Compaq Deskpro 4000 Technical Reference Manual page 74

Chapter 4 System Support
RTC Control Register A, Byte 0Ah
Bit Function
7 Update in Progress. Read only.
0 = Time update will not occur before 2444 us
1 = Time update will occur within 2444 us
6..4 Divider Chain Control. R/W.
00x = Oscillator disabled.
010 = Normal operation (time base frequency = 32.768 KHz).
11x = Divider chain reset.
3..0 Periodic Interrupt Control. R/W. Specifies the periodic interrupt interval.
0000 = none
0001 = 3.90625 ms
0010 = 7.8125 ms
0011 = 122.070 us
0100 = 244.141 us
0101 = 488.281 us
0110 = 976.562 us
0111 = 1.953125 ms
RTC Control Register B, Byte 0Bh
Bit Function
7 Time Update Enable/disable
0 = Normal operation,
6 Periodic Interrupt Enable/Disable.
0 = Disable,
5 Alarm Interrupt Enable/disable
0 = Disabled,
4 End-of-Update Interrupt Enable/Disable
0 = Disabled,
3 Reserved (read 0)
2 Time/Date Format Select
0 = BCD format,
1 Time Mode
0 = 12-lhour mode,
0 Automatic Daylight Savings Time Enable/Disable
0 = Disable
1 = Enable (Advance 1 hour on 1
RTC Status Register C, Byte 0Ch
Bit Function
7 If set, interrupt output signal active (read only)
6 If set, indicates periodic interrupt flag
5 If set, indicates alarm interrupt
4 If set, indicates end-of-update interrupt
3..0 Reserved
RTC Status Register D, Byte 0Dh
Bit Function
7 RTC Power Status
0 = RTC has lost power
1 = RTC has not lost power
6..0 Reserved
4-28 Compaq Deskpro 4000 and 6000 Personal Computers
featuring the Pentium II Processor
1000 = 3.90625 ms
1001 = 7.8125 ms
1010 = 15. 625 ms
1011 = 31.25 ms
1100 = 62.50 ms
1101 = 125 ms
1110 = 250 ms
1111 = 500 ms
1 = Disable time updating for time set
1 = Enable interval specified by Register A
1 = Enabled
1 = Enabled
1 = Binary format
1 = 24-hour mode
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Sunday in April, retreat 1 hour on last Sunday in October).
First Edition - October 1997