3.1.4
Real Time Clock
The RTC is a low-power clock that provides a time-of day clock and a 100-year calendar with alarm
features and battery backed operation. The time-of-day function includes 14 control registers. Other
features include maskable interrupt sources and 242 byte of general purpose CMOS RAM. Valid RAM
data and time can be maintained after power down through the use of an external battery source. The
RTC is software compatible to the Dallas DS1287 and the Motorola MC146818. The RTC function is
mapped to I/O locations $70 (index) and $71 (data).
The table below shows the address location of the clock registers in the battery backed RAM area.
Name
Adr. (Index)
Length
current second in BCD
$00
1
second alarm in BCD
$01
1
current minute in BCD
$02
1
minute alarm in BCD
$03
1
current hour in BCD
$04
1
hour alarm in BCD
$05
1
current day of week in 1-7 (1=sunday)
$06
1
current day of month in 1-31 BCD
$07
1
current month in 1-12 BCD (1=January)
$08
1
current year in BCD (00=1900)
$09
1
control register A
$0A
1
control register B
$0B
1
control register C
$0C
1
control register D
$0D
1
BCD value for current century (19 or 20)
$032
1
Apart from the RTC information the RAM also contains system information as listed below:
Name
Adr. (Index)
Length
AT errors
$0E
1
shutdown status byte
$0F
1
diskette drive types
$10
1
fixed disk types
$12
1
equipment byte
$14
1
amount of base memory
$15
2
expansion memory
$17
2
extended fixed disk type
$19
2
Checksum
$2E
2
The general purpose CMOS RAM is reserved and must not be changed. Any change of the reserved
RAM area may result in undesirable functionality of the system.
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