Hello everybody, does anybody know what the maximum external clock frequency that we can use is? Because on GitHub, I found that it is higher than 60 MHz. Then I read that 7 months ago, Tim Edwards answered that officially it was 25 MHz, but that we can get more by raising the 1.8V supply. Finally, I read in the Caravel documentation (Caravel and Caravan use the same SoC) that if we use the PLL, the external clock * divisor should be between 90 MHz and 214 MHz, and I don't know what frequency to set as the maximum.

Hi! I am not sure what the maximum frequency is that Caravel can run on. But for the PLL you need to make sure that the internal frequency is in the possible range. On an actual chip this range is a bit different than originally expected, see [1]. Since the input clock is 10MHz, you need to set the feedback divider (see [2]) to a value to get into that possible range (10MHz * feedback = 55 to 105 MHz). Then by setting the output dividers you can reduce this frequency to get the core and secondary clock. So if you use a tool like [3], make sure the internal pll frequency falls within this range. [1] https://github.com/efabless/caravel/pull/537 [2] https://caravel-harness.readthedocs.io/en/latest/housekeeping-spi.html#housekeeping-spi-registers [3] https://github.com/kbeckmann/caravel-pll-calculator

As far as i know Max frequency was 50 MHz

Fantastic, thank you !

@CELINA BOSSA: There are various numbers depending on what you're talking about. The rated specification for the SkyWater GPIO pad outputs is 50MHz. Inputs can go somewhat higher; I forget what the official spec is, but it's at least 60MHz. But that's just the pads. If you're asking about how fast you can run the part without the processor failing, then my statement about 25MHz is the rating for the processor based on what the timing specification was for synthesis (actually it's higher; it changes every time we resynthesize the core, and the 25MHz number sounds about right for some of the earlier versions, like MPW-2); that rating is across all corners. Under typical conditions (typical temperature and voltage), the current processor (since MPW-6) runs up to about 50MHz. That's a measured value, but I was not exercising many features of the processor, so it's possible that some functions break at that frequency. And as I mentioned, you can get the processor to run faster by raising the 1.8V supply (up to 2V is within spec).