Technical Reference Guide
Compaq Deskpro EN Series of Personal Computers
Desktop and Minitower Form Factors
Third Edition - September 1998
4-31
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 1000 = 3.90625 ms
0001 = 3.90625 ms 1001 = 7.8125 ms
0010 = 7.8125 ms 1010 = 15. 625 ms
0011 = 122.070 us 1011 = 31.25 ms
0100 = 244.141 us 1100 = 62.50 ms
0101 = 488.281 us 1101 = 125 ms
0110 = 976.562 us 1110 = 250 ms
0111 = 1.953125 ms 1111 = 500 ms
RTC Control Register B, Byte 0Bh
Bit Function
7 Time Update Enable/disable
0 = Normal operation, 1 = Disable time updating for time set
6 Periodic Interrupt Enable/Disable.
0 = Disable, 1 = Enable interval specified by Register A
5 Alarm Interrupt Enable/disable
0 = Disabled, 1 = Enabled
4 End-of-Update Interrupt Enable/Disable
0 = Disabled, 1 = Enabled
3 Reserved (read 0)
2 Time/Date Format Select
0 = BCD format, 1 = Binary format
1 Time Mode
0 = 12-hour mode, 1 = 24-hour mode
0 Automatic Daylight Savings Time Enable/Disable
0 = Disable
1 = Enable (Advance 1 hour on 1
st
Sunday in April, retreat 1 hour on last Sunday in October).
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
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