User`s manual

62 CMX34GS cpuModule BDM-610000082 Rev A
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 Port 2.0 connector (CN5) 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. Except for a few well-known CMOS
locations which are automatically populated for legacy compatibility, the CMOS memory is largely unused and
should be considered “Reserved”.
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 41 below. This is not an exhaustive list. Refer to the AMD A55E
datasheet for more detail.
.
Note Alternative battery inputs reside on the Optional RTC Battery Input (CN13) as well as the PCIe/104
Type 2 Bus connectors (CN1 & CN2).
WARNING Only one RTC battery input connection should be used at a time. The other should remain
unconnected.
Table 41 Real Time Clock Registers
Registers
(hex)
Registers
(decimal)
Function
00h 0 RTC Seconds (BCD format)
02h 2 RTC Minutes (BCD format)
04h 4 RTC Hours (BCD format)
06h 6 RTC Day of Week (BCD format)
07h 7 RTC Day of Month (BCD format)
08h 8 RTC Month (BCD format)
09h 9 RTC Year (BCD format)