ADC + SPI + PPI + TIMER - 500khz+ data rate required

[jtf]

I have a device on my desk that performs the data transfer I need, so I know it's possible. I've attached a screenshot of the successful data transfer.

I tried to replicate the setup using a 2-byte buffer with PPI and Timer1. On the oscilloscope, the signals look similar, but I'm having issues processing the data. The SPI event handler (in this case) needs to be called every 2 microseconds to transfer data to a larger buffer. 

Do you have any suggestions on how to approach this? Below is the code that transfers the data but loses all the data at the same time.

nrfx_err_t err_code;
    uint8_t ppi_channel1, ppi_channel2;

    nrfx_timer_t timer_inst = NRFX_TIMER_INSTANCE(1);
    timer1_init(&timer_inst);

    // Allocate PPI channels
    err_code = nrfx_gppi_channel_alloc(&ppi_channel1);
    if (err_code != NRFX_SUCCESS)
    {
        LOG_ERR("PPI channel allocation error for channel 1");
        return;
    }
    err_code = nrfx_gppi_channel_alloc(&ppi_channel2);
    if (err_code != NRFX_SUCCESS)
    {
        LOG_ERR("PPI channel allocation error for channel 2");
        return;
    }


    uint8_t task_channel;
    err_code = nrfx_gpiote_channel_alloc(&task_channel);
    if (err_code != NRFX_SUCCESS)
    {
        LOG_ERR("Channel alloc failed: %d", err_code);
        return;
    }

    nrfx_gpiote_output_config_t config = NRFX_GPIOTE_DEFAULT_OUTPUT_CONFIG;
    nrfx_gpiote_task_config_t task_config = {
        .task_ch = task_channel,
        .polarity = NRF_GPIOTE_POLARITY_TOGGLE,
        .init_val = NRF_GPIOTE_INITIAL_VALUE_HIGH
    };

    err_code = nrfx_gpiote_output_configure(ADC_CS_PIN, &config, &task_config);
    if (err_code != NRFX_SUCCESS)
    {
        LOG_ERR("Output configure failed: %d", err_code);
        return;
    }

    nrfx_gpiote_out_task_enable(ADC_CS_PIN);


    uint32_t timer_addr = nrfx_timer_compare_event_address_get(&timer_inst, NRF_TIMER_CC_CHANNEL0);
    uint32_t cs_set_addr = nrfx_gpiote_set_task_address_get(ADC_CS_PIN);
    uint32_t cs_clear_addr = nrfx_gpiote_clr_task_address_get(ADC_CS_PIN);
    uint32_t spim_end_event_addr = nrf_spim_event_address_get(spim_inst.p_reg, NRF_SPIM_EVENT_END);
    uint32_t spim_start_task_addr = nrf_spim_task_address_get(spim_inst.p_reg, NRF_SPIM_TASK_START);
    
    nrfx_gppi_channel_endpoints_setup(ppi_channel1, timer_addr, cs_clear_addr);
    nrfx_gppi_fork_endpoint_setup(ppi_channel1, spim_start_task_addr);
    nrfx_gppi_channel_endpoints_setup(ppi_channel2, spim_end_event_addr, cs_set_addr);
      
    err_code = nrfx_spim_xfer(&spim_inst, &xfer_desc, NRFX_SPIM_FLAG_HOLD_XFER| NRFX_SPIM_FLAG_REPEATED_XFER);
    if (err_code != NRFX_SUCCESS)
    {
        LOG_ERR("Initial SPI transfer failed: %d", err_code);
        return;
    }

[Elfving]

Hello,

I have a device on my desk that performs the data transfer I need, so I know it's possible. I've attached a screenshot of the successful data transfer.

Could you expand a bit on this part? What is this device, and what protocols is it using to do this data transfer?

Or is the idea that you do not know what it is doing precisely?

Regards,

Elfving

[jtf]

The device (which has an nRF52840) is communicating with the ADC using 32MHz SPI. The yellow trace represents the clock signal, the green trace shows the data, and the blue trace is the CS pin, which needs to be toggled between each 16-bit transfer. I’m not sure what model the ADC is, but the one I’m using requires the CS pin to be toggled.

I am studying the NRFX_SPIM_FLAG_RX_POSTINC flag to see if it could help me here.

[Elfving]

Still not sure if I understand.

Do you want to achieve the same thing with just PPI, without using an SPI driver?

[jtf]

Any solution is acceptable as long as the 500 kHz data rate is maintained. The code I posted "works," but it only stores the data in 2-byte chunks, essentially losing all of it because there is no CPU time to copy it to a larger buffer.

[Elfving]

Well you can get more than that with SPI, so I guess that is an option. If you really want a high datarate, you can also look into QSPI.

Regards,

Elfving