ADS1258

Hi Charlie, 

Answers to your questions are below in blue

1) If I read the datasheet correctly one could start the converter in autoscan mode with all 16 channels selected and never stop it. 
   And then read the channel data as needed with Channel Data Read Commands. Is this correct?

Yes, you can use that method. Note that the ADS1258 architecture is a single A/D converter with a built in mux. For that reason, the ADC has a single output register that is updated with the conversion result every time that /DRDY pulses. For that reason, you need to monitor the /DRDY pulse and use the channel data read command every time the /DRDY pulses. You also have the option to run the ADC in channel data read direct which in includes no command. Therefore, once /DRDY goes low, you begin sending SCLKs and the ADC will output the data. See the data sheet for more information on operating modes

2) Very little is said about the Channel Data Read Command. 
   For Register Read Command automatic incrementing of register address is discussed. 
   But there is no mention of a similar index for Channel Data Read Command. I assume that the device behaves this way.
   Is this correct? If so, does Channel Data Read Direct also work with a auto-increment mechanism?

This specific ADC has a single output register that is updated with the conversion result from every channel. Table 11 on page 33 of the data sheet breaks down the channel pointer index showing the sequence that the output register is updated in. You will need to make sure that you read out the converted data before /DRDY pulses again indicating that the output reg has been updated. If you do not monitor /DRDY, you may miss a conversion result. 

3) I seem to be having a problem with the Register Write Command. I follow each of these commands with a Register Read Command but
   I do not always get the value that I just wrote. The device behavior also seems to indicate that the write did not occur properly.

Do you have an issue writing to set up the registers and then trying to read them back? Make sure you review the command byte configuration on page 34 of the data sheet (enable or disable the MUL bit, etc.). Also, make sure you are using the correct edge of SCLK as the critical edge. The rising edge of SCLK is the critical edge so make sure that the input data is changing on the falling edge of SCLK so it is stable at the rising edge of SCLK.  Do you always have problems writing to registers, or just some of the time?

4) In Autoscan mode it appears that DRDY asserts at the end of each conversion cycle - if one channel is selected after it is converted 
   (approx. 43.75 us) - if 16 channels are selected after it has converted all sixteen channels (approx. 700 us, 1.4 ms if CHOP is enabled).
   Is this correct?

You are correct. The ADS1258 is a single converter with a mux in front of it. So the 23.7kSPS data rate is the speed for a single conversion of a single channel. So to read back all 16 channels single ended, it would take 700uS (1.4 ms with CHOP enabled). 

Regards,

Tony Calabria

Hi Charlie, 

Looks like your SCLK speed may be too fast for this converter. The speed of your master clock (fclk) sets the limit for the SCLK speed. If you look at page 6 of the data sheet, we spec the SCLK period as min 2 tclk (master clock) periods. The fastest the master clock can run is 16 MHz (pg 3 of data sheet), and if it is run at that speed, the SCLK can be a max of 8 MHz. You may not be violating the setup and hold times but you are violating the max SCLK frequency if you are using a 24MHz signal. Try slowing down your SCLK, this should fix your issue. 

Regards,

Tony