Simple SPI Tests

spitest is a trivial Python tool for interacting with the SPI device code in the hello_world test program running on top_earlgrey on the FPGA board.

The same FTDI interface is used for SPI as for JTAG, so this tool cannot be used at the same time as an OpenOCD debugger. The top_earlgray_nexsysvideo pin mux has been modified to use one of the FTDI GPIOs to select between the two interfaces, so it is possible to dynamically switch between the tools. This status bit is also presented as GPIO[16] (which was previously unused).

The example commands assume $REPO_TOP is set to the toplevel directory of the repo.

Setup

If packages have not previously been installed you will need to set a few things up. First use apt and pip3 to install some required packages:

$ sudo apt-get install libusb-1.0
$ pip3 install --user pyftdi

You also need the udev rules to be set correctly by following the instructions for setting up the Xilinx tools.

SPI/JTAG selection GPIO

The tool can be used to test the SPI/JTAG selection gpio. This is done by providing the --flippy or -f argument which causes the selection to be flipped 10 times with 2 second pauses between. The hello_world program will see the bit flip and print a message.

$ cd $REPO_TOP/util/simplespi
$ ./spitest.py -f

SPI Protocol Test

The SPI protocol used in the test is a simple exchange of 4-byte messages. When data is received from the SPI Device interface the hello_world program will receive and print it out on the UART. The first four bytes have their bottom bit inverted and are returned in the next SPI transaction. If the sender is faster than the FPGA then messages are aggregated in the receive buffer and will appear as a single long message to hello_world which will only return a single 4-byte message.

A simple test should therefore just use 4 characters.

$ cd $REPO_TOP/util/simplespi
$ ./spitest.py 1234
$ ./spitest.py 5678

Long messages will be padded to a multiple of 4 bytes before being sent, and there may be some garbage return messages.

$ cd $REPO_TOP/util/simplespi
$ ./spitest.py the quick brown fox jumps

Messages of any length (the length given is used even if not a multiple of 4) may be generated by the tool. The message is formed from repeating the ascii characters 0123456789abcdef (to fill the required length) and sent, and there may be some garbage return messages.

$ cd $REPO_TOP/util/simplespi
$ ./spitest.py -l 120