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pulseIn and pulseInLong implemented using hw peripherals: NRF_TIMER2, PPI_CHANNEL and GPIOTE events
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‎cores/arduino/wiring_pulse.cpp

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#include "Arduino.h"
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#include <hal/nrf_timer.h>
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#include <hal/nrf_gpiote.h>
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#include <hal/nrf_gpio.h>
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#include <hal/nrf_ppi.h>
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#include "nrfx_gpiote.h"
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#include "nrfx_ppi.h"
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/* Hot encoded peripherals. Could be chosen with a more clever strategy */
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#define PULSE_TIMER (NRF_TIMER2)
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#define TIMER_FIRST_CHANNEL (NRF_TIMER_CC_CHANNEL1)
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#define TIMER_SECOND_CHANNEL (NRF_TIMER_CC_CHANNEL2)
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#define TIMER_FIRST_CAPTURE (NRF_TIMER_TASK_CAPTURE1)
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#define TIMER_SECOND_CAPTURE (NRF_TIMER_TASK_CAPTURE2)
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#define TIMEOUT_US (0)
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/* GPIOTE configuration for pin PPI event */
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static nrfx_gpiote_in_config_t cfg =
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{
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.sense = NRF_GPIOTE_POLARITY_TOGGLE,
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.pull = NRF_GPIO_PIN_NOPULL,
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.is_watcher = false,
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.hi_accuracy = true,
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.skip_gpio_setup = true // skip pin setup, the pin is assumed to be already configured
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};
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/*
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* This function enables the pin edge detection hardware and tries to understand the state of the pin at the time of such activation
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* If the hardware detection event is enabled on an edge of the pin, it's not possible to understand if that edge would be detected or not
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* In such case, TIMEOUT_US constant is returned to indicate this extreme condition
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* Else, if the function is able to understand the state of the pin at the time of activation, it returns the index of the desired pulse
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*/
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static uint8_t measurePulse(PinName pin, PinStatus state, nrf_ppi_channel_group_t firstGroup)
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{
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/* three different reads are needed, because it's not easy to synchronize hardware and software */
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uint32_t firstState, secondState, thirdState;
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core_util_critical_section_enter();
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firstState = nrf_gpio_pin_read(pin);
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/* Enable the hardware detection of the pin edge */
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nrf_ppi_group_enable(firstGroup);
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secondState = nrf_gpio_pin_read(pin);
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__NOP();
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thirdState = nrf_gpio_pin_read(pin);
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core_util_critical_section_exit();
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uint8_t pulseToTake = 0;
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/* If no changes on the pin were detected, there are no doubts */
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if (firstState == secondState && firstState == thirdState) {
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if (firstState != state) {
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pulseToTake = 1;
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} else {
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pulseToTake = 2;
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}
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} else {
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pulseToTake = TIMEOUT_US;
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}
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return pulseToTake;
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}
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/*
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* The pulse pin is assumed to be already configured as an input pin and NOT as an interrupt pin
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* Also the serial_api.c from NRF sdk uses ppi channels, but there shouldn't be any issue
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* The disadvantage of this approach is that some pulses could be missed, and more time could be necessary for retrying the measure.
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* Using two different events for rising and falling edge would be way better
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*/
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unsigned long pulseIn(PinName pin, PinStatus state, unsigned long timeout)
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{
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/* Configure timer */
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nrf_timer_mode_set(PULSE_TIMER, NRF_TIMER_MODE_TIMER);
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nrf_timer_task_trigger(PULSE_TIMER, NRF_TIMER_TASK_STOP);
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nrf_timer_frequency_set(PULSE_TIMER, NRF_TIMER_FREQ_1MHz);
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nrf_timer_bit_width_set(PULSE_TIMER, NRF_TIMER_BIT_WIDTH_32);
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nrf_timer_cc_write(PULSE_TIMER, TIMER_FIRST_CHANNEL, 0);
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nrf_timer_cc_write(PULSE_TIMER, TIMER_SECOND_CHANNEL, 0);
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nrf_timer_task_trigger(PULSE_TIMER, NRF_TIMER_TASK_CLEAR);
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/* Configure pin Toggle Event */
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nrfx_gpiote_in_init(pin, &cfg, NULL);
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nrfx_gpiote_in_event_enable(pin, true);
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/* Allocate PPI channels for starting and stopping the timer */
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nrf_ppi_channel_t firstPPIchannel, firstPPIchannelControl;
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nrf_ppi_channel_t secondPPIchannel, secondPPIchannelControl;
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nrf_ppi_channel_t thirdPPIchannel, thirdPPIchannelControl;
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nrfx_ppi_channel_alloc(&firstPPIchannel);
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nrfx_ppi_channel_alloc(&firstPPIchannelControl);
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nrfx_ppi_channel_alloc(&secondPPIchannel);
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nrfx_ppi_channel_alloc(&secondPPIchannelControl);
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nrfx_ppi_channel_alloc(&thirdPPIchannel);
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nrfx_ppi_channel_alloc(&thirdPPIchannelControl);
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/* Allocate PPI Group channels to allow activation and deactivation of channels as PPI tasks */
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nrf_ppi_channel_group_t firstGroup, secondGroup, thirdGroup;
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nrfx_ppi_group_alloc(&firstGroup);
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nrfx_ppi_group_alloc(&secondGroup);
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nrfx_ppi_group_alloc(&thirdGroup);
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/* Insert channels in corresponding group */
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nrfx_ppi_channel_include_in_group(firstPPIchannel, firstGroup);
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nrfx_ppi_channel_include_in_group(firstPPIchannelControl, firstGroup);
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nrfx_ppi_channel_include_in_group(secondPPIchannel, secondGroup);
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nrfx_ppi_channel_include_in_group(secondPPIchannelControl, secondGroup);
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nrfx_ppi_channel_include_in_group(thirdPPIchannel, thirdGroup);
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nrfx_ppi_channel_include_in_group(thirdPPIchannelControl, thirdGroup);
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/* Configure PPI channels for Start and Stop events */
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/* The first edge on the pin will trigger the timer START task */
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nrf_ppi_channel_endpoint_setup(firstPPIchannel,
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(uint32_t) nrfx_gpiote_in_event_addr_get(pin),
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(uint32_t) nrf_timer_task_address_get(PULSE_TIMER, NRF_TIMER_TASK_START));
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nrf_ppi_channel_and_fork_endpoint_setup(firstPPIchannelControl,
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(uint32_t) nrfx_gpiote_in_event_addr_get(pin),
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(uint32_t) nrfx_ppi_task_addr_group_enable_get(secondGroup),
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(uint32_t) nrfx_ppi_task_addr_group_disable_get(firstGroup));
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/* The second edge will result in a capture of the timer counter into a register. In this way the first impulse is captured */
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nrf_ppi_channel_endpoint_setup(secondPPIchannel,
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(uint32_t) nrfx_gpiote_in_event_addr_get(pin),
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(uint32_t) nrf_timer_task_address_get(PULSE_TIMER, TIMER_FIRST_CAPTURE));
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nrf_ppi_channel_and_fork_endpoint_setup(secondPPIchannelControl,
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(uint32_t) nrfx_gpiote_in_event_addr_get(pin),
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(uint32_t) nrfx_ppi_task_addr_group_enable_get(thirdGroup),
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(uint32_t) nrfx_ppi_task_addr_group_disable_get(secondGroup));
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/* The third edge will capture the second impulse. After that, the pulse corresponding to the correct state must be returned */
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nrf_ppi_channel_and_fork_endpoint_setup(thirdPPIchannel,
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(uint32_t) nrfx_gpiote_in_event_addr_get(pin),
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(uint32_t) nrf_timer_task_address_get(PULSE_TIMER, NRF_TIMER_TASK_STOP),
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(uint32_t) nrf_timer_task_address_get(PULSE_TIMER, TIMER_SECOND_CAPTURE));
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nrf_ppi_channel_endpoint_setup(thirdPPIchannelControl,
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(uint32_t) nrfx_gpiote_in_event_addr_get(pin),
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(uint32_t) nrfx_ppi_task_addr_group_disable_get(thirdGroup));
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uint8_t pulseToTake = TIMEOUT_US;
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auto startMicros = micros();
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pulseToTake = measurePulse(pin, state, firstGroup);
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while (pulseToTake == TIMEOUT_US && (micros() - startMicros < timeout)) {
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/* In case it wasn't possible to detect the initial state, disable the hardware detection control and retry */
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nrf_ppi_group_disable(firstGroup);
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nrf_ppi_group_disable(secondGroup);
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nrf_ppi_group_disable(thirdGroup);
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/* Stop the timer and clear its registers */
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nrf_timer_task_trigger(PULSE_TIMER, NRF_TIMER_TASK_STOP);
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nrf_timer_task_trigger(PULSE_TIMER, NRF_TIMER_TASK_CLEAR);
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nrf_timer_cc_write(PULSE_TIMER, TIMER_FIRST_CHANNEL, 0);
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nrf_timer_cc_write(PULSE_TIMER, TIMER_SECOND_CHANNEL, 0);
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/* Retry to enable the detection hardware figuring out the starting state of the pin */
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pulseToTake = measurePulse(pin, state, firstGroup);
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}
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unsigned long pulseTime = TIMEOUT_US;
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unsigned long pulseFirst = TIMEOUT_US;
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unsigned long pulseSecond = TIMEOUT_US;
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/* Optionally the time reference could be restarted because here the actual wait for the pulse begins */
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//startMicros = micros();
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if (pulseToTake >= 1) {
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while (!pulseFirst && (micros() - startMicros < timeout) ) {
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pulseFirst = nrf_timer_cc_read(PULSE_TIMER, TIMER_FIRST_CHANNEL);
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}
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pulseTime = pulseFirst;
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}
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if (pulseToTake == 2) {
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while (!pulseSecond && (micros() - startMicros < timeout) ) {
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pulseSecond = nrf_timer_cc_read(PULSE_TIMER, TIMER_SECOND_CHANNEL);
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}
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pulseTime = pulseSecond ? pulseSecond - pulseFirst : TIMEOUT_US;
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}
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/* Deallocate all the PPI channels, events and groups */
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nrf_timer_task_trigger(PULSE_TIMER, NRF_TIMER_TASK_SHUTDOWN);
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nrfx_gpiote_in_uninit(pin);
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nrfx_ppi_group_free(firstGroup);
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nrfx_ppi_group_free(secondGroup);
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nrfx_ppi_group_free(thirdGroup);
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nrf_ppi_channel_group_clear(firstGroup);
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nrf_ppi_channel_group_clear(secondGroup);
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nrf_ppi_channel_group_clear(thirdGroup);
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nrfx_ppi_channel_free(firstPPIchannel);
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nrfx_ppi_channel_free(firstPPIchannelControl);
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nrfx_ppi_channel_free(secondPPIchannel);
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nrfx_ppi_channel_free(secondPPIchannelControl);
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nrfx_ppi_channel_free(thirdPPIchannel);
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nrfx_ppi_channel_free(thirdPPIchannelControl);
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/* The timer has a frequency of 1 MHz, so its counting value is already in microseconds */
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return pulseTime;
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}
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unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout)
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{
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return pulseIn(digitalPinToPinName(pin), (PinStatus)state, timeout);
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}
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unsigned long pulseInLong(uint8_t pin, uint8_t state, unsigned long timeout)
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{
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return pulseIn(digitalPinToPinName(pin), (PinStatus)state, timeout);
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}
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unsigned long pulseInLong(PinName pin, PinStatus state, unsigned long timeout)
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{
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return pulseIn(pin, state, timeout);
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}

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