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DP8572AN Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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DP8572AN Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 22 page  Functional Description (Continued) INITIAL POWER-ON of BOTH VBB and VCC VBB and VCC may be applied in any sequence In order for the power fail circuitry to function correctly whenever power is off the VCC pin must see a path to ground through a maximum of 1 MX The user should be aware that the con- trol registers will contain random data The first task to be carried out in an initialization routine is to start the oscillator by writing to the crystal select bits in the Real Time Mode Register If the DP8572A is configured for single supply mode an extra 50 mA may be consumed until the crystal select bits are programmed The user should also ensure that the RTC is not in test mode (see register descriptions) REAL TIME CLOCK FUNCTIONAL DESCRIPTION As shown in Figure 2 the clock has 10 bytes of counters which count from 1100 of a second to years Each counter counts in BCD and is synchronously clocked The count se- quence of the individual byte counters within the clock is shown later in Table VII Note that the day of week day of month day of year and month counters all roll over to 1 The hours counter in 12 hour mode rolls over to 1 and the AMPM bit toggles when the hours rolls over to 12 (AM e 0 PM e 1) The AMPM bit is bit D7 in the hours counter All other counters roll over to 0 Also note that the day of year counter is 12 bits long and occupies two addresses Upon initial application of power the counters will contain random information 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 RTC 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 ) The crystal frequency that can be selected are 32 kHz 32768 kHz 49152 MHz and 4194304 MHz TLF9980 – 8 FIGURE 3 Programmable Clock Prescaler Block 9 |
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