This is explained very clearly (with figures) on pages 38 and 39 of
the CC2420 datasheet.
http://focus.ti.com/lit/ds/symlink/cc2420.pdf
--
Jonathan Hui
[EMAIL PROTECTED]
On 9/28/06, Tie Luo <[EMAIL PROTECTED]> wrote:
Dr Hui, at first I thank you very much for your explanation.
Based on your answer to 1.2, buf[ length ] is crc. But why buf[ length ]
>> 7 being one indicates a good crc?
On 9/28/06, Jonathan Hui <[EMAIL PROTECTED]> wrote:
> On 9/28/06, Tie Luo <[EMAIL PROTECTED]> wrote:
> > 1) CC2420ReceiveP.nc , in event void RXFIFO.readDone(uint8_t* rx_buf,
uint8_t
> > rx_len, error_t error )
> > 1.1) line 197:
> > if ( length > 10 ) {...} (where length is header[0])
> > Q: why use constant 10? Does it mean sizeof(cc2420_header_t)? If
so,
> > will it always be true, since length include a 11-byte header and a
payload?
>
> This is simply for timestamping information. The CC2420 is doing
> hardware address rejection, so any packets that are rejected should
> not be considered.
>
> > 1.2) line 207,208:
> > // pass packet up if crc is good
> > if ( ( buf[ length ] >> 7 ) && rx_buf ) {...}
> > Q: buf[ length ] is the byte beyond the last byte of
header+payload,
> > so why judge on that? and what does it being shift by 7 bits mean? How
does
> > this related to crc (see the comment above)
>
> The crc comes after the payload. Please see the CC2420 datasheet.
>
> > 2) in message_t, length=header+payload+footer, right? But in
> > CC2420TransmitP.nc line 243 is:
> > call TXFIFO.write( (uint8_t*)header, header->length - 1 );
> > Q: why -1?
>
> The length byte is not included in the length of the packet. Please
> see the 802.15.4 spec.
>
> > 3) related to (2), in the same file at line 421: msg_metadata->rssi =
> > ack_buf[ length - 1 ]; while rssi is define as the 2nd field of
> > typedef nx_struct cc2420_metadata_t {
> > nx_uint8_t tx_power;
> > nx_uint8_t rssi;
> > nx_uint8_t lqi;
> > nx_bool crc;
> > nx_bool ack;
> > nx_uint16_t time;
> > } cc2420_metadata_t;
> > So why ack_buf[ length - 1 ] is rssi?
>
> The crc is replaced with rssi when in auto crc mode. Please see the
> CC2420 datasheet.
>
> > These questions really make us blue, can you help us out? Thanks a ton!
>
> If not obvious already, I *strongly* recommend that you read the
> CC2420 datasheet and the 802.15.4 spec. They will clear a lot of these
> issues up.
>
> http://www-s.ti.com/sc/ds/cc2420.pdf
>
http://standards.ieee.org/getieee802/download/802.15.4-2003.pdf
>
> --
> Jonathan Hui
> [EMAIL PROTECTED]
>
--
Regards,
Tie
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