Interface speeds should be more restricted

[jonas-schievink]

The JLinkGDBServer will reject speeds below 4 kHz saying they're too low. After setting a 1 kHz speed I've observed the generated clock to be extremely bad, so we should forbid that too:

[jonas-schievink]

It turns out the 1 kHz speed works perfectly fine despite the highly irregular clock signal. The issue in the above trace is that the parity bit was wrong, so the device doesn't reply.

I think we should still mirror what the official tools are doing and forbid speeds below 4 kHz though. There might be other issues and reasons to forbid this; perhaps some J-Link will completely fail to generate lower clocks.

[jonas-schievink]

We should also figure out what the max speed is, and figure out what happens if you set a speed above that. I suspect the max speed is just the base frequency divided by the min. divisor, which we can already fetch, but this isn't documented and needs confirmation.

[Nable80]

We should also figure out what the max speed is, and figure out what happens if you set a speed above that. I suspect the max speed is just the base frequency divided by the min. divisor, which we can already fetch, but this isn't documented and needs confirmation.

"Reference manual for J-Link USB Protocol" (RM08001) explicitly states: "Maximum JTAG speed 12 MHz" and "The transmission speed is fixed to 12MHz and allows transmission of up to app. 1MByte/sec". I'm reading "RM08001-R7" now, newer revision may contain more details.