Real Time Clock Control

Overview

The cpuModule is equipped with a Real Time Clock (RTC) which provides system date and time functions. When the cpuModule is turned off, a battery must be attached to the utility connector to provide power to the RTC. Without power, the RTC will lose the date/time information when the system is turned off.

The RTC also provides an “alarm” function. This may be used to generate an interrupt at a particular time and day. This feature is commonly used to wake up the system from Sleep/Standby to run a scheduled task (defragment the hard drive, back up files, etc.).

In addition to the date/time/alarm functions, the RTC contains several bytes of battery-backed RAM, commonly called CMOS memory. In a typical desktop PC, the CMOS memory is used by the BIOS to store user settings. This RTD cpuModule uses onboard flash to store user BIOS settings. To preserve compatibility with traditional PCs, the RTD Enhanced BIOS also mirrors the user settings from flash in CMOS. Therefore, the contents of CMOS may be overwritten at boot time, and should be treated as “read only”.

Accessing the RTC Registers

You may access the RTC date/time and CMOS memory using the Index and Data Registers located at I/O addresses 70h and 71h.

•Address 70h is the Index register. It must be written with the number of the register to read or write. Valid values are 00h to 7Fh.

•Address 71h is the Data register. It contains the contents of the register pointed to by the Index.

To read/write an RTC register, you must first set the Index register with the register number, and then read/write the Data register.

A list of key RTC registers is shown in Table 52 below:

.

 

 

Table 52 Real Time Clock Registers

 

 

 

Registers

Registers

Function

(hex)

(decimal)

 

 

 

 

00h

0

RTC Seconds

 

 

 

02h

2

RTC Minutes

 

 

 

04h

4

RTC Hours

 

 

 

06h

6

RTC Day of Week

 

 

 

07h

7

RTC Day of Month

 

 

 

08h

8

RTC Month

 

 

 

09h

9

RTC Year

 

 

 

0Ah

10

RTC Status Register A

• Bit 7: RTC Update In Progress (Read Only) - RTC registers should not be accessed when this bit is high.

• Bits 6-4:Divider for 32.768 KHz input (should always be 010)

• Bits 3-0:Rate select for periodic interrupt.

BDM-610000049 Rev G

Chapter 4: Using the cpuModule 69

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IBM BDM-610000049 user manual Real Time Clock Control, Overview, Accessing the RTC Registers, Real Time Clock Registers
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