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DAC8741H: Framing Error reported

Pran Haran
Prodigy 40 points
Part Number: DAC8741H

With some HART transmitters, we are getting random framing errors. (Roughly 1 % of the times). The modem status register shows 0x4806.

There are still a couple of bytes left (we know this based on the HART frame format) after the framing error which are not coming through the FIFO.

The modem status register does not show CD off for 20 ms more. (Modem status register then goes to 0x4804)

I have two questions:

1. Does the modem stops sending data out after the framing error? 

2. Can the modem handle baud rate variation of at least plus or minus 2%

  • 0
    TI__Guru* 93485 points

    Pran,

    Just to make sure that I understand, is question 1 on receiving or sending the data after the framing error? Regardless, I'll need to check with the digital designer on that one.

    For the modem baud rate variation, I would guess that the device should receive signals with a frequency error as high as 4-5%. The receive has a 3rd order filter that should be moderately tolerant to frequency variation. I would note that receiving a frequency with a variation at 9% may be very problematic because if the "0" FSK bit reaches 2400Hz, the bit is at a harmonic of the "1" bit.

    Joseph Wu 

  • 0 in reply to Joseph Wu
    TI__Guru* 93485 points

    Pran,

    While I'm thinking of it, how do you have the MODEM_IRQ_MASK set up?

    Joseph Wu

  • 0
    TI__Guru* 93485 points

    Pran,


    I did hear back from one of the digital designers. The device will continue to receive data after a FRAME error, indicating that the STOP bit was not correct. If the data continues to come into the demodulator, the device will continue to try to decode the data, but it's likely that the data is corrupt. Normally, the FRAME error is critical, and you should throw away the data after the CD de-asserts.

    For the modem baud rate variation, the digital designer pointed out that the variation spec is really about ±1%. He had pointed out that the timing error accumulates through the reception of the byte. He thinks that we may tolerate about 2.5%. With that variation, after 11 bits, we'd be off by over 25% at the STOP bit (and that might be tolerable). We can't tolerate 4-5%.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 40 points

    The modem IRQ has been set to  ,~(FIFO_M2D_LEVEL + CD + FRAME )

    enabling CD, and M2D fifo IRQ along with frame error irq.

    I am working to verify if the frame error is caused by the other modem either due to baud rate difference or premature ending of the carrier signal. 

    I will keep you posted.

  • 0 in reply to Pran Haran
    TI__Guru* 93485 points

    Pran,

    Ok, thanks. you should be able to see the HART signal with the oscilloscope to see if the other modem is ending the signal. You may also be able to get a reading of the FSK bit frequencies also. Let me know what you find.

    Joseph Wu