Intel® Pentium® Silver and Intel® Celeron® Processors Datasheet, Volume 1
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The PCH supports a timer off-load feature that allows the HPET timers to remain operational during very low power S0 operational modes when the PCH’s XTAL clock is disabled. The clock source during this off-load is the Real Time Clock’s 32.768 kHz clock. This clock is calibrated against the PCH’s XTAL clock during boot time to an accuracy that ensures the error introduced by this off-load is less than 10 ppb (.000001%).
When the PCH’s XTAL clock is active, the 64-bit counter will increment by one each cycle of the PCH’s XTAL clock when enabled. When the PCH’s XTAL clock is disabled, the timer is maintained using the RTC clock. The long-term (> 1 msec) frequency drift allowed by the HPET specification is 500 ppm. The off-load mechanism ensures that it contributes < 1ppm to this, which will allow this specification to be easily met given the clock crystal accuracies required for other reasons.
Timer off-load is prevented when there are HPET comparators active.
The HPET timer in the PCH runs typically on the PCH’s XTAL crystal clock and is off-loaded to the 32 kHz clock once the processor enters C10. This is the state where there are no C10 wake events pending and when the off-load calibrator is not running. HPET timer re-uses this 28-bit calibration value calculated by PMC when counting on the 32-kHz clock. During C10 entry, PMC sends an indication to HPET to off-load and keeps the indication active as long as the processor is in C10 on the 32 kHz clock. The HPET counter will be off-loaded to the 32 kHz clock domain to allow the PCH’s XTAL MHz clock to shut down when it has no active comparators.
Theory of Operation
The Off-loadable Timer Block consists of a 64b fast clock counter and an 82b slow clock counter. During fast clock mode the counter increments by one on every rising edge of the fast clock. During slow clock mode, the 82-bit slow clock counter will increment by the value provided by the Off-load Calibrator.
The Off-loadable Timer will accept an input to tell it when to switch to the slow RTC clock mode and provide an indication of when it is using the slow clock mode. The switch will only take place on the slow clock rising edge, so for the 32 kHz RTC clock the maximum delay is around 30 microseconds to switch to or from slow clock mode. Both of these flags will be in the fast clock domain.
When transitioning from fast clock to slow clock, the fast clock value will be loaded into the upper 64b of the 82b counter, with the 18 LSBs set to zero. The actual transition through happens in two stages to avoid metastability. There is a fast clock sampling of the slow clock through a double flop synchronizer. Following a request to transition to the slow clock, the edge of the slow clock is detected and this causes the fast clock value to park. At this point the fast clock can be gated. On the next rising edge of the slow clock, the parked fast clock value (in the upper 64b of an 82b value) is added to the value from the Off-load Calibrator. On subsequent edges while in slow clock mode the slow clock counter increments its count by the value from the Off-load Calibrator.
When transitioning from slow clock to fast clock, the fast clock waits until it samples a rising edge of the slow clock through its synchronizer and then loads the upper 64b of the slow clock value as the fast count value. It then de-asserts the indication that slow clock mode is active. The 32 kHz clock counter no longer counts. The 64-bit MSB will be over-written when the 32 kHz counter is reloaded once conditions are met to enable the 32 kHz HPET counter but the 18-bit LSB is retained and it is not cleared out during the next reload cycle to avoid losing the fractional part of the counter.
After initiating a transition from fast clock to slow clock and parking the fast counter value, the fast counter no longer tracks. This means if a transition back to fast clock is requested before the entry into off-load slow clock mode completes, the Off-loadable Timer must wait until the next slow clock edge to restart. This case effectively performs the fast clock to slow clock and back to fast clock on the same slow clock edge.