SPI_ISOLATE add-on board needs logic level conversion on CTRL line
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I just tested this out, and when using a 5V target, the CRTL line will be tied to 5V. If you want to control this with a GPIO off of the pi, then we will need to add logic level conversion from 3.3V to logic level of the MUX IC.
A work around I just tried out: I added a 10K pull down resistor to CRTL, so that it sits at 1/2 of VCC. In my use case, this puts it at 2.5V. It still effectively enables/disables the MUX, and I can use a GPIO off the pi (36 for me), to control the switch.
If this MUX ever gets baked into the MAIN (aka "HAT"), It' be better to put proper logic conversion between the GPIO and the CTRL line.
I revised the MAIN board to include the iso switch. It also converts the logic level of the pi GPIO to the target logic level.