UBX Protocol Parsing

[nburkitt]

Subject of the issue

When a UBX protocol packet is received, it appears to be parsed incorrectly.

Your workbench

Cypress PSoC 6

Steps to reproduce

Unless I'm missing something obvious, in SFE_UBLOX_GPS::process(uint8_t incoming, ubxPacket *incomingUBX, uint8_t requestedClass, uint8_t requestedID), when incoming is 0xB5, we set the following values:

ubxFrameCounter = 0;
currentSentence = UBX;
packetBuf.counter = 0;
activePacketBuffer = SFE_UBLOX_PACKET_PACKETBUF;

A bit later in the method...

if (currentSentence == UBX)
{

The only clause that is satisfied by the fact that ubxFrameCounter == 0 is

    if ((ubxFrameCounter == 0) && (incoming != 0xB5))      //ISO 'μ'
        currentSentence = NONE;                              //Something went wrong. Reset.

so we end up at

...
else // if (activePacketBuffer == SFE_UBLOX_PACKET_PACKETBUF)
  processUBX(incoming, &packetBuf, requestedClass, requestedID);

Since activePacketBuffer == SFE_UBLOX_PACKET_PACKETBUF, we call processUBX(uint8_t incoming, ubxPacket *incomingUBX, uint8_t requestedClass, uint8_t requestedID), with incoming == 0xB5 and incomingUBX == &packetBuf.

In SFE_UBLOX_GPS::processUBX()

if (incomingUBX->counter == 0) // a.k.a. packetBuf.counter
{
    incomingUBX->cls = incoming; // incoming == 0xB5
}

This assigns the packet class the value of the first sync byte, 0xB5.

Expected behavior

The packet class should be assigned the value of the third byte of the packet.

Actual behavior

The packet class is assigned the first byte of the packet, which is always 0xB5.

[PaulZC]

Hi @nburkitt ,
Great detective work!
Everything you've said is true... But the .counter gets reset to zero again when the class byte does arrive.
Are you just trying to understand the code - or is there something else you're investigating?
(Please close this issue if you're happy with this answer.)
Best wishes,
Paul

[nburkitt]

Hi Paul.
Thanks, but you're being generous. Great detective work would have discovered that .counter was subsequently reset to 0.
I appreciate the quick reply - it saved me a lot of head scratching.
-Nick