how to get DMA to do scheduled short burst SPI

[Randy Lee]

I'm using an nRF52840 and SPI 15.3 here..

What I'd like to do is to have SPI write (at 10 Mbps) 8 transactions of 2 bytes each every 91uS and do that for say 512 times before telling me it's done (i.e. all controlled by DMA/hardware)

Is this possible on this chipset? Otherwise, I suppose I have to crank up a timer and run the bursts manually in the ISR?

[Håkon Alseth]

Hi,

What I'd like to do is to have SPI write (at 10 Mbps) 8 transactions of 2 bytes each every 91uS and do that for say 512 times before telling me it's done (i.e. all controlled by DMA/hardware)

SPIM can be used with up to 8 MHz, while the nRF52840's SPIM3 can do up to 32 MHz:

https://infocenter.nordicsemi.com/topic/ps_nrf52840/spim.html?cp=5_0_0_5_24_5#topic

 

Is this possible on this chipset? Otherwise, I suppose I have to crank up a timer and run the bursts manually in the ISR?

You can setup a high-speed timer instance to trigger every 'n' us, together with a PPI channel, and use EasyDMA Array List (https://infocenter.nordicsemi.com/topic/ps_nrf52840/easydma.html?cp=5_0_0_3_5_1#arraylist) feature, but you will have to count the amount of transactions in order to reset the auto-increment.

Here's an example for NCS that does this (fairly low-level): https://github.com/too1/ncs-spi-arraylist-example/blob/master/src/main.c

 

Kind regards,

Håkon

[Randy Lee]

Great!  Thanx! This will get me down the right direction...