clock input specification improvements

[sbourdeauducq]

Currently the the external clock selection on ARTIQ boards (KC705, ZC706, Kasli, Kasli-SoC) is inconsistent, limited in features, and difficult to use with firmware recompilations and reflashing required to change Si5324 settings on Kasli.

There are currently two clock settings, the rtio_clock config key used by KC705, ZC706 and Kasli, and the JSON options used by Kasli.

I propose replacing them in ARTIQ-7 with a single rtio_clock config key that can take values such as:

• ext0_synth0_10to125: 10MHz clock input to 125MHz RTIO clock, using a PLL (e.g. Si5324).
• ext0_synth0_100to125: same as above with 100MHz input.
• ext0_synth0_125to125: 125MHz clock input to 125MHz RTIO clock, through a PLL, which can filter jitter. The properties of the PLL would be board-dependent.
• ext0_bypass_125: 125MHz clock input to 125MHz RTIO clock, any PLL bypassed
• int_125: 125MHz RTIO clock, internally generated

If we ever have a board with several clock inputs, the input could be selected with ext0/ext1/ext2... Synthesizers (e.g. WRPLL vs. Si5324) could similarly be selected using synth0/synth1...

There can be board-dependent extensions for these strings. For example, PLL loop bandwidth could be selected using ext0_synth125to125_bw606 / ext0_synth125to125_bw200.

Any given board would only support its own set of strings.

[dhslichter]

Would it be better to have a series of key, value pairs indicating: clock input, synthesizer (incl. bypass), input frequency, output frequency, loop bandwidth? I just worry about the long concatenated strings above being fragile and unwieldy. Or is the idea that only a small subset of combinations will actually be supported, so making a small list of possible strings allows users to choose from among the only available combinations?

[sbourdeauducq]

Or is the idea that only a small subset of combinations will actually be supported, so making a small list of possible strings allows users to choose from among the only available combinations?

Yes, that's the idea. While the strings share a common format for easier understanding, each board would only support a limited set of clocking configurations and corresponding strings. The strings would make it easier to see what is supported and what is not.