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Publication No.
500-9300007876-000
Rev. C.0
Functional Description 71
The following table gives an overview of the 8254 timer functions.
The counter/timers are programmed by I/O accesses. A single control word
register controls the operation of all three counters. For more information on
programming and a detailed register description, see the 8254 data sheet.
6.10 Real Time Clock
The two key functions of the Real Time Clock (RTC) are:
1. Tracking time of day
2. Storing system data, even when the system is powered down
The PCH provides the RTC module that is equipped with a battery backed-up
date and time keeping device. The RTC is powered from the +5V_STDBY or VBAT
supply. When either of these supplies is present, the system clock is maintained. If
all power is removed, the RTC is reset. Valid RAM data and time can be
maintained after power down through the use of an external battery source.
The RTC module is comprised of a clock with 1 second resolution and 242 bytes of
general purpose CMOS RAM used by system UEFI Firmware. The three maskable
interrupts’ features are:
• Time of day alarm with once a second to once a month range
• Periodic rates of 122
s to 500 ms
• End of update cycle notification
The lower 14 bytes on the lower RAM block has very specific functions. The first
ten are for time and date information. The next four (0Ah to 0Dh) are registers,
which configure and report RTC functions. In addition to providing time of day,
the RTC provides a 100 year calendar with alarm features and battery backed
operation. The time of day function includes fourteen control registers.
NOTE
The leap year determination for adding a 29th day to February does not take into account the end-of-the-
century exceptions.
Table 6-1 Interval Timer Functions
Interval Timer Function
Description
Function
Counter 0 (System Timer)
Gate
Always on
Clock In
1.193 MHz (OSC/12)
Out
IRQ0 (INT1)
Function
Counter 1 (Refresh Request)
Gate
Always on
Clock In
1.193 MHz (OSC/12)
Out
Refresh Request
Function
Counter 2 (Speaker Tone)
Gate
Programmable via Port $061
Clock In
1.193 MHz (OSC/12)
Out
Speaker