Fast ADC using the PRU to bitbang SPI

Prodigy 80 points

Baptiste Pierrat

Replies: 3

Views: 1212

Hi,

I would like to know if someone here has already tried bitbanging SPI with the PRU units? 

I have a 12bit ADC (TI ADS7883) which takes a clock signal and outputs 12 bits of data, and I would like to use the PRU to achieve high sampling rate.

See http://www.ti.com/lit/ds/symlink/ads7883.pdf for the ADC documentation with the interface timing diagram.

Basically, the PRU program would do the following (assuming a cycle time of 5ns and one cycle for each line of the loop):

- Initiate 3 GPIO channels (2 outputs: SCLK and CS, 1 input: SDO)

- Allocate memory (a lot! I'd like to capture millions of samples)

Loop for N samples:

- Pull CS high

- Wait 3 cycles

- Pull SCLK high

- Wait 2 cycles

- Pull CS low (initiate output)

- Wait 2 cycles

- Pull SCLK low

- Wait 5 cycles

- Pull SCLK high

- Wait 5 cycles

- Pull SCLK low

- Wait 5 cycles

- Pull SCLK high

- Wait 5 cycles

- Pull SCLK low

- Read SDO (first bit)

- Store it somewhere

- Wait 3 cycles

- Pull SCLK high

- Wait 5 cycles

- Pull SCLK low

- Read SDO (2nd bit)

- Store it somewhere

- Wait 3 cycles

- Pull SCLK high

- Wait 5 cycles

- Pull SCLK low

- Read SDO (3rd bit)

- Store it somewhere  ... and so on until 12th bit

- Wait 60ns (acquisition time) while always pulling CLK high and low every 6 cycles

End of loop

The problem is that I have never programmed in assembly code. I have tried looking at examples but have a really hard time understanding the simplest commands (ex: what is a simple MOV r3, 1<<7 doing?).

I'd like to have a thoroughly commented, simple example to start if possible. 

Memory issues are also a big problem. How can I store all the captured data so that it is fast and doesn't overflow the PRU memory?

Many thanks for your help!

Touste

• ADC
• PRU
• ADS7883
• SPI

Prodigy 80 points

Baptiste Pierrat

• Jan 23, 2015 6:56 PM
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3 Replies

• Prodigy 393215 points

  

  Biser Gatchev-XID

  

  • Jan 26, 2015 7:17 AM

  Hi Baptiste,

  What is the sampling rate you want to achieve? Isn't it possible to do this with standard McSPI peripheral? I don't think there are ready examples of doing this with PRU.

   

  Best Regards

  Biser

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• Prodigy 80 points

  

  Baptiste Pierrat

  

  • Jan 26, 2015 9:57 AM

  In reply to Biser Gatchev-XID:

  Hi Biser,

  Thanks for your help. I would like to achieve a minimum sampling rate of 1 MSPS, and if possible in a "real-time" way, i.e. the time between two samples should always be the same. 

  Is it possible with standard McSPI peripheral? If so, it would make things easier I think. Do you have a link to relevant online resources to get me started?

  Many thanks

  Baptiste

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• Prodigy 393215 points

  

  Biser Gatchev-XID

  

  • Jan 26, 2015 11:08 AM

  In reply to Baptiste Pierrat:

  The McSPI can go up to 48MHz clock, however I don't think that what you want to achieve will be possible under Linux. You might want to check the Starterware package, which is an OS-less environment: http://processors.wiki.ti.com/index.php/StarterWare

   

  Best Regards

  Biser

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