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 PCH 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

 

82801

FFh

 

 

0Ch

Register C

 

 

 

 

 

0Bh

Register B

 

Extended Config.

 

0Ah

Register A

 

Memory Area

 

09h

Year

 

(128 bytes)

 

08h

Month

 

 

80h

07h

Date of Month

 

 

 

 

7Fh

06h

Day of Week

 

 

 

Standard Config.

 

05h

Hours (Alarm)

 

 

04h

Hours (Timer)

 

Memory Area

 

03h

Minutes (Alarm)

 

(114 bytes)

0Eh

 

 

02h

Minutes (Timer)

 

 

 

RTC Area

0Dh

01h

Seconds (Alarm)

 

 

(14 bytes)

00h

00h

Seconds (Timer)

 

 

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 (XBT1) 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 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.

✎The above procedure does not clear the Power On Password. To clear the Power-On Password refer to section 4.5.1.

4-8

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Technical Reference Guide