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I'm developing a USB DAC using XMOS XHRA-2HPA as USB->I2S interface. XHRA-2HPA is a sort of highly specialized uC which requires a firmware and configuration data to be stored on external SPI flash chip.

I want to install a service connector on the board to be able to reprogram this flash. First I need to be able to power up flash chip without powering up the entire system. It can be done with a sort of manual jumper. But I'm in doubt if unpowered USB interface chip connected to the same SPI bus in parallel can affect proper bus operation.

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Why can't you still power the rest of the board up while you are programming the audio signal processor via the service connector? You could hold your processor in reset so it doesn't interfere with anything -- all pins should look like inputs.

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