HP Vectra Technical Reference Manual page 382

C.2 Real-Time Clock (CMOS Address
OOH-ODH)
The real-time clock (RTC) chip maintains the current date and time, even when the system is
powered off. Four registers are initialized by the SETUP program when the user sets the current
date and time. These are detailed in tables C.2, C.3, C.4 and C.5.
Table C.2
CMOS_RTC_REGA (CMOS Address OAH)
Bit
7
6-4
3-0
Data
o
1
Definition
The current date and time is available to read
The current date and time are not available to read because an
update of these values is in progress
Time divider selection bits to indicate what time-base frequency is
being used. This field is set to 2H to indicate that a 32,768 hertz
crystal is providing the time-base.
Rate selection bits to specify output square wave frequency. This field
is set to 06H to select a square wave frequency of 1 .024K Hertz or a
periodic interrupt rate of 976.562 microseconds.
Table C.3
CMOS_RTC_REGB (CMOS Address OBH)
Bit
Data
7 0
1
6 0
1
5 0
,.
1
4 0
1
3 0
1
Definition
Update clock normally (default)
Suspend clock updates
Disable periodic interrupts (default)
Enable periodic interrupts
Disable alarm interrupts (default)
Enable alarm interrupts
Do not generate an interrupt when the current update cycle
completes (default)
Generate an interrupt each time a clock update completes
Disable square wave output (default)
Enable square wave output
CMOS Memory Layout and Real-Time Clock 373