Functional Description
202 Datasheet
Periodic Mode
Timer 0 is the only timer that supports periodic mode. Refer to 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.
The Timer 0 Comparator Value register cannot be programmed reliably by a single
64-bit write in a 32-bit environment except if only the periodic rate is being changed
during run-time. If the actual Timer 0 Comparator Value needs to be reinitialized, then
the following software solution will always work regardless of the environment:
1. Set TIMER0_VAL_SET_CNF bit.
2. Set the lower 32 bits of the Timer0 Comparator Value register.
3. Set TIMER0_VAL_SET_CNF bit.
4. Set the upper 32 bits of the Timer0 Comparator Value register.
5.17.4 Enabling the Timers
The BIOS or operating system PnP code should route the interrupts. This includes the
Legacy Rout bit, Interrupt Rout bit (for each timer), interrupt type (to select the edge
or level type for each timer)
The Device Driver code should do the following for an available timer:
1. Set the Overall Enable bit (Offset 10h, bit 0).
2. Set the timer type field (selects one-shot or periodic).
3. Set the interrupt enable.
4. Set the comparator value.
5.17.5 Interrupt Levels
Interrupts directed to the internal 8259s are active high. See Section 5.9 for
information regarding the polarity programming of the I/O APIC for detecting internal
interrupts.
If the interrupts are mapped to the 8259 or I/O APIC and set for level-triggered mode,
they can be shared with PCI interrupts. They may be shared although it is unlikely for
the operating system to attempt to do this.
If more than one timer is configured to share the same IRQ (using the
TIMERn_INT_ROUT_CNF fields), then the software must configure the timers to level-
triggered mode. Edge-triggered interrupts cannot be shared.
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