Power-up considerations when connecting external components to the XS Boards
Q:
Would it be possible to damage the XS Board by leaving an external cicuit connected (and possibly powered) before the FPGA is configured or while downloading a new configuration i.e. before putting the microcontroller and RAM pins into the high impedance state?
A:
This should be OK because the FPGA pins are in a high-impedance state during the configuration process anyway.
Q:
Even if the trainer board is switched off, various pins on the XS40 would be shorted via common grounded switches etc.
A:
This will be OK as long as none of the pins that are held high or low are involved in the configuration process (such as DIN, CCLK, /PROGRAM, etc.).
Q:
Also, would there be any problem if the XS board was switched off while the external circuit was still powered?
A:
This might be a problem if there are protection diodes on the FPGA pins: any logic high from the external circuit would be dragged low to the grounded Vcc of the FPGA through the diode.