Tutorial on debugging SPI with numerical scope

This tutorial explain how to debug SPI message exchanges using a numerical scope (refer to http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus for more details on this protocol). It follows the previous one ('how to debug I2C master/slave exchange').

1. Software configuration

  • Use a GPIO associated to /CS (master mode) to trigger the beginning of the SPI read/write operation.
  • The SPI operation to debug (write and read) is placed into a single call to the method SPI::write()

2. Probes assignment

  • CH1: On /CS trigger (Yellow signal)
  • CH2: On SCK line (blue signal)
  • CH3: On SDO line (pink signal)
  • CH4: On SDI line (green signal)

The screenshot below shows a SPI frame sent to a Microchip Digital Potentiometer. The 4 scope channels are used. As expected in a such architecture (mbed as master), SDI line is not used,


3. Scope settings

  • All channels are set on 1V/div in 3.3V power supply (or 2V/div in 5V power supply),
  • Time base is set on 10 us/div,
  • The horizontal trigger threshold level is set on the left position,
  • Triggering is set on CH1, in sample mode, on falling edge, level is set to 50%

4. Signal acquisition

  • Launch the application in debug mode
  • Captured signals shall be displayed by the scope
  • Press on Single Seq to capture one SPI frame
  • Reduce the time base around 2.5us/dev to visualize different part of the SPI frame:
  • External trigger (/CS) is set to 0 to enable the slave, because of mode (0,0) (Yellow signal)
  • The clock is started when data is written into the SPI buffer (see NXP datasheet LPC1769/68/67/66/65/64/63, Clause 7.17 SPI serial I/O controller)
  • External trigger (/CS) is set to 1 to enable the slave (Yellow signal)

5. Frame analyzing

  • Activate Cursor functionality on CH2 (SCK line/blue signal)
  • Verify the SCK frequency. In this case study, it should be around 1MHz
  • Place the cursor to each SCKhigh level and read the SDI value (pink signal)
  • Compare capture timing diagram with expected timing diagram from SPI IC datasheet

The screenshot below shows the SPI frame with the time base set to around 2.5us/dev,



3 comments on Tutorial on debugging SPI with numerical scope:

24 Jan 2013

If you want to debug digital signals the logic from Saleae is a nice thing : http://www.saleae.com/logic/features

You can also decode I2C, SPI and serial protocols. The max sampling rate is 24MHz. Only fast SPI will be to fast on the mbed.

28 Jan 2013

Hi Peter,

Thanks for this information. One of my colleague got it. It looks really good tool!

28 Jan 2013

The Logic8 has a max samplefreq of 24MHz, but the Logic16 can sample 2 channels at 100MHz, 4 channels at 50MHz, 8 channels at 25MHz, or all 16 channels at 12.5MHz.

It works real well for decoding I2C or SPI.

Here is picture used for SPI debugging


Here is picture used for I2C debugging


It will detect start, stop and display databytes as hex, ascii etc

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