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PCI/LPC Bridge Description
15-10 Intel® 460GX Chipset Software Developer’s Manual
15.4 Timer/Counters
The IFB contains three counters that are equivalent to those found in the 82C54 programmable
interval timer. The three counters are contained in one IFB timer unit, referred to as Timer-1. Each
counter output provides a key system Function. Counter 0 is connected to interrupt controller IRQ0
and provides a system timer interrupt for a time-of-day, diskette time-out, or other system timing
Functions. Counter 1 generates a refresh request signal and Counter 2 generates the tone for the
speaker. The 14.31818 MHz counters normally use OSC as a clock source.
Counter 0, System Timer
This counter Functions as the system timer by controlling the state of IRQ0 and is typically
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.
Counter 1, Refresh Request Signal
This counter provides the refresh request signal and is typically programmed for Mode 2 operation.
The counter negates refresh request for one counter period (838 ns) during each count cycle. The
initial count value is loaded one counter period after being written to the counter I/O address. The
counter initially asserts refresh request, and negates it for 1 counter period when the count value
reaches 1. The counter then asserts refresh request and continues counting from the initial count
value.
Counter 2, Speaker Tone
This counter provides the speaker tone and is typically programmed for Mode 3 operation. The
counter provides a speaker frequency equal to the counter clock frequency (1.193 MHz) divided by
the initial count value. The speaker must be enabled by a write to port 061h (see NMI Status and
Control ports).
15.4.1 Programming the Interval Timer
The counter/timers are programmed by I/O accesses and are addressed as though they are
contained in one 82C54 interval timer. A single Control Word Register controls the operation of all
three counters.
The interval timer is an I/O-mapped device. Several commands are available:
The Control Word Command specifies:
• Which counter to read or write.
• The operating mode.
• The count format (binary or BCD).