[Tinyos Core WG] arbiter issues

[Philip Levis]

On Oct 23, 2006, at 9:45 AM, Joe Polastre wrote:

>> 7) We need a radio timestamp abstraction. I am tempted to add it to
>> Packet. Thoughts?
>
> This can be tricky, especially in a platform independent manner.  For
> example, if atmega* doesn't support 32khz stamping, but msp430* does,
> what does the Packet interface expose?  Can you insert timestamps into
> a message, or is it only for SFD capture on receive?  Lots of
> questions here...

Both chips provide 32kHz timestamps.

I think we went over the issues a few months ago: the timestamps are  
gathered locally at a point which the RX and TX nodes agree on. For  
the CC2420, the SFD interrupt is the common place to do this, but the  
CC1000 (or XE1205 or TDA5250) might pick a time besides the first  
data bit of the packet. It is up to the radio stack to decide when  
this point is, based on the radio's capabilities and the stack's  
software structure.

The reason why I'd argue it belongs in Packet is because it is the  
generic interface that all packet-based protocols use. It doesn't  
make sense to turn L2 acks on/off on routing-layer packets, for  
example: that's up to the routing layer. However, as long as packets  
are moving through the data path, timestamps are important, and  
adding another interface seems unnecessary.

Given that these timestamps will suffer from inaccuracies due to  
atomic sections, I think that a fixed precision but variable accuracy  
might be palatable.

Phil