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DAC8760 - Communication CRC Problem

Jens Koehler
Prodigy 30 points
Other Parts Discussed in Thread: DAC8760

Hi,

we're using DAC8760 in a configuration with another device on same SPI. At system startup we initiate the following sequence:

1.) Write SW-Reset to DAC8760 (otherwise we get CRC Error on first command after activating CRC Check)

2.) Activate CRC Check in DAC8760

3.) Setup DAC8760 to output e.g. 1V

4.) Write to the other device on SPI 2 Bytes

5.) Setup DAC8760 to output e.g. 2V --> at this point we get a CRC Fault (in STATUS Reg CRC-FLT is set)

If we're sendig two dummy bytes with 0's after step 4 we get no error. But this is not a good solution for us. In the datasheet we read, that only the last 32 bits before a low to high transition at the LATCH Pin are relevant to the chip. But this doesn't seem to be true.

Is this a known bug? Do we do something wrong?

Kind Regards

Jens

  • 0
    TI__Mastermind 46290 points
    Jens,

    I think the problem may be that during step 4 when you are writing to the other device with 16-bit frame size the internal counter is still toggling and therefore becomes misaligned by 16 clock cycles for the internal CRC. The device doesn't actually latch and take action on that data, but the counter becomes corrupt. One experiment to verify that would be if you simply toggle LATCH in between step 4 and step 5.
  • 0 in reply to Kevin Duke
    Prodigy 30 points

    Kevin,

    Thanks for your reply. I think you're right. I found the answer yesterday in the "Precision Data Converters Wiki" e2e.ti.com/.../3251.crc-frame-error-checking

    My question is: Why it isn't mentioned in the datasheet??? There's no explanation how the CRC unit works.

    Kevin Duke said:
    One experiment to verify that would be if you simply toggle LATCH in between step 4 and step 5.

    If I do so I think I'll get a CRC Error too. Or does the DAC only take action if 32bits received AND it has a low to high transition on latch pin?

    I'll give it a try but I'm not sure if it is a reliable solution for the Problem.

    Jens

  • 0 in reply to Jens Koehler
    Prodigy 30 points

    Kevin,

    I tried it and it works. I guess the shift-register is cleared if I toggle LATCH - correct?

    Jens

  • 0 in reply to Jens Koehler
    TI__Mastermind 46290 points

    Jens,

    Toggling LATCH with no data issues a NO OP which resets the internal counter and recovers the CRC calculation.