READING THE CLOCK VALIDATED READ

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

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 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

INITIALIZING AND WRITING TO THE

CALENDAR-CLOCK

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

PRESCALEROSCILLATOR 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 )

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-

tance

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

ground)

TLF9981 – 9

FIGURE 4 Oscillator Circuit Diagram

6