Hi Xiaohui,
I strongly suggest that you read the CC2420 chip manual available from
Texas Instruments. TinyOS' CC2420 implementation can only be understood
when you know the hardware.
> status = m_cca ? call STXONCCA.strobe() : call STXON.strobe();
> if ( !( status & CC2420_STATUS_TX_ACTIVE ) ) {
> //why check status twice? congestion means after strobe STXON or
> STXONCCA, packet is not sent, am I right? Under what condition will
> congestion happen?
CC2420 outputs the status byte WHILE the STXON(CCA) command is received.
Therefore, it cannot turn on the transmitter and set the corresponding
bit in the status byte before the complete command was received. So,
usually you need the second query for the status byte. The TX_ACTIVE bit
is only on in the first status byte if the transmitter was already on.
> //What does this command do?
> call CaptureSFD.captureFallingEdge();
It tells the ATmega128 to issue the next interrupt (and thus call
CaptureSFD.captured()) when SFD goes from high to low ("falling edge").
In general, SFD goes high when the SFD byte was sent/received and goes
low again when the complete MAC protocol data unit was sent/received.
> //What does this command do and what does its return value mean?
> if ( call SFD.get() ) {
It queries if the SFD pin is high or low. Meaning s.a.
> 3.
> What does the boolean variable /m_receiving/ represent? Whether the SFD
> is caused by packet reception or transmission?
Mainly, m_state controls the meaning of the SFD pin. But if we are not
in sending state (S_SFD, S_EFD) m_receiving tells us if we are currently
receiving a packet.
> //what is ack turn-around here and why do we have to wait even
> channel is clear?
When a msg is received the receiver may send back an ACK. But the switch
from RX to TX mode takes some time, and in this time the channel is
free. So, if we would start immediately we would destroy an ACK.
Therefore, we wait a little bit longer.
Best,
Daniel
--
Daniel Minder
University of Duisburg-Essen, Networked Embedded Systems
Bismarckstr. 90, 47057 Duisburg, Germany
Skype: d.minder
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