307013-003 Datasheet, PDF (198/848 Pages) Intel Corporation – Intel I/O Controller Hub 7

Functional Description
The main counter is clocked by the 14.31818 MHz clock, synchronized into the 66.666
MHz domain. This results in a non-uniform duty cycle on the synchronized clock, but
does have the correct average period. The accuracy of the main counter is as accurate
as the 14.3818 MHz clock.
5.18.2 Interrupt Mapping
Mapping Option #1 (Legacy Replacement Option)
In this case, the Legacy Replacement Rout bit (LEG_RT_CNF) is set. This forces the
mapping found in Table 5-45.
Table 5-45. Legacy Replacement Routing
Timer
8259 Mapping
APIC Mapping
Comment
0
IRQ0
IRQ2
In this case, the 8254 timer will
not cause any interrupts
1
IRQ8
IRQ8
In this case, the RTC will not cause
any interrupts.
2
Per IRQ Routing Field. Per IRQ Routing Field
Mapping Option #2 (Standard Option)
In this case, the Legacy Replacement Rout bit (LEG_RT_CNF) is 0. Each timer has its
own routing control. The supported interrupt values are IRQ 20, 21, 22, and 23.
5.18.3 Periodic vs. Non-Periodic Modes
Non-Periodic Mode
Timer 0 is configurable to 32 (default) or 64-bit mode, whereas Timers 1 and 2 only
support 32-bit mode (See Section 20.1.5).
All three timers support non-periodic mode.
Consult Section 2.3.9.2.1 of the IA-PC HPET Specification for a description of this
mode.
Periodic Mode
Timer 0 is the only timer that supports periodic mode. Consult Section 2.3.9.2.2 of the
IA-PC HPET Specification for a description of this mode.
The following usage model is expected:
1. Software clears the ENABLE_CNF bit to prevent any interrupts
2. Software Clears the main counter by writing a value of 00h to it.
3. Software sets the TIMER0_VAL_SET_CNF bit.
4. Software writes the new value in the TIMER0_COMPARATOR_VAL register
5. Software sets the ENABLE_CNF bit to enable interrupts.
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Intel ^® ICH7 Family Datasheet