HP dc7900 manual Real-Time Clock and Configuration Memory, Clearing Cmos

System Support

4.4 Real-Time Clock and Configuration Memory

The Real-time clock (RTC) and configuration memory (also referred to as “CMOS”) functions are provided by the 82801 component and is MC146818-compatible. As shown in the following figure, the 82801 ICH10 component provides 256 bytes of battery-backed RAM divided into two 128-byte configuration memory areas. The RTC uses the first 14 bytes (00-0Dh) of the standard memory area. All locations of the standard memory area (00-7Fh) can be directly accessed using conventional OUT and IN assembly language instructions through I/O ports 70h/71h, although the suggested method is to use the INT15 AX=E823h BIOS call.

0Dh	Register D
0Ch	Register C
0Bh	Register B
0Ah	Register A
09h	Year
08h	Month
07h	Date of Month
06h	Day of Week
05h	Hours (Alarm)
04h	Hours (Timer)
03h	Minutes (Alarm)
02h	Minutes (Timer)
01h	Seconds (Alarm)
00h	Seconds (Timer)

82801

Extended Config.

Memory Area

(128 bytes)

Standard Config.

Memory Area

(114 bytes)

RTC Area (14 bytes)

FFh

80h

7Fh

0Eh

0Dh

00h

CMOS

Figure 4 4. Configuration Memory Map

A lithium 3-VDC battery is used for maintaining the RTC and configuration memory while the system is powered down. During system operation a wire-Ored circuit allows the RTC and configuration memory to draw power from the power supply. The battery is located in a battery holder on the system board and has a life expectancy of three or more years. When the battery has expired it is replaced with a CR2032 or equivalent 3-VDC lithium battery.

4.4.1 Clearing CMOS

The contents of configuration memory (including the Power-On Password) can be cleared by the following procedure:

1.Turn off the unit.

2.Disconnect the AC power cord from the outlet and/or system unit.

3.Remove the chassis hood (cover) and insure that no LEDs on the system board are illuminated.

4.On the system board, press and hold the CMOS clear button (switch SW50, colored yellow) for at least 5 seconds.

5.Replace the chassis hood (cover).

6.Reconnect the AC power cord to the outlet and/or system unit.

7.Turn the unit on.

To clear only the Power-On Password refer to section 4.5.1.

Technical Reference Guide www.hp.com 4-9