Real-Time Requirements | EasyDMA

[LeviRand]

Hello Nordic,

I am developing for a project that requires real-time accuracy. We are using ncs v2.9.1 Zephyr, an nRF54L15 and an IMU (w/ SPI interface) to acquire positional data. 

An observed issue is that we cannot reliably sample our IMU during BLE communications due to BLE interrupts. The interrupts prevent our main CPU thread and block us from calling spi_transceive_dt(). Because the IMU is tracking real-time data, any missed communications with the IMU cause significant accuracy loss in our system.

In theory, this should be solvable if we can use the DMA to cyclically service and buffer IMU communications. From the examples on the forum, I have found no other posts that contain this use case and as such no suitable answers. From my understanding the SPI peripheral is intrinsically using EasyDMA, but I cannot tell how to take advantage of this. Is it assumed while calling spi_transceive_dt() other threads can be run during this function call? If so, is there a way to schedule a SPI transceive that will not get interrupted by Bluetooth communication? 

Is this problem solvable using EasyDMA or otherwise?

Thank you,
Levi

[LeviRand]

Hi Edvin,

The bus is serviced every 5ms, so setting and unsetting manually is not ideal.

It looks like you can set multiple channels from the same event. I was thinking of the following:

First,

IMU Interrupt (GPIOTE)  ---> GPPI Channel --->  SPI Transmission Start

                                       ----> GPPI Channel ---> Chip Select High-to-Low (GPIOTE)

Then,

SPI Transmission End ----> GPPI Channel ---> Chip Select Low-to-High (GPIOTE)

                   

Would this work reliably?                

[Edvin]

That is up to you to test. The last one (hooked on the SPI Transmission End) is fine. The question is whether or not it is enough to set the CSN pin at the same time as you start the transmission. Is it enough for the SPI slave to react to it? Does it behave the way you want when you do it this way?

If not, you probably need to set it a little bit sooner. Perhaps you need to include a timer, first set the CSN, start the timer, and when the timer reaches a certain value, then start the SPI transmission.

Best regards,

Edvin

[LeviRand]

Ok, thank you for the instruction.

One final question: When testing the module, I did not do as the driver instructed. The driver states to leave "Chip select must be configured to @ref NRF_SPIM_PIN_NOT_CONNECTED and managed outside the driver."  Instead, I configured it to pin 69. See below.

#define SPI_CLK_PIN_201      65
#define SPI_MOSI_PIN_202     66
#define SPI_MISO_PIN_204     68
#define SPI_CS_PIN_205       69
#define SPI_INSTANCE_IDX     00
const nrfx_spim_t spiDev = NRFX_SPIM_INSTANCE(SPI_INSTANCE_IDX);

nrfx_spim_config_t spiConfig = {
        .sck_pin = SPI_CLK_PIN_201,
        .mosi_pin = SPI_MOSI_PIN_202,
        .miso_pin = SPI_MISO_PIN_204,
        .ss_pin = SPI_CS_PIN_205,
        .ss_active_high = false,
        .irq_priority = NRFX_SPIM_DEFAULT_CONFIG_IRQ_PRIORITY,
        .orc = 0x00,
        .frequency = NRFX_MHZ_TO_HZ(8),
        .mode = NRF_SPIM_MODE_0,
        .bit_order = NRF_SPIM_BIT_ORDER_MSB_FIRST,
        .miso_pull = NRF_GPIO_PIN_NOPULL,
        .ss_duration = 5UL,
        .dcx_pin = NRF_SPIM_PIN_NOT_CONNECTED,
        .use_hw_ss = true,  // TODO LR: Not sure if this should be true
        .rx_delay = 2UL,
    };

    // Perform Reconfigure
    nrfx_err_t err = nrfx_spim_reconfigure(&spiDev, &spiConfig);
    if (err != NRFX_SUCCESS) {
        printk("[SPI] FAILURE: Reconfiguration Failed\n");
    }

Despite this, the device seems to operate fine. When scoping the lines, the CS functions appropriately. Is there any consequence or negative side effect if I keep it this way? Why does this work?

Channel 4 (Yellow) - SPI CS

Channel 5 (Green) - IMU Interrupt

Channel 6 (Blue) - SPI MOSI

Channel 7 (Purple) - SPI MISO

[Edvin]

I guess the SPIS device that you are using accepts that the CS pin is set at the same time as the MOSI starts outputting data. Some devices uses this as a wakeup source to trigger some task, and is not ready to receive data before a certain amount of time has passed. 

How do you trigger the CS pin? Is it being done by the driver, or do you do it using the fork and PPI?

BR,

Edvin

[LeviRand]

Yes, that makes sense. my device only needs 40ns for CS setup/hold time. Upon closer inspection of the waveform there is a 60ns hold time.

 

For my code, the CS is being trigger by the driver. Is the CS being triggered (almost) at the same time as the MOSI why the documentation states to handle the CS outside the driver? Can I leave it like this?