What is Reddit's opinion of Adafruit FT232H Breakout - General Purpose USB to GPIO+SPI+I2C?
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By popularity on Reddit, this product is:

#10191 in Computer Internal Components

FT232H (1 serial port) FT2232H (2 serial ports) is a good start. You can get a dev board like this one from Adafruit

Ebay also has FT232H/FT2232H dev boards.

These ICs are commonly used to implement JTAG debuggers which are very similar to the SPI protocol.

They're generally well supported by Windows and Linux as well

I use a Adafruit FT232H Breakout dev board connected to the debug-enabled USB port and a PL2303TA USB to TTL Serial Cable plugged into another Linux computer. On this second computer, after running modprobe usb_debug, start minicom or screen, you can see the coreboot debug logs assuming coreboot is built with the debug config parameters specified with the in the tutorial.

$100 - $200

< $50