Confusion with List in EasyDMA

[Antonio]

Hi,

The samples for using list are BUFFER = 4, but it does not specify is that is the only size that works, and what value do you place on NRF_SPIM0->TXD.LIST

In my case I need a buffer size of 8, and 46 samples. I implemented the following, among other code, which does not seem to have any effect. Am I missing something?

#define BUFFER 8

typedef struct ArrayList {
	uint8_t buffer[BUFFER];
} ArrayList_type;

ArrayList_type MyArrayList[46];
ArrayList_type MyReceiveList[46];

NRF_SPIM0->TXD.PTR    = (uint32_t)MyArrayList;
NRF_SPIM0->TXD.MAXCNT = 46;

NRF_SPIM0->RXD.PTR    = (uint32_t)MyReceiveList;
NRF_SPIM0->RXD.MAXCNT = 46;

nrf_spim_event_clear(NRF_SPIM0, NRF_SPIM_EVENT_ENDTX);
NRF_SPIM0->TXD.LIST = 8;  //I tried as per documentation to only place a 1 here.
NRF_SPIM0->RXD.LIST = 8;
nrf_spim_task_trigger(NRF_SPIM0, NRF_SPIM_TASK_START);	

[Tien]

Hi, Antonio

The RXD.LIST only used to enable/diasble the EasyDMA list. It means the RXD.PTR will automatic add a offset of RXD.MAXCNT value.

And EasyDMA List will not check the PTR boundary, you should do the check in SPI_IRQHandler.

In your case, the config should like this:

uint8_t buffer_list[46][8];
RXD.PTR = buffer_list;
RXD.MAXCNT = 8;
RXD.LIST = 1;

SPI_IRQHandler()
{
    if ( RXD.PTR < bufferlist boundary )
       start next transmission.
}

[Antonio]

I have done it this way too. What I need is to transmit 46 * 8 bytes without an interrupt. In my example I am doing [8][46] and List = 1 and it does not send the 368 bytes. How does the SPIM knows that it has to send 46 bytes 8 times or viceversa?

[Tien]

SPIM doesn't know how many times it has to repeat. It only understands where the data (PTR) is and how many bytes (MAXCNT) to transfer. And when EasyDMA list is enabled, it will automatic change the PTR for next transmission. Then you can just start the next transmission without manually change the PTR .

[Antonio]

Thank you Tien for the comment. When you say that I start the next transmission, is at the interrupt when the END event was triggered?, so in my example, if MAXCOUNT is 8, I will get the interrupt after 8 bytes have been sent and I will just restart for the next batch?

[Tien]

1.You should start the next transmission in IRQHandler, and don't forget check the PTR boundary. 2.Yes, if MAXCOUNT is 8, you will get the interrupt after 8 bytes have been sent.

So, if you need to send 46 bytes 8 times, you can set MAXCNT = 46. Then you start the first transmission , after that you start the next 7 times transmisson in IRQHandler.

[Tien]

Yes, you are right. Using PPI+GPIOTE+TIMER pure periphs will have best power efficiency. I have thought about it and found SPI CS pin will need a little more tricks. e.g.If there is no Ext_INT( such as Data_Ready ) for triggering the transmission, you will need another TIMER CC[x] = 1 to trigger the next transmission and toggle CS pin.