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No output from ADS1278 Frame Sync Mode

Royke
Intellectual 510 points
Other Parts Discussed in Thread: ADS1278, OMAP-L138

Hi,

I have a problem. I have connected a ADS1278 to a OMAP L138. The OMAP gives 2 signals, a clock signal from 2,67 MHz and a Framesync signal from 10 KHz.
I'm working on it now over a week and can't get it to work. Tried almost everything but can't get any output out of the ADC. I'm using the ADS1278EVM borad for now.

My settings are:
PWDN1 to 8 1
Mode0 1 High-Resolution mode
Mode1 0
Format0 0 Frame-Sync, TDM, Fixed Output data format
Format1 0
Format2 1

Vref = 2.5V
DVDD = 1.8V 
IOVDD = 3.3V
AVDD = 5V

At start SYNC is taking low and than high again and stays high.
Din is fixed to ground, 0.
both testpins are connected to ground.
CLKDIV is1. 

Timings:
CLK = SCLK = 2.67 MHz => 374 ns
t cpw = 190 ns
t frame = 10 KHz => 100us = 267 t clk
t fpw = 2 t clk

In SPI -mode it looks like there is output data, but we want to use Frame sync mode.
It is very importend to me that I get this to work very soon. 

In the datasheet (SBAA367E) on page 24 is written that the ADC is clocking 24 bits out. That is 8 channels * 24 bits = 192 bits.
In the table on page 9 is written that t frame is minimal 256 clocks. Why not 192?? Or is he giving 8 * 32bits = 256?

added: 
Also tried
t clk: 189 ns
t frame: 528 t clk
in table 8 on page 26 says that in High-Resolution, f clk/ f data is 512. Is this the minimum?
I have now 528.

  • TI__Guru**** 180821 points

    Hi Royke,

    Is it possible for you to describe how you have the ADS1278 hardware setup?  How are the various switches on the EVM set?  I'd like to ensure that you don't have any issues with the hardware before trying to tackle any software concerns.

    When you run the ADS1278 in Frame Sync mode, you have to have the FS synchronized to the data rate of the device.  You noticed on table 8 the fclk/fdata relationship is 512.  You have to have exactly 512 conversion clocks between frames.  Since you were running at 2.67MHz SCLK=CLK, your data rate is going to be 374ns * 512 fclk = 5.215Ksps.  Pulsing FS at a 10K rate, your data output is getting corrupted, which might explain why you have nothing on the output. 

    Once you have the frame sync matched to the data rate, you should get the eight channels of data (starting with the MSB of CH0) coming out of the part on the first rising SCLK after FS goes low.  All eight channels worth of data will be clocked out in the first 192 clocks between frames.

  • in reply to Tom Hendrick
    Intellectual 510 points

    Hi Tom,

    Thanks for responding. I will give you my hardware settings and connections, but I measured all signals on the chip. 
    I tried  your advice to give 512 clocks between the frames but still no output. I think the chip is defect.
    Today I will receive a new one and will test it then again.

    My connections:
    J3A-3 5V
    J3A-5 and J3A-6 GND 
    J3A-9 3.3V
    J3A-10 5V

    J5A-5 Clock
    J5A-9 FSYNC
    J5A-11 GND
    J5A-13 Data out

     

    S10  All HI
    S11 1,5 an 6 HI rest low
    S12 FS

    J6 FSR
    J19 IOVDD
    J17 closed
    J4 closed 
    J18 open
    J10,J11,J13,J14 closed 

     

  • in reply to Royke
    TI__Guru**** 180821 points

    Hi Royke,

    I've lookoed over the HW connections for your ADS1278 setup and everything looks fine.  I suspect you might be right in that the chip could be damaged somehow - even if you don't have the proper number of clock cycles between frames, you should still get something out of the serial port, even though the data might be corrupted.  Let me know how you make out with the replacement chip or if you need any samples. 

    I'm attaching a sample project for the C6713 that was put together a few years ago - not sure if it will help you at all with your OMAP-L138 or not, but your welcome to try it out.

     

    ADS1278-C6713-CCS3v1.zip
  • in reply to Tom Hendrick
    Intellectual 510 points

    Hi Tom,

    I received a new ADC from our supplier and it worked! I couldn’t check if the data was correct because the ADC last just 5 minutes and then there was no output any more.
    The only thing I did is disconnected the sync from the +3,3V and connected it for 1 second to ground and then to 3,3V again. After that the ADC was dead.
    Thanks for the sample project, I will look into that, but my OMAP L138 is running and the McBSP works also with interrupt and all that.

    It would be nice to receive some samples. Maybe you can email me for this.

    Kind regards,
    Roy

  • in reply to Royke
    TI__Guru**** 180821 points

    Hi Roy,

    There is no reason for the SYNC on the ADS1278 going to GND that should cause the part to fail - please double check you power connections and try to verify that there is no large voltage differential between your analog and digital grounds.  I'll try to get your contact details and see what I can do about samples.