ADS1158 Channel read

Hi Anthony,

There are two types of "read" commands on the ADS1158.

1. Channel data read direct (no command)
   In this mode, data is read right after it becomes available (i.e. /DRDY goes low). Sending this command retrieves the status byte (if enabled) and conversion result with only 16 or 24 SCLKs (rising edges). Once this command has been performed and data has been retrieved, /CS must be toggled before sending additional commands. Note that reading data in this mode requires that all data bytes be retrieved from the output shift register before /DRDY goes low again, otherwise results will be corrupted by the next conversion's result.
   
    
2. Channel data read command (register format)
   In this mode, the data in the output shift register is preserved in another register to avoid possible corruption from following conversions; therefore, reading data in this mode does not require synchronization with the /DRDY signal. This mode requires a total of 32 SCLKs (8 for the command, 8 for the STATUS byte which is always enabled in this mode, and 16 for the data).

If you're monitoring /DRDY with an interrupt and able to serve the interrupt quickly, then the "read direct" mode is the fastest way to retrieve data.

If you don't monitor /DRDY, than you'll need to use the "read command" mode which requires additional SCLKs.

Regarding which channel's data you read back depends on the register settings...

• In fixed-channel mode, you select the input channel with the MUXSCH register. Once configured, you'll start a conversion, wait for /DRDY to go low, then retrieve the data for that channel using one of the two methods listed above.
  
  
• In auto-scan mode, you select which channels you would like the ADC to cycle through with the MUXDIF, MUXSG0, MUXSG1, and SYSRED registers. After starting conversions, you'll get data out for one of the selected channels, each time /DRDY goes low. The order of the channel scanning is shown in Table 10. Additionally, you can read the STATUS byte to check which of the channel's data you are currently reading.

Unfortunately, you cannot output the data from all channels in one-shot...You'll need to retrieve data one channel at a time, the sequence will look something like this...

• Write to all registers with the "register write command" (sets the initial configuration - let's assume you decide to use auto-scan mode)
• Start converting by setting the START pin high.
• Wait for /DRDY to go low
• Read the 1st channel's data with the "channel data read direct" command (you'll need to be able to retrieve all data before /DRDY goes low, otherwise you would need to use the fixed-channel mode).
• Wait for /DRDY to go low
• Read the 2nd channel's data with the "channel data read direct" command
• ....repeat waiting for /DRDY and reading data for all remaining channels...
• Stop conversions and enter power-down mode OR repeat channel cycling.

I hope that helps!

Best Regards,
Chris

Hi Anthony,

If you are not monitoring /DRDY, then you could do one of two things...

1. Operate the ADS1158 in fixed-channel mode and read data with the register read mode (Correct, this would take 32 SLKs to send the read command and retrieve the STATUS + Data bytes).  You would then change the MUXSCH setting and restart conversion to switch channels. You wouldn't need to worry about data being overwritten when using the register read mode.
   
   
2. Additionally, you could still operate the ADS1158 in auto-scan mode, but use the pulse convert command to only run a single conversion. Then you would retrieve the data with either read command and check to make sure the "NEW" bit in the STATUS byte is set. If so, just send the pulse convert command again and the ADC will continue to index through all 16 channels. This method also avoids overwriting data, because only one conversion will complete.

Regarding your last question...

Anthony Scarnici said:

If I understand this correctly... when I'm in auto-scan mode, I pulse the start pin low for some nominal amount of time, and after all 16 channels are converterd (I set it up to scan all 16), the /DRDY pin will go low, prompting me to read the data. I send down the command 000 and the MULT bit =1, and will receive one byte as invalid data, followed by the status byte, then I send 32 more bytes of 0s to get the 16 channels of data... do I have that right?

No, in auto-scan mode the ADC will only measure 1 channel at a time. After measuring a channel, /DRDY will go low to notify you that a 16-bit conversion result has completed. Meanwhile, the ADC automatically switches channels and starts measuring the next channel. This process repeat until you've read data 16 times (one channel's data for each time /DRDY goes low).

If you don't read data in time (before /DRDY goes low again) then you will lose the measurement result for that channel (at least until auto-scan has cycled back through to that channel again). However, if you instead were to use the pulse convert command, you could slow down this process by making the ADC wait until the next pulse convert command before starting the next conversion, that way you don't lose data OR get corrupted data.

(To read out data from more than one channels at a time would require a simultaneously sampling ADC that has more than one modulator, such as the ADS1178)

Best Regards,
Chris