Compaq 850 manual System Clock Distribution, REAL-TIME Clock and Configuration Memory

Chapter 4 System Support

4.5SYSTEM CLOCK DISTRIBUTION

These systems use an Intel CK-type clock generator and crystal for generating the clock signals required by the system board components. Table 4-10 lists the system board clock signals and how they are distributed.

Table 4-10.Clock Generation and Distribution

Table 4-10

Clock Generation and Distribution

Frequncy	Source	Destination
400 MHz	MCH	Processor, RIMM sockets
100 MHz	CK	MCH
66 MHz	CK	MCH, ICH2, AGP slot
48 MHz	CK	ICH2, I/O Cntlr.
33 MHz	CK	Processor, ICH2, PCI Slots
14.31818 MHz	Crystal	CK

Certain clock outputs are turned off during reduced power modes to conserve energy. Clock output control is handled through the SMBus interface by BIOS.

4.6REAL-TIME CLOCK AND CONFIGURATION MEMORY

The Real-time clock (RTC) and configuration memory (also referred to as “CMOS”) functions are provided by the 82801 ICH component and is MC146818-compatible. As shown in the following figure, the 82801 ICH 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.

		82801
0Dh	Register D
0Ch	Register C
0Bh	Register B	Extended Config.
0Ah	Register A	Memory Area
09h	Year	(128 bytes)
08h	Month
07h	Date of Month
06h	Day of Week
05h	Hours (Alarm)	Standard Config.
04h	Hours (Timer)	Memory Area
03h	Minutes (Alarm)	(114 bytes)
02h	Minutes (Timer)
		RTC Area
01h	Seconds (Alarm)
00h	Seconds (Timer)	(14 bytes)

FFh

80h

7Fh

0Eh

0Dh

00h

CMOS

Figure 4-10.Configuration Memory Map

A lithium 3-VDC battery is used for maintaining the RTC and configuration memory while the system is powered down. The battery is located in a battery holder on the system board is replaced with a Renata CR2032 or equivalent 3-VDC lithium battery.

4-20Compaq Deskpro EXS and Workstation 300 Personal Computers Featuring the Intel Pentium 4 Processor

First Edition - December 2000