[Tinyos-devel] cc2420 packet timestamping, FTSP, and LPL

Miklos Maroti mmaroti at math.u-szeged.hu
Mon Mar 2 01:32:41 PST 2009


>> =========================================================================
>> 2) timestamp queue
>>
>> i think we do agree that cc2420 timestamping is tricky. summarized in
>> the attached email from jonathan hui and also discussed by me&miklos a
>> while ago:
>> http://mail.millennium.berkeley.edu/pipermail/tinyos-devel/2008-August/0033
>>46.html
>>
>> one suggestion to drop the queue was by miklos:
>> http://mail.millennium.berkeley.edu/pipermail/tinyos-devel/2008-August/0033
>>56.html i tested it and it didn't work, but it can be a good starting point.
>
> As far as I could follow the code, the whole time stamp queue is dead code:
> the real time stamp is maintained in the packet meta data. Besides the
> slightly smaller RAM footprint of the meta data I do not see any advantages,
> not even from a beautiful programming point of view to put the time stamp in a
> separate queue.

I think the original idea was to track the RXFIFO content with time
stamps. Since more than one message can be in the RXFIFO and the time
stamp is generated at the microcontroller, you need match incoming
messages with time stamps. But this algorithm cannot possibly work
reliably if the radio chpi is allowed to filter out bad messages since
then the frame arrival interrupt is called even for messages that get
discarded, and then you cannot match up timestamps and messages. It is
I think much better to remove all that time stamping queue stuff and
have valid time stamps when we know that no other messages are/were in
the RXFIFO.

Miklos


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