Programming over USB

[thatoddmailbox]

Currently, the board requires an external JTAG adapter to program the flash. It would be nice if some sort of serial bootloader was implemented, like on Arduino, so that the built-in USB serial adapter could be used to program the chip. There already is support for using a bootloader (the HiFive has one present at the start of SPI flash at 0x20000000, and the linker script is set up to build user code at 0x20400000), but the existing bootloader is only used to allow the user to reset the FE310 before running the program, just in case there's a 'bad' program that puts the chip in some weird state that JTAG can't rescue it from.

The HiFive solves this by using an FT2232 USB-to-JTAG chip, but that currently costs $6.71 in quantity 1 from DigiKey, vs. the ~$0.50 for the Holtek part currently used, and there doesn't seem to be any cheaper alternative USB-to-JTAG chip.