PacketTimeStamp CC2420 bug

PacketTimeStamp Bug in CC2420 radio chip

Parent Project: FTSP

Related TEP: PacketTimeStamp TEP

Description of PacketTimeStamp

PacketTimeStamp interface provides accurate timestamping of radio messages low in the radio stack (MAC layer). this is required to allow for accurate time-synchronization algorithms built on top of PacketTimeStamp interface.

CC2420 radio chip implementation of PacketTimeStamp uses SFD interrupt for both reception and transmission timestamps. Time difference between the SFD interrupt firing at sender and receiver is less then 1 microsecond, theoretically allowing for sub-microsecond accuracy. To eliminate interrupt handling delays, mote platforms connect capture timer to the SFD interrupt line, recording the timestamp irrespective of when the interrupt is handled in software. MicaZ uses Timer1 (driven by external 32khz oscillator). Telos uses TimerX (driven by xxx).

Timestamps recorded at SFD interrupt are saved in cc2420_metadata_t.timestamp and are not transmitted in the air. Specifically, saving timestamps is done using PacketTimeStamp.set() in tos/chips/cc2420/CC2420TransmitP.nc (TX side) and tos/chips/cc2420/CC2420ReceiveP.nc (RX side). Client application can read this timestamp from Receive.receive() or AMSend.SendDone() events.

One problem is that multiple messages may be received before the client application has a chance reading the timestamp in receive() event. Therefore, CC2420ReceiverP.nc maintains a FIFO buffer for all received timestamps and serves them to client app as needed.