Scott Dixon describes on his website how he used the Bus Pirate to connect to a Raspberry Pi:
The other day I found myself with a Rasberry Pi that I wanted to use but I had forgotten my FTDI UART cable. What I did have is my Bus Pirate v3.6 and I found it was pretty easy to use it’s transpartent UART bridge macro to connect to the Pi.
More details at 0x20 homepage.
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