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ADS1258 Sequential Sampling Skips Even Channels

Reza Naima95461
Intellectual 280 points

Other Parts Discussed in Thread: ADS1258

I have the ADS1258 configured to sample channels 0-15 in single-ended mode.  However, it seems to only sample every other channel.  I've verified that the following channels are configured:

MUXSCH:0x00
MUXDIF:0x00
MUXSG0:0xFF
MUXSG1:0xFF

I toggle the start bit at a fixed rate, and read the data registers when DREADY is toggled.  However, the status byte indicates that every other channel is being read (i.e channel 9, 11, 13, etc..).  

Also, the DREADY is doing something strange.  It goes high, then low, then goes high again after I start the data transfer and lasts a bit.. I have no clue what it is doing.   You can see a snapshot of the logic analyzer output here: http://imgur.com/a/bFL6k

The top picture is zoomed in, the bottom is zoomed out.  The bottommost trace is DREADY.  

What am I doing wrong?  

Thx,
Reza 

  • 0
    TI__Guru**** 182601 points

    Hi Reza,

    Thank you for the screen shots from your ADS1258 setup!  The zoomed channel shows the expected behavior of the DRDY when you read back data - the pin should go HIGH after the ADS1258 receives the first DCLK.  The bottom picture tells me that you may have an ISR that is configured for a rising transition of the DRDY.  What you see in the bottom picture is the DRDY going low, followed by a high pulse that is one clock wide.  If you never read data, the short high pulse on DRDY is what you would expect to see.  What's happening there is a conversion finished - DRDY goes low but data is never read.  DRDY then pulses high and you start the data transfer, so you are skipping every other channel.  If you are using an interrupt on your processor, please make sure it is set for falling edge sensitivity.

  • 0 in reply to Tom Hendrick
    Intellectual 280 points

    Yes, the ISR was setup to detect low->high transitions.  I'll switch it over and test.  Thanks!!

    Reza