[Tinyos-host-mote-wg] [Tinyos-2.0wg] Timers in 2.x

[David Gay]

Working on the NSDI paper, I came up with a few of issues related to
2.x's timers:

- there's a lot of semi-independent, not very large components (hal
alarm, hal counter, 32 kHz 32-bit alarm, 32kHz 32-bit counter, 1 kHz
32-bit alarm, 1 kHz 32-bit counter, virtualized 32kHz timer,
virtualized 1kHz timer)

- there are opportunities for optimizing the hal->32-bit alarm/counter
transformation if they are done together
 Basically: if the alarm deadline is further away than one full cycle
of the hal timer, you can avoid setting a low-level alarm and rely
instead on the overflow event to know when it's time to actually set
an alarm. This reduces interrupt overhead from 3
interrupts/hal-timer-cycle (two of which set an alarm) to 1 (which
typically just needs to increment or decrement a couple of variables).

- even with the above optimization, using the 8-bit timer to provide a
32kHz timer on the mica family keeps the processor busy an additional
~0.1% of the time, significantly affecting maximum mote lifetime

So there's two questions/proposals:
- I'd like to suggest merging the components which offer the Alarm and
Counter interfaces (this basically means cutting the number of
components above by two). We would keep the option of using a hardware
timer as a Counter only, though.
- Do we want to mandate the presence of a 32kHz timer in TinyOS 2.x?
  If so:
  - how precise should it be?
  - should we expect low-power operation to be possible while it's in use?

  (if you answer "not too precise" and "no", you can use one of the
ATmega128's 16-bit timers to get approximate 32kHz timing -
approximate means something between 28 and 31kHz depending on whether
you use the external crystal or the internal oscillator)

  Another option is to make the 32kHz timer's precision configurable
by applications. In practice, on mica's, this means having 8 or 1ms
precision)

David

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