Hi, Just to clarify, my difficulty of understanding mainly arises from the fact that the following code snippet seems only related to reception and thus reception timestamping (see highlighted parts). But somehow transmission timestamp is invalidated under some circumstances.
Please help me understand and thank you in advance. On Thu, Oct 25, 2012 at 1:00 AM, Xiaohui Liu <[email protected]> wrote: > Hi everyone, > > When is transmission timestamp considered invalid for a packet? AFAIK, the > only place to invalidate a packet being transmitted is in > CC2420TransmitP$CaptureSFD$captured() as the following code shows: > > * /* this is the SFD for received messages */ > ** if ( !m_receiving && sfdHigh == FALSE ) { > ** sfdHigh = TRUE; > ** call CaptureSFD.captureFallingEdge(); > ** // safe the SFD pin status for later use > ** sfd_state = call SFD.get(); > ** call CC2420Receive.sfd( time32 ); > ** m_receiving = TRUE; > ** m_prev_time = time; > ** if ( call SFD.get() ) { > ** // wait for the next interrupt before moving on > ** return; > ** } > ** // if SFD.get() = 0, then an other interrupt happened since we > ** // reconfigured CaptureSFD! Fall through > ** } > ** > ** if ( sfdHigh == TRUE ) { > ** sfdHigh = FALSE; > ** call CaptureSFD.captureRisingEdge(); > ** m_receiving = FALSE; > ** /* if sfd_state is 1, then we fell through, but at the time of > ** * saving the time stamp the SFD was still high. Thus, the timestamp > ** * is valid. > ** * if the sfd_state is 0, then either we fell through and SFD > ** * was low while we safed the time stamp, or we didn't fall through. > ** * Thus, we check for the time between the two interrupts. > ** * FIXME: Why 10 tics? Seams like some magic number... > ** */ > ** if ((sfd_state == 0) && (time - m_prev_time < 10) ) { > ** call CC2420Receive.sfd_dropped(); > ** if (m_msg) > ** call PacketTimeStamp.clear(m_msg); > ** } > ** break; > ** } > ** }* > Can someone please help me understand what is going here, especially the > condition to regard a tx timestamp as invalid? Your help will be highly > appreciated. > > -Xiaohui Liu > TelosB > TinyOS 2.1.2 >
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