[Tinyos-help] CRC in CC2420

[Razvan Musaloiu-E.]

Hi!

On Thu, 19 Feb 2009, Avinash Sridharan wrote:

> Dear Razvan,
> I am using Tiny OS-2.0.2, I wanted to follow up on this question. I
> have enabled software ACK in my system and see quite a few false
> positives (for topologies with link qualities as bad as 30%). My guess
> is that though the CRC is failing, the CRC field is not checked before
> the complete packet is read. While the software acknowledgment is sent
> out the minute the header information is read (which could still be
> correct though the data is corrupted).  Please correct me if my
> description of the behavior of software acknowledgment is not exact.

The specs [1] (page 42, first paragraph after Figure 23) says that SACK 
and SACKPEND will not generate an ACK if the CRC is not valid. So the 
current implementation should work properly.

[1] http://www.ti.com/lit/gpn/cc2420

--
Razvan ME

> Given my above description is correct, just to see if I can see any
> reduction in the number of false positive ACK's I wanted to move the
> software acknowledgment to be generated after performing the CRC test.
>
> I was planning to modify the current code from :
> if ( ( buf[ rxFrameLength ] >> 7 ) && rx_buf ) {
>        uint8_t type = ( header->fcf >> IEEE154_FCF_FRAME_TYPE ) & 7;
>        signal CC2420Receive.receive( type, m_p_rx_buf );
>        if ( type == IEEE154_TYPE_DATA ) {
>          post receiveDone_task();
>          return;
>        }
>      }
>
>
> to ------
>
> if ( ( buf[ rxFrameLength ] >> 7 ) && rx_buf ) {
>        uint8_t type = ( header->fcf >> IEEE154_FCF_FRAME_TYPE ) & 7;
>        signal CC2420Receive.receive( type, m_p_rx_buf );
>        if ( type == IEEE154_TYPE_DATA ) {
>           call CSN.set();
>          call CSN.clr();
>          call SACK.strobe();
>          call CSN.set();
>          call CSN.clr();
>
>          post receiveDone_task();
>          return;
>        }
>      }
>
> Of course I will turn of the current initiation of software
> acknowledgment.   Wanted your feedback on these code changes.
>
> Thanks and regards,
> Avinash
>
>
> On Tue, Feb 17, 2009 at 12:18 PM, Razvan Musaloiu-E. <razvanm at cs.jhu.edu> wrote:
>> Hi!
>>
>> On Tue, 17 Feb 2009, Jordi Pérez wrote:
>>
>>> Hi!
>>>
>>> I have just tried it, and it works correctly. I see the bad and the correct
>>> packets.
>>>
>>
>> There was a bug in my patch (Mike point it out to me). I attached the
>> correct one now. :-)
>>
>> --
>> Razvan ME
>>
>>> 2009/2/17 Razvan Musaloiu-E. <razvanm at cs.jhu.edu>
>>>
>>>> Hi!
>>>>
>>>> On Mon, 16 Feb 2009, Jordi Pérez wrote:
>>>>
>>>>  Hi Razvan!
>>>>>
>>>>> I tryed to understand the code to disable the filtering you said, but I
>>>>> can't. Do you mind explaining me the things I have to change?
>>>>>
>>>>> In RXFIFO.readDone event when I modify something, the program doesn't work.
>>>>> So I don't know how disable it.
>>>>>
>>>>> In the SubReceive.receive event I changed the word Snoop to Receive, but I
>>>>> don't really know if it is correct.
>>>>>
>>>>> Can you help me please?
>>>>>
>>>>
>>>> I attached a patch that takes out the parts that discards the packets with
>>>> bad CRC. It's against the latest CVS and you can apply it with this requence
>>>> of commands:
>>>>       cd $TOSROOT
>>>>       patch -p1 < cc2420-ignore-crc-patch.diff
>>>>
>>>> Warning: I didn't have time to test this! I only made sure that it compiles
>>>> fine. :-) Give it a try and if it doesn't work at it should I'll look more
>>>> into it.
>>>>
>>>> --
>>>> Razvan ME
>>>>
>>>>
>>>>  2009/2/12 Razvan Musaloiu-E. <razvanm at cs.jhu.edu>
>>>>>
>>>>>  Hi!
>>>>>>
>>>>>>
>>>>>> On Wed, 11 Feb 2009, Jordi Pérez wrote:
>>>>>>
>>>>>>  Dear all,
>>>>>>
>>>>>>>
>>>>>>> For CC2420, if AUTOCRC is set, will the hardware reject CRC-error packet automatically?
>>>>>>> If true, then is it possible to receive those packets but with AUTOCRC still
>>>>>>> being set? I want to receive this packets and rebuild them.
>>>>>>>
>>>>>>>
>>>>>>  The CC2420 is not discarding the packets with bad CRC. What does is to
>>>>>> indicate using a bit that the packet was bad (page 38, section 16.4 from
>>>>>> [1]). The TinyOS driver for CC2420 inspects this bit and discards the
>>>>>> packets if that bit is not 1. You can see this happening in the
>>>>>> RXFIFO.readDone event from CC2420ReceiveP.nc. The switch branch of interest
>>>>>> is 'case S_RX_PAYLOAD:'.
>>>>>>
>>>>>> The other filtering that you'll have to disable is in the
>>>>>> SubReceive.receive event from CC2420ActiveMessageP.nc.
>>>>>>
>>>>>> [1] http://focus.ti.com/lit/ds/symlink/cc2420.pdf
>>>>>>
>>>>>> --
>>>>>> Razvan ME
>>>>>>
>>>>>
>>>
>>
>