Count Clock Pulses with PPI + Gpiote Then Change GPIO Depending on Count (NRF52810)

Question

I'm attempting to use PPI and Timer2 on the NRF52810 to count the clock pulses coming out of the SPIM driver. As there is no way to do this internally (as far as I know) I tied another GPIO pin to the clock pin. The idea being here is that I want to toggle a separate "latch" pin at different places during the serialization process. With the latch pin active at different clock cycles, it signifies a different command in the LED driver I'm using. Ex. Latch pin high for 16 bits is one while only active for the last two bits is a completely different command. 
 I've had some success with but not 100%. Here's some code: 
 This is for initializing PPI, gpiote pins and timer (in counter mode) uint32_t err;

 // change to coutner
 nrfx_timer_config_t tmr_config = NRFX_TIMER_DEFAULT_CONFIG;
 tmr_config.mode = NRF_TIMER_MODE_COUNTER;
 tmr_config.bit_width = NRF_TIMER_BIT_WIDTH_32;

 // TODO: Configure counter. No timeout handler.
 err = nrfx_timer_init(&m_counter, &tmr_config, timer_handler_count);
 VERIFY_SUCCESS(err);

 // Input & output configuration
 nrfx_gpiote_in_config_t in_config = NRFX_GPIOTE_CONFIG_IN_SENSE_LOTOHI(true);
 nrfx_gpiote_out_config_t out_config = NRFX_GPIOTE_CONFIG_OUT_TASK_HIGH;

 // Init GPIO pins
 err = nrfx_gpiote_in_init(LIS2DH_INT_PIN, &in_config, clk_in_pin_handler);
 VERIFY_SUCCESS(err);

 err = nrfx_gpiote_out_init(LED_TRANS_PIN, &out_config);
 VERIFY_SUCCESS(err);

 // Enable tasks
 nrfx_gpiote_in_event_enable(LIS2DH_INT_PIN, false);
 nrfx_gpiote_out_task_enable(LED_TRANS_PIN);

 // Configure GPIO high on counter compare event
 err = nrfx_ppi_channel_alloc(&m_latch_in_channel);
 VERIFY_SUCCESS(err);

 // Configure output after counter compare
 err = nrfx_ppi_channel_alloc(&m_latch_out_channel);
 VERIFY_SUCCESS(err);

 // Assign the channel, input event, then counts the timer
 err = nrfx_ppi_channel_assign(
 m_latch_in_channel, nrfx_gpiote_in_event_addr_get(LIS2DH_INT_PIN),
 nrfx_timer_task_address_get(&m_counter, NRF_TIMER_TASK_COUNT));
 VERIFY_SUCCESS(err);

 // Also capture the register
 err = nrfx_ppi_channel_fork_assign(
 m_latch_in_channel,
 nrfx_timer_task_address_get(&m_counter, NRF_TIMER_TASK_CAPTURE0));

 // Sets the state of the trans pin after count compare
 err = nrfx_ppi_channel_assign(m_latch_out_channel,
 nrfx_timer_compare_event_address_get(
 &m_counter, NRF_TIMER_EVENT_COMPARE0),
 nrfx_gpiote_out_task_addr_get(LED_TRANS_PIN));
 VERIFY_SUCCESS(err);

 return NRF_SUCCESS; 
 Then the code below is used to start the enable the counter, enable PPI before writing the SPI commands: 
 // Enable these channels
 nrfx_ppi_channel_enable(m_latch_in_channel);
 nrfx_ppi_channel_enable(m_latch_out_channel);

 // With reset.
 nrfx_timer_compare(&m_counter, NRF_TIMER_CC_CHANNEL0, 5, false);
 nrfx_timer_enable(&m_counter);
 nrfx_timer_clear(&m_counter);

 // The data!
 static uint8_t data[] = {0xaa, 0xaa};

 nrfx_spim_xfer_desc_t xfer_desc = NRFX_SPIM_XFER_TX(data, 2);

 // Transfer test 16 bits
 nrfx_spim_xfer(&spi, &xfer_desc, 0); 
 If I fork the assignment for m_latch_in_channel I get 15 ticks (when it should be 16). If I don't, I get zero or the value that's reflected in nrfx_timer_compare(&m_counter, NRF_TIMER_CC_CHANNEL0, 5, false); no matter how many clock pulses there are. 
 I'm stumped to say the least. Any insight is appreciated!

haakonsh · Accepted Answer

nrfx_timer_task_address_get(&m_counter, NRF_TIMER_TASK_CAPTURE0)); I think you'll need to use another capture register since you're using capture/compare register 0 for the compare event. What I think is happening is that the compare register is overwritten at every clock cycle by the capture task. Why do you need to capture the timer value anyways? It will trigger a compare event when it reaches the value of the capture/compare register 0.

Count Clock Pulses with PPI + Gpiote Then Change GPIO Depending on Count (NRF52810)

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