The knowledge that lp_ticker runs in deep sleep was hard-coded with a
comparison check of a ticker_data_t pointer against get_lp_ticker_data.
Remove this hard-coded check, which adds a linker dependency against
the low power ticker even if not being used - put a flag into the
ticker_interface_t.
A future extension might be to move this flag into the ticker_info_t
provided by the HAL, but for the moment keep the assumption that
lp_ticker does run, us_ticker doesn't.
Update the low power ticker wrapper code so it does not violate any
properties of the ticker specification. In specific this patch fixes
the following:
- Prevent spurious interrupts
- Fire interrupt only when the ticker times increments to or past the
value set by ticker_set_interrupt
- Disable interrupts when ticker_init is called
Some low power tickers take multiple cycles of the low power clock
to set a compare value. Because of this if the compare value is set
twice back-to-back these implementations will block until that time
has passed. This can cause system stability issues since interrupts
are disabling for this time.
To gracefully support this kind of hardware this patch adds code
to prevent back-to-back writes to the hardware. It does this by
recording the low power clock cycle of the initial write. If any
writes come in too soon after this initial write the microsecond
ticker is used to schedule the new write in the future when the
hardware is ready to accept a new value.
To enable this feature on a target the macro LOWPOWERTIMER_DELAY_TICKS
must be set to the number of low power clock cycles that must elapse
between writes to the low power timer.
Kevin Bracey
Martin Kojtal
Russ Butler
jeromecoutant