SH3100 Datasheet, PDF (60/80 Pages) Semtech Corporation – Supervisory IC with I2C Interface and PWM

SH3100
POWER MANAGEMENT
Functional Descriptions (continued)
Real Time Clock (RTC)
The RTC is a 47-bit Binary+BCD counter clocked by a 256 Hz clock derived from the internal 32.768 kHz clock. The RTC
is always enabled and continues to run during battery backup and WDT resets. The RTC runs for 99 years before over-
flowing, and incorporates leap-year adjustment up to the year 2100. For synchronization purposes, the RTC should
be read LSB first – this causes all seven bytes to be latched into the serial interface simultaneously in order to avoid
problems of byte-overflow between individual byte reads. The RTC should also be written MSB first. Only when the LSB
is written are the entire six bytes loaded into the RTC counter. The RTC incorporates an alarm register which causes
an interrupt at a specified time with 1 minute resolution. When the alarm is not being used, the register may be used
as a scratchpad, as it remains throughout brownout and WDT events.
Note: The RTC read value may not be updated until one 256Hz cycle after an RTC write. Also, the shadow register used to
synchronize both reads and writes between the two clock domains is shared with the PIT and the DCOCode, so the implication
of this is that after writing to any one of these registers, the microcontroller software must wait for a period of the relevant
counter before writing any one of the others. (e.g., having written to the RTC, the software can not write to the PIT or DCO
Code within one 256Hz cycle (4ms).
Note: RTC accuracy is dependent on the accuracy of the crystal oscillator or internal 32.268kHz clock, whichever is being
used as the internal system clock.
Register
RTC0
RTC1
RTC2
RTC3
RTC4
RTC5
RTC6
Format
Binary
BCD
BCD
BCD
BCD
BCD
BCD
No. of Bits
8
7
7
6
6
5
8
Description
256ths seconds
Seconds
Minutes
Hours
Day of the month
Month
Year
Range
0x00 – 0xFF
0x00 – 0x59
0x00 – 0x59
0x00 – 0x23
0x00 – 0x31
0x01 – 0x12
0x00-0x99
© 2006 Semtech Corp.
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