Add basic support for JTAG over FTDI in bitbang mode #118
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This does make some small architectural changes to the pif layer, but I hope it's acceptable.
Assorted notes
jtbitbang_set_fast_baud
function is required, but the mehfet driver used it and it seemed like it was neededFurther work will be to look into MPSSE for the FT4232H to increase speed more. The mspdebug backend seems inefficient (though reliable), especially since every state change, even those that can reasonably be batched, needs to written out over USB. I tried batching all writes unless TCK or RST was strobed, but it broke device identification - I suspect there's some early step that requires pin toggles to be synchronous.
Programming in general on the MSP430 architecture seems hugely inefficient. Each word takes over 16 bytes of instructions over the wire. I have yet to verify this, but I suspect the MSP-FET might upload a small bit of assembly to locally program data from RAM, then use the auto-incrementing RAM load feature to program the data in chunks.