RE: SAADC acquisition time jitter even when used with PPI

Florian Zierer — Tue, 04 Jan 2022 09:48:29 GMT

Hi Einar,

I tried to reproduce it but had problems because the stripped down firmware worked as expected.

Finally I realized, that the SAADC_CONFIG_LP_MODE was different.

When SAADC_CONFIG_LP_MODE = 1, I see the jitter in the events and also the acquisition time is increased to ~32µS. When disabling low power mode, acquisition time is ~21.6µs and no jitter can be measured.

RE: SAADC acquisition time jitter even when used with PPI

Einar Thorsrud — Wed, 22 Dec 2021 14:13:36 GMT

Hi,

I do not see how the acquisition time should be affected by using SoftDevice or not, this should be unrelated. As you write, PPI is unelated to the CPU, and also the SAADC sampling (once started it is independent and writes data to memory using DMA without CPU intervention).

If you have not ensured that the HFXO always runs, the SoftDevice will enable it whenever needed, and disable it in other cases. When disabled, the HFINT (RC oscillator) is used instead. This should only give you a variation of worst case ±8 % though, so it also cannot explain what you see.

Can you upload a full example project that I can run on a DK to test this on my end, both with and without SoftDevice?

PS: I will be OoO during the holidays, and DevZone is not fully staffed so it may take some time before you get replies in this thread.