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VT82885 Datasheet(PDF) 6 Page - List of Unclassifed Manufacturers |
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VT82885 Datasheet(HTML) 6 Page - List of Unclassifed Manufacturers |
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6 / 16 page  6 VT82885 Real Time Clock VIA Technologies, Inc. should be written to a logic one to prevent updates from occurring while access is be- ing attempted. In addition to writing the ten time, calendar and alarm registers in a se- lected format (binary or BCD), the data mode bit (DM) of Register B must be set to the appropriate logic level. All ten time, cal- endar and alarm bytes must use the same data mode. The set bit in Register B should be cleared after the data mode bit has been written to allow the real time clock to update the time and calendar bytes. Once initial- ized, the real time clock makes all updates in the selected mode. The data mode can-not be changed without reinitializing the ten data bytes. Table 2 shows the binary and BCD formats of the ten time, calendar and alarm locations. The 24-12 bit cannot be changed without reinitializing the hour loca-tions. When the 12-hour format is selected, the high order bit of the hours byte repre-sents PM when it is a logic one. Once per second the ten bytes are advanced by one second and checked for an alarm condition. If a read of the time and calendar data oc-curs during an update, a problem exists where seconds, minutes, hours, etc. may not correlate. The probability of reading in-correct time and calendar data is low. Sev-eral methods of avoiding any possible incor-rect time and calendar reads are covered later in this text. The three alarm bytes can be used in two ways. First, when the alarm time is written in the appropriate hours, minutes and seconds alarm locations, the alarm interrupt is initi- ated at the specified time each day if the alarm enable bit is high. The second use condition is to insert a “don’t care” state in one or more of the three alarm bytes. The “don’t care” code is any hexadecimal value from C0 to FF. The two most significant bits of each byte set the “don’t care” condition when at logic 1. An alarm will be generated each hour when the “don’t care” bits are set in the hours byte. Similarily, an alarm is generated every minute with “don’t care” codes in the hours and minute alarm bytes. The “don’t care” codes in all three alarm bytes create an interrupt every second. TABLE 2: TIME. CALENDAR AND ALARM DATA MODES ADDRESS DECIMAL RANGE LOCATION FUNCTION RANGE BINARY DATA MODE BCD DATA MODE 0 Seconds 0-59 00-3B 00-59 1 Seconds Alarm 0-59 00-3B 00-59 2 Minutes 0-59 00-3B 00-59 3 Minutes Alarm 0-59 00-3B 00-59 4 Hours-12-hr Mode 1.12 01-0C AM, 81-8C PM 01-12AM,81-92PM Hours-24-hr Mode 0-23 00-17 00-23 5 Hours Alarm-12-hr 1-12 01-0C AM, 81-8C PM 01-12AM,81-92PM Hours Alarm-24-hr 0-23 00-17 00-23 6 Day of the Week Sunday = 1 1-7 01-07 01-07 7 Date of the Month 1-31 01-1F 01-31 8 Month 1-12 01-0C 01-12 9 Year 0-99 00-63 00-99 NONVOLATILE RAM |
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