There are usually two ways to read a data sheet. The first is to take each specification at face value, assuming the engineers have considered everything and presented each number as the absolute limit that will prevent the Magic Smoke from escaping. The other way is to throw away the spec sheet and try whatever you want, assuming the engineers have played it as safe as possible.
This last case seems to have been the motivation behind pushing a very small path, WAY beyond what the datasheet says is possible. According [SM6VFZ], the ATtiny817 specs show that the 12-bit timer/counter D (TCD) should be limited to a maximum frequency of 32 MHz, above which one is supposed to use the counter’s internal prescaler. But using a 10MHz precision frequency generator as an external clock, [SM6VFZ] found that inputs up to slightly above 151MHz were countable with 1Hz precision. Above that point things started to drift, but it’s still a pretty good performance of something tinkered with on a rating board in a decidedly sub-optimal way.
We imagine this result could lead to some interesting projects, as the undocumented limit of this timer puts it well within range of several amateur radio assignments. Even if it doesn’t turn out to be useful, that’s okay – just seeing how far things can be taken is also cool. And it’s not like it’s the first time we catch [SM6VFZ] Nor can persuading an ATtiny to do unusual things.