How Do I Get the SPI Host Adapter to Drive the SPI Clock?

Question from the Customer:

I have been working with your Aardvark I2C/SPI Host Adapter and it’s been working great on all my projects. I have a question for you.

Using the Aardvark adapter for SPI, I programmed a script in python to handle a more complex communication with a µC-System. Here is the problem I want to solve:

  • I’m using a GPIO of the Aardvark adapter as the CS, the original SPI CS (chip select) is ignored
  • The communication is handled by sending single bytes - to be able to set a delay between two bytes
  • The SPI-Mode should be Mode 0
  • The problem is, the Aardvark adapter seems to set Mode 0 right before every transmission. SCK is changing from always idle high to Mode 0 (idle low) right before the original CS, the Aardvark adapter transmits the payload, deselects the slave, and then releases SCK back to idle high.
  • As I’m using a separate GPIO controlled CS, I cannot use Mode 0.

Here’s the diagram of what I need to fix:

 user's example of using aa_api_write


Response from Technical Support:

Thanks for your question! The project may be complex, but in this case the solution is simple: an external pull-up resistor.

In general, the Aardvark adapter starts driving the SPI clock only where a transactions occurs.  Until then, that signal may be in high impedance tri-state mode. With a pull-up resister attached from the SPI clock line to VCC, the Aardvark adapter will detect a logical ‘1’ and then start driving the SPI clock. You won’t need much current – a 100k resistor should do the job.

For more information, please refer to section Pin Driving of the Aardvark I2C/SPI Host Adapter User Manual.

Additional resources that you may find helpful include the following:

We hope this answers your questions. If you have other questions about our protocol analyzers or other Total Phase products, feel free to email us at, or if you already own one of our devices and have a technical question, please submit a request for technical support.

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