SPI master and DMA

RE: SPI master and DMA

MartinBL — Tue, 25 Sep 2018 13:01:38 GMT

Hi,

Unfortunately we don't have any examples for SPIS with DMA.

RE: SPI master and DMA

PartTimeCode — Tue, 25 Sep 2018 07:06:15 GMT

Hi, I am using SDK 15.0 with PCA10040 Board. I am trying to use DMA for SPI slave instance. Is there any example in SDK that can guide me to start? I found many examples of SPIM DMA and Arraylist but none on SPIS.

RE: SPI master and DMA

erltot — Tue, 19 Sep 2017 08:08:44 GMT

Great, thanks

RE: SPI master and DMA

MartinBL — Tue, 19 Sep 2017 07:53:40 GMT

You only need it if you want to do multiple transfers triggered on some sort of event and don't want to use the CPU to set up each transfer. Such events can be generated by timers, external signals, etc. If you only need one transfer at the time I suggest that you just enable the event handler and let the CPU sleep until the event handler wakes it up after starting a transfer.

We don't have any good examples in the SDK, but I posted an example in the comment section here that might be useful.

RE: SPI master and DMA

erltot — Tue, 19 Sep 2017 07:41:19 GMT

So i need to set up PPI then? My app is built on ble nus peripheral example. Is there any tutorials to set up ppi for beginners?

RE: SPI master and DMA

MartinBL — Tue, 19 Sep 2017 06:41:08 GMT

Hi,

If you don't use an event handler the SPI driver is waiting in a while() loop until the transfer is completed and you get an SPI transfer END event.

In SDK 14 this happens at line 575 in nrf_drv_spi.c:

if (!p_cb->handler)
{
    while (!nrf_spim_event_check(p_spim, NRF_SPIM_EVENT_END)){}
    if (p_cb->ss_pin != NRF_DRV_SPI_PIN_NOT_USED)
    {
        nrf_gpio_pin_set(p_cb->ss_pin);
    }
}

So yes, the CPU is waiting for the transfer to complete while wasting time and current. As you say, the point of DMA is to let the CPU sleep or do other things, but you are not actually using DMA if you don't use the SPIM as explained in the Advanced Usage section of the SPI driver documentation.

RE: SPI master and DMA

erltot — Mon, 18 Sep 2017 12:51:31 GMT

I am using the SPI Master driver from the NRF5 SDK (nrf_drv_spi.c?) So if I understand you right the CPU is busy clocking out the bytes on the SPI? I thought the point of DMA was to let the CPU sleep or do something else instead of driving the SPI bus on a much slower frequency?

RE: SPI master and DMA

endnode — Mon, 18 Sep 2017 12:42:38 GMT

Any particular reason why you don't use SPI Master driver from nRF5 SKD? It shouldn't have any flaws for battery powered app... also formulation "what is the CPU doing while waiting for the SPI slave to clock in the bytes" doesn't make sense because SPI Master is not waiting for anything normally, it must driver CLK signal so slave can output data on MISO line. If you have another (usually called "interrupt") line from Slave to Master which signals that now Slave want's to talk (so Master must provide CLK and listen on MISO) then simply put the MCU to POWER ON SLEEP and wait for GPIOTE interrupt...)