-
cillation This load capcitance is the series combination of
capacitance on each side of the crystal (with respect to
TLF9981 – 9
FIGURE 4 Oscillator Circuit Diagram

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