IBM CMX58886CX Real Time Clock Control, Real Time Clock Registers, Registers Number Function

Real Time Clock Control

The cpuModule is equipped with a real time clock (RTC) which provides system date and time functions, and also provides 128 nonvolatile memory locations. The contents of these memory locations are retained whenever an external backup battery is connected, whether or not system power is connected.

You may access the RTC date, time, and memory using an index and data register 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. Refer to the map below for valid choices for the index. Data is then written to or read from the selected register by writing or reading (respectively) the data register at address 71h.

WARNING Do NOT change values stored in the RTC registers listed as RESERVED in the table below. Doing so will interfere with proper cpuModule operation.

Registers of the RTC are shown below.

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Table 47 Real Time Clock Registers

Registers	Registers	Number of	Function
(hex)	(decimal)	Bytes
00h	0	1	BCD Seconds
02h	2	1	BCD Minutes
04h	4	1	BCD Hours
06h	6	1	Day of Week
07h	7	1	Day of Month
08h	8	1	Month
09h	9	1	Year
0A–0Dh	10–13	4	RTC Control Registers
0E–31h	14–49	36	RESERVED—Do not modify!
32h	50	1	BCD Century
33–3Fh	51–63	13	RESERVED—Do not modify!
40–7Fh	64–127	64	User RAM

BDM-610000050 Rev A Chapter 4: Using the cpuModule 63