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ADS1258: About command sending in fixed channel mode for ADS1258

user6151539
Prodigy 30 points
Part Number: ADS1258

I have implemented the function in auto-scan mode, using the instruction of 80+30+FF*4, periodic collection, channel conversion time of 80us (that is, the interval between 80 and 30 instructions). In the fixed mode, the speed is increased to 125ksps, the spi clock frequency is maintained at 6.25mhz, and the 80 and 30 instruction interval is still 80us. The same instruction can achieve the function, but the function cannot work after the instruction interval is adjusted to 8us.

Is it a problem of sending instructions?

0x80+delay8us+0x30+0xFF+0xFF+0xFF+0xFF

  • 0
    TI__Guru** 112855 points

    Hi user6151539,

    Just so that I'm clear, you are issuing the pulse convert start command in fixed channel mode, then waiting 8us and attempting to read back the data is that correct?  You can monitor DRDY with a scope to see if you are attempting to read the result before the conversion has completed.  Consider that you are in an idle state when the Pulse Convert Start command is issued.  There will need a specific amount of time to delay before the conversion completes with settled data.  See the topic Initial Delay on page 28 of the ADS1258 datasheet.  Also see the timing in Table 11.  As the ADS1258 uses a sinc5 digital filter, you need to complete 5 conversions before you have settled data.  Plus you need initial startup time to get the ADS1258 ready to start the conversion.  The delay is expressed in fclk cycles in Table 11.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 30 points

    Thank you for your reply. I have a question. It is impossible to complete the sampling rate of 125ksps in fixed mode only with 80+30+ff*4 instructions (used in auto-scan mode). Do I have to use start and drdy signal to complete the data sampling function?

  • 0 in reply to user6151539
    TI__Guru** 112855 points

    Hi user6151539,

    It is not totally clear to me what you are asking and perhaps I'm not clear in my response.  Some of the actions of the START pin can also issued by command which is what you are doing when you send 0x80 (Pulse Convert Command).  On page 27 of the ADS1258 datasheet is the section Pulse Convert Command.  When using the command (and START pin is low), the ADS1258 is placed into conversion mode and one channel is converted.  If auto-scan is used, the mux position is advanced to the next channel to be converted.  In fixed channel operation, there is no change for the mux. 

    You do not need to use DRDY, but it is very useful when determining when the conversion has completed and you have valid data available to be read from the ADS1258.

    In the section on page 28 of the datasheet called Initial Delay, there is a discussion regarding the timing delay for the initial conversion following Standby or Sleep mode.  The ADS1258 is converting at the selected data rate, but the conversion data are not available until the data are fully settled and at which time DRDY will transition from high to low.  Using the Pulse Convert Command when in fixed channel mode will not achieve 125ksps.

    If you use the START pin and hold it high (continuous conversions), the first available conversion will still take the same amount of time as it does in pulse convert mode, but after the initial conversion delay the following conversions will be available at the 125ksps data rate.

    Best regards,

    Bob B