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ADS1246: SYNC command not working

Peter Schmuckal
Intellectual 350 points
Part Number: ADS1246


We have an application using an ADS1246 with an external amux.  So we need the capability to "restart" the conversion after the amux has been properly setup and settled and don't want the next conversion to begin immediately.  Presently we are using a 160 sps rate.  START is tied high.  It was my understanding that issuing a SYNC SPI command would restart the conversion from that point.

So what we do is:

- Wait for DRDY to fall & read result regs (RDATA cmd)

- Set our ext amux w/ a little delay

- Issue SYNC command to restart conversion

However, when I look at it on the scope, I see only a rock solid 160 sps rate between DRDY pulses, whereas I would expect to see 6.25 ms (1/160) delay from the time the SYNC cmd is sent to the next DRDY pulse. 

I also tried issuing a SLEEP/WAKE combination in another attempt to "restart" it, but that also made no apparent difference.

What am I doing wrong here?

Thanks,

- Peter Schmuckal

  • 0
    TI__Guru* 98055 points
    Peter,


    Your understanding of the SYNC command is correct, and the device should stop the conversion in progress and start a new conversion when it receives the command. If you've made a change to the MUX and sent the SYNC command you should see some sort of blip or delay in the trail of /DRDY pulses that would normally come every 6.25ms.

    You mention that you've written the SLEEP/WAKE command, did the device shut off and start up again? The device should have received the SLEEP command, completed the conversion in progress, shutdown, and then started up again after the WAKE command. Just to make sure, I wanted to see that the device was properly receiving the command.

    At this point, my guess would be that the device isn't receiving the SYNC command or it's being missed on the oscilloscope. I'd start by looking at the SPI communications, including the /CS line. Can you post a pic of the digital communications for the SYNC command? If you're using multiple devices on the SPI bus, make sure you have selected the correct /CS for the ADS1246. I'd like to see that the command is sent correctly through SPI.

    Another test would be to start issuing a series of SYNC commands every 5ms. This would tie up the device forever and the ADS1246 would stop sending /DRDY pulses. This would just test to see that it is receiving the command and interpreting it correctly.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 350 points

    Thanks Joseph,

    My bad - I was raising CS before shift was complete.  Works now.  You can delete this.