Multiple SPI transfers, PPI, EasyDMA ArrayList, GPIOTE

RE: Multiple SPI transfers, PPI, EasyDMA ArrayList, GPIOTE

Torsten Robitzki — Fri, 01 Mar 2024 09:14:02 GMT

In the SPIM peripheral, there is an additional register, named LIST, where you can enable the feature. My understanding (of the still very vague description of the feature) is, that basically, the TXD.PTR and RXD.PTR are incremented by MAXCNT at the end of a transfer.

Basically, you can draw a timeline with all your required SPI transfers and assign a PPI channel for every point in time, where something interesting should happen. This would probably 3 times, the start of the SPI transfer (SPIM.TASKS_START), assertion of CS1, deassertion of CS1, assertion of CS2, deassertion of CS3, assertion of CS3, deassertion of CS3 and resetting the timer to. This are 10 points in time, which would require 10 capture / compare registers, which you do not have in any timer, so you need to use 2 timers. And then, you probably also need a 3rd timer to switch read buffers and to interrupt the CPU to do something with the collected data.

RE: Multiple SPI transfers, PPI, EasyDMA ArrayList, GPIOTE

Ars Morozov — Fri, 01 Mar 2024 08:44:54 GMT

Hi, thanks for answer.
Actually we have different sizes of buffer, but we can use for example maximum for all transfers. The question is how to use List feature? Is it in using flag for post increment of the buffer?

Regarding cs signal.For me it's clear how to use GPIOTE and timer for one transfer with one device. But if I have few devices, how we can configure system to automaticaly manages CS signals without CPU?

RE: Multiple SPI transfers, PPI, EasyDMA ArrayList, GPIOTE

Torsten Robitzki — Fri, 01 Mar 2024 08:14:48 GMT

I'm working on something similar. Basically, I use a timer to create the necessary timings to trigger the start of the SPI transfer. If you have similar sizes for the buffers, that needs to be transferred, you could use the LIST feature of EasyDMA.

For the CS signal, you could use the GPIOTE peripheral and the timer to assert and deassert the CS signal. In case, you need to update some registers, you could use the EGU peripherals to generate interrupts with relative high priority to do that. If you have more points in time, than capture compare registers, you could use an other timer and synchronize them.

EGU also comes handy, if you need to duplicate events or to combine (OR) events.