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DS1339A Datasheet(PDF) 9 Page - Maxim Integrated Products |
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DS1339A Datasheet(HTML) 9 Page - Maxim Integrated Products |
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9 / 19 page  DS1339A Low-Current, I2C, Serial Real-Time Clock 9 Maxim Integrated Clock Accuracy The accuracy of the clock is dependent upon the accuracy of the crystal and the accuracy of the match between the capacitive load of the oscillator circuit and the capacitive load for which the crystal was trimmed. Additional error is added by crystal frequency drift caused by temperature shifts. External circuit noise coupled into the oscillator circuit may result in the clock running fast. Figure 6 shows a typical PC board layout for isolating the crystal and oscillator from noise. Refer to Application Note 58: Crystal Considerations with Dallas Real-Time Clocks for detailed information RTC Address Map Table 3 shows the address map for the DS1339A registers. During a multibyte access, when the address pointer reaches the end of the register space (10h), it wraps around to location 00h. On an I2C START or address pointer incrementing to location 00h, the current time is transferred to a second set of registers. The time information is read from these secondary registers, while the clock may continue to run. This eliminates the need to re-read the registers in case of an update of the main registers during a read. Time and Date Operation The time and date information is obtained by reading the appropriate register bytes. Table 3 shows the RTC registers. The time and date are set or initialized by writing the appropriate register bytes. The contents of the time and date registers are in the BCD format. The DS1339A can be run in either 12-hour or 24-hour mode. Bit 6 of the HOURS register is defined as the 12- or 24-hour mode-select bit. When high, the 12-hour mode is selected. In the 12-hour mode, bit 5 is the AM/PM bit with logic high being PM. In the 24-hour mode, bit 5 is the 20-hour bit (20 to 23 hours). All hours values, including the alarms, must be re-entered whenever the 12/24-hour mode bit is changed. The Century bit (bit 7 of the MONTH register) is toggled when the YEAR register overflows from 99 to 00. If the Century bit is logic 0, the year will be designated as a Leap Year and February will contain 29 days. If the Century bit is logic 1, the year will not be designated as a Leap Year and February will contain 28 days. The Day-Of-Week register increments at midnight. Values that correspond to the day of week are user-defined, but must be sequential (i.e., if 1 equals Sunday, then 2 equals Monday and so on). Illogical time and date entries result in undefined operation. When reading or writing the time and date registers, secondary (user) buffers are used to prevent errors when the internal registers update. When reading the time and date registers, the user buffers are synchronized to the internal registers on a START or when the address pointer rolls over to 00h. The countdown chain is reset whenever the seconds register is written. Write transfers occurs on the acknowledge pulse from the device. To avoid rollover issues, once the countdown chain is reset, the remaining time and date registers must be written within one second. If enabled, the 1Hz square-wave output transitions high 500ms after the seconds data transfer, provided the oscillator is already running. |
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