Guzunty Pi loading gz_test.xsvf problem

The very first time I tried to load an xsvf file to Guzunty Pi, I had bad luck because the CPLD was faulty, and it took me hours troubleshooting to accidentally find that the CPLD is faulty.

This morning I had a similar experience.  When I tried to load the third GPi, I found that I could load the xsvf file, implying that the CPLD and JTAG should be more or less working.  But then the gz_test.c could not run.  I thought might be the SPI wiring had a problem.  So I removed cable by cable during testing.  Finally I found that even the first GPi could not load the gz_test.xsvf file.

It took me 2 hours to find the cause.  Since the CPLD is configured as 16o8i with output and input hardware connected to the CPLD.  If I loaded the gz_test which is 24o, then even I did not switch on the hardware connected to the old 8i ports, the high level by high impedance (LV245) would output to CPLD which has all ports output, therefore causing a crash.

It was by accident that I found the CPLD very hot to my fingers, then I realized that I made a careless mistake.  I disconnected the input to the CPLD, then everything works again.

The lesson learned is that when configuring CPLD ports as input, disconnect everything from the port first.

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