What is the recommendation for how big buffers should be to drive out a ~30-50MHz clock out of Caravel

gpio_analog[]

pads? Is

sky130_fd_sc_hd__clkbuf_8

strong enough to drive whichever capacitance is in the pad, or should I use more than that?

You will need to use like 400 of them. Most likely you want to drive io_out and io_oeb instead.

how much cap should I assume on one of those gpio_analog[] pads? A few pF ?

at least 2pf + 150 ohms

my circuit is a VCO running at around 3GHz, followed by a few divide-by-2 which divide the frequency down by 128 or 256 to get around ~30MHz and ~15MHz respectively. The idea is that on the bench I want to measure the ~30MHz frequency and multiply it by 128, and roughly that should give an indication that the VCO is running at ~3.2GHz or something like that.

Depends. It might or might not be able to. If wire is long, then no. If wire is short, then answer is still "depends"

yes I mean I want to drive out the ~30MHz out (3.2GHz divided by 128) but I won't attempt to drive out the 3.2GHz.

So how you divide the output if not a flip flop?

oh I see your point regarding the flip flop. I have designed a custom divider to take in the 3.2GHz straight from VCO, that divider can run above 3.5GHz in my post layout sims. Then after that I keep dividing down and I keep lowering the frequency with more dividers. Until I get down to divide-by-128 and 256

Oh okay, custom divider is great. Try to use biggest buffer you can and directly connect it to pad as close as possible. It should be able to drive io_out. Keep in mind without setting io_oeb low, it will not be active

which is the ~30MHz or ~15MHz that I want to drive out of Caravel pad. Then on the bench I will try to see those ~30MHz and multiply by 128 to guess how many GHz my VCO is running at internally

@User: If the output is for verification only, you should know that I've been able to measure the frequency of a 100MHz signal on the digital I/O passed through the standard digital output. It's not full swing, definitely (!), but it can still be measured. I have an HP 5385A frequency counter which was able to pick up the signal with no problem. And I could read it from an oscilloscope; it just comes out as a low amplitude triangle wave.