Cmos Reg A

CMOS Register A

CMOS register A has these bits.

Bit(s) Meaning
7 RO UIP or Update-in-progress bit. This bit is set by the CMOS processor when it is about to update its stored time and date information. The bit is cleared once the update has finished. During the period that the bit is set reads of the CMOS time and date information may return invalid results and attempts to set any of that information may fail. However, CMOS locations that do not store date and time information can be accessed even if this bit is set.
6::4 RW Input clock rate. These bits should be set to match the frequency of the clock that is supplied to the CMOS chip. In a PC the frequency is 32,768 KHz so the only correct value for these bits is 010.
3::0 RW Periodic interrupt rate. If you want to use the periodic interrupt function of the CMOS chip these bits are used to determine the rate at which that interrupt will occur. Possible values are in the table below.
The periodic interrupt rate can be one of these values.
Bit(s) Interrupts per Second Approximate Period
0000 n/a None
0001 256 3.9 ms
0010 128 7.8 ms
0011 8192 0.122 ms
0100 4096 0.244 ms
0101 2048 0.488 ms
0110 1024 0.976 ms
0111 512 1.95 ms
1000 256 3.9 ms
1001 128 7.8 ms
1010 64 15.6 ms
1011 32 31.2 ms
1100 16 62.5 ms
1101 8 125 ms
1110 4 250 ms
1111 2 500 ms
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