[Tinyos-help] Apparent Timing Bug in RssiSample

[Tal Rusak]

Hi,
     I have noticed an apparent bug the tinyos-2.x-contrib application
RssiSample. In
/tinyos-2.x-contrib/stanford-sing/apps/RssiSample/NoiseSampleP.nc, on
lines 197-201, the Alarm0.fired() even is implemented as follows:
  197 	async event void Alarm0.fired()
  198 	{
  199 		call Resource.request();
  200 		call Alarm0.start(ALARM_PERIOD);
  201 	}
     I believe that calling Resource.request() before
Alarm0.start(ALARM_PERIOD) causes a time lag since the next alarm
does not start until after the call to Resource.request() returns.
     There are two possible patches that I have come up with.

1)   Switch the order of the calls, i.e.,

  197 	async event void Alarm0.fired()
  198 	{
  199 		call Alarm0.start(ALARM_PERIOD);
  200 		call Resource.request();
  201 	}

     It may be the case that this introduces a race condition.

2)   Use a periodic timer (see attached code).

     I have audited collection time for 61440 samples (1 minute clock
time) for each of the approaches with the following results (four
runs of each):

Original
000:01:08.593
000:01:08.730
000:01:08.781
000:01:08.699

Swapped calls (correction 1)
000:01:03.311
000:01:03.229
000:01:03.193
000:01:03.339

Periodic Timer (correction 2)
000:01:00.238
000:01:00.250
000:01:00.324
000:01:00.255

      These results were collected using the stopwatch program
(http://expect.nist.gov/stopwatch/) by looking at the LEDs on a telosb mote.

Thanks,
Tal
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