Stroboscopic ADC- follow up, VERY LOW capacitors

 I've spent a lot of times on this due to a silly bug. The sillier it is , the harder it is to find out.

Now, it works fine.

For reference, we sample a cyclic waveform, repeating every N cycles by sampling it every N+x cycles.

That gives an apparent sampling period of x.

In our case, for the STM32/GD32, the higher sampling frequency is 72M/PWM divider (1 or 2) 

(96M for the GD32). That gives an equivalent smallest sampling period  of 14nS (10 nS for GD32), compared to the 500 ns maximum using the "regular" method.

Let's inject that into our capacitor measurement stuff and check the results :

C	Me	M328
0	N/A	N/A
3	2,54	N/A
10	9,65	N/A
20	19,4	N/A
47	47,6	52
56	55,6	60
68	70	75
100	96,8	101
220	231	232
310	323	330

The first column is the value written on the tin

The second column is the value measured by our tester

The 3rd column is the value measured by a M328 transistor tester (cheap chinese version)

Not too bad, and we can go down to ~ 2pF.

The main drawback of that method is that it is slow.

E.g. if we  use a  waveform at 1khz and want 512 samples, it takes 0.5s to collect them.