Datasheet

ACPI Devices
Intel
®
Atom™ Processor E6xx Series Datasheet
201
11.0 ACPI Devices
11.1 8254 Timer
The 8254 contains three counters which have fixed uses. All registers are in the core
well and clocked by a 14.31818 MHz clock.
11.1.1 Counter 0, System Timer
This counter functions as the system timer by controlling the state of IRQ0 and is
programmed for Mode 3 operation. The counter produces a square wave with a period
equal to the product of the counter period (838 ns) and the initial count value. The
counter loads the initial count value one counter period after software writes the count
value to the counter I/O address. The counter initially asserts IRQ0 and decrements the
count value by two each counter period. The counter negates IRQ0 when the count
value reaches 0. It then reloads the initial count value and again decrements the initial
count value by two each counter period. The counter then asserts IRQ0 when the count
value reaches 0, reloads the initial count value, and repeats the cycle, alternately
asserting and negating IRQ0.
11.1.2 Counter 1, Refresh Request Signal
This counter is programmed for Mode 2 operation and impacts the period of the
NSC.RTS. Programming the counter to anything other than Mode 2 results in undefined
behavior.
11.1.3 Counter 2, Speaker Tone
This counter provides the speaker tone and is typically programmed for Mode 3
operation.
11.1.4 Timer I/O Registers
11.1.5 Offset 43h: TCW - Timer Control Word Register
This register is programmed prior to any counter being accessed to specify counter
modes. Following reset, the control words for each register are undefined and each
counter output is 0. Each timer must be programmed to bring it into a known state.
Table 297. Timer I/O Registers
Port Register Name/Function Default Value Type
40h/50h
Counter 0 Interval Time Status Byte Format 0XXXXXXXb Read Only
Counter 0 Counter Access Port Register Undefined Read/Write
41h/51h
Counter 1 Interval Time Status Byte Format 0XXXXXXXb Read Only
Counter 1 Counter Access Port Register Undefined Read/Write
42h/52h
Counter 2 Interval Time Status Byte Format 0XXXXXXXb Read Only
Counter 2 Counter Access Port Register Undefined Read/Write
43h/53h
Timer Control Word Register Undefined Write Only
Timer Control Word Register Read Back XXXXXXX0b Write Only
Counter Latch Command X0h Write Only