NRF52 HX711

HX711.c

// For more information, please refer to https://devzone.nordicsemi.com/f/nordic-q-a/34112/best-approach-to-interface-hx711-with-nordic-nrf52832
// Feel free to comment any questions or suggestions for improvements

//TODO you will have to define the following:
// HX711_TIMER   (the index of the timer, 2 for me)
// HX711_TIMER_INTERRUPT  (the IRG for the timer, TIMER2_IRQn for me)
// Loadcell_Dout_Pin (the pin number for the HX711 data line)
// Loadcell_Sck_Pin (the pin number for the HX711 clock line)
#include "HX711.h"
#include "nrf_drv_gpiote.h"
#include "nrf_drv_timer.h"
#include "nrf_gpio.h"
#include "nrf_gpiote.h"
#include "nrfx_ppi.h"

const HX711_GAIN gain                   = GAIN_A_128;
static const nrf_drv_timer_t hx711Timer = NRF_DRV_TIMER_INSTANCE(HX711_TIMER);
// HX711 ADC resolution in bits
#define ADC_RES 24
#define HX711_CONTINUOUS 1
#define NUM_HX711_BUFFERED 2
uint32_t hx711_results[NUM_HX711_BUFFERED];
uint16_t sampleCount   = 0;    // The index we're reading from
uint8_t is_loadcell_on = 0;
uint8_t step           = 0;
nrf_ppi_channel_t ppi_channel_0;
nrf_ppi_channel_t ppi_channel_1;

static void dout_high2low_handler(nrf_drv_gpiote_pin_t pin, nrf_gpiote_polarity_t action) {
  if (unlikely(!is_loadcell_on))
    return;
  //   DOUT going low means that the hx711 is ready to send us a result
  //   Turn off listening for DOUT changes (for now)
  nrf_drv_gpiote_in_event_disable(Loadcell_Dout_Pin);

  // clear working datum
  hx711_results[(sampleCount + 1) % NUM_HX711_BUFFERED] = 0;
  step                                                  = 0;
  //  Start the timer that will pwm the clock and read out
  nrf_drv_timer_resume(&hx711Timer);
}
void hx711_timer_callback(nrf_timer_event_t event_type, void* ctx) {
  if (event_type != NRF_TIMER_EVENT_COMPARE1)
    return;

  if (step < ADC_RES) {
    // We read the current datum
    uint32_t datum = nrf_gpio_pin_read(Loadcell_Dout_Pin);
    // Store datum
    uint32_t* res = &hx711_results[(sampleCount + 1) % NUM_HX711_BUFFERED];
    res[0] |= datum << (ADC_RES - 1 - step);
  } else if (step == ADC_RES) {
    // Continue ticking
    // We do have data ready though
    sampleCount++;
  } else if (step == gain) {
    //  hit the end, stop ticking for now
    if (is_loadcell_on)
      nrf_drv_gpiote_in_event_enable(Loadcell_Dout_Pin, true);
    return;
  }

  step++;
  nrf_drv_timer_resume(&hx711Timer);
}
void hx711_init(void) {
  ret_code_t err_code;
  is_loadcell_on = 0;    // Leave off for now
  if (!nrfx_gpiote_is_init()) {
    err_code = nrfx_gpiote_init();
    APP_ERROR_CHECK(err_code);
  }

  nrf_gpio_pin_set(Loadcell_Sck_Pin);
  nrf_gpio_cfg(Loadcell_Sck_Pin,
               NRF_GPIO_PIN_DIR_OUTPUT,
               NRF_GPIO_PIN_INPUT_DISCONNECT,
               NRF_GPIO_PIN_NOPULL,
               NRF_GPIO_PIN_S0S1,
               NRF_GPIO_PIN_NOSENSE);

  // Configure DOUT for reading
  nrf_gpio_cfg_input(Loadcell_Dout_Pin, NRF_GPIO_PIN_NOPULL);

  // Control the clock line with a PPI event
  const bool startClockHigh       = true;
  nrfx_gpiote_out_config_t config = NRFX_GPIOTE_CONFIG_OUT_TASK_TOGGLE(startClockHigh);
  err_code                        = nrfx_gpiote_out_init(Loadcell_Sck_Pin, &config);
  APP_ERROR_CHECK(err_code);
  nrfx_gpiote_out_task_enable(Loadcell_Sck_Pin);
  uint32_t toggle_clock_task_addr = nrfx_gpiote_out_task_addr_get(Loadcell_Sck_Pin);

  // Set interrupt to low priority, this method is robust to variable latency
  NVIC_SetPriority(HX711_TIMER_INTERRUPT, 7);
  // Configure the main timer
  nrf_drv_timer_config_t timer_cfg = NRF_DRV_TIMER_DEFAULT_CONFIG;
  timer_cfg.frequency              = NRF_TIMER_FREQ_1MHz;
  timer_cfg.mode                   = NRF_TIMER_MODE_TIMER;      // Mode is timer, not counter
  timer_cfg.bit_width              = NRF_TIMER_BIT_WIDTH_8;    // TODO I think 08 would be fine
  err_code                         = nrfx_timer_init(&hx711Timer, &timer_cfg, hx711_timer_callback);
  APP_ERROR_CHECK(err_code);
  nrf_drv_timer_pause(&hx711Timer);

  // Channel 0 turns the clock high
  nrfx_timer_compare(&hx711Timer, NRF_TIMER_CC_CHANNEL0, 1, true);
  // Channel 1 turns the clock low and clears/stops the timer + calls the interrupt
  nrfx_timer_extended_compare(&hx711Timer,
                              NRF_TIMER_CC_CHANNEL1,
                              2,
                              NRF_TIMER_SHORT_COMPARE1_STOP_MASK | NRF_TIMER_SHORT_COMPARE1_CLEAR_MASK,
                              true);

  // Turn clock high on tick 1
  err_code = nrfx_ppi_channel_alloc(&ppi_channel_0);
  APP_ERROR_CHECK(err_code);
  uint32_t compare_evt_addr = nrfx_timer_event_address_get(&hx711Timer, NRF_TIMER_EVENT_COMPARE0);
  err_code                  = nrfx_ppi_channel_assign(ppi_channel_0, compare_evt_addr, toggle_clock_task_addr);
  APP_ERROR_CHECK(err_code);
  err_code = nrfx_ppi_channel_enable(ppi_channel_0);
  APP_ERROR_CHECK(err_code);
  // Turn clock low on tick 2
  err_code = nrfx_ppi_channel_alloc(&ppi_channel_1);
  APP_ERROR_CHECK(err_code);
  compare_evt_addr = nrfx_timer_event_address_get(&hx711Timer, NRF_TIMER_EVENT_COMPARE1);
  err_code         = nrfx_ppi_channel_assign(ppi_channel_1, compare_evt_addr, toggle_clock_task_addr);
  APP_ERROR_CHECK(err_code);
  err_code = nrfx_ppi_channel_enable(ppi_channel_1);
  APP_ERROR_CHECK(err_code);

  nrf_drv_gpiote_in_config_t gpiote_config = GPIOTE_CONFIG_IN_SENSE_HITOLO(false);
  err_code |= nrf_drv_gpiote_in_init(Loadcell_Dout_Pin, &gpiote_config, dout_high2low_handler);
  nrf_drv_gpiote_in_event_disable(Loadcell_Dout_Pin);
  APP_ERROR_CHECK(err_code);
  // printf("hx711 init\n");
}
void hx711_power_on(void) {
  if (is_loadcell_on)
    return;
  is_loadcell_on = true;
  // Turn the hx711 on
  nrfx_gpiote_out_task_trigger(Loadcell_Sck_Pin);

  // If DOUT is low, we're already ready to read
  if (nrf_gpio_pin_read(Loadcell_Dout_Pin) == 0) {
    // printf("hx711 starting on\n");
    hx711_results[(sampleCount + 1) % NUM_HX711_BUFFERED] = 0;
    step                                                  = 0;
    nrf_drv_timer_resume(&hx711Timer);
  } else {
    // printf("hx711 wait for DOUT\n");
    nrfx_gpiote_in_event_enable(Loadcell_Dout_Pin, true);
  }
}
void hx711_power_off(void) {
  if (!is_loadcell_on)
    return;
  is_loadcell_on = false;
  nrf_drv_gpiote_in_event_disable(Loadcell_Dout_Pin);
  // Turn hx711 off
  nrfx_gpiote_out_task_trigger(Loadcell_Sck_Pin);
}

static int32_t convert_sample(uint32_t sample) {
  return (int32_t)(sample << 8) >> 8;
}
bool hx711_try_read_result(int32_t* result, uint16_t* lastSampleNum) {
  if (*lastSampleNum == sampleCount)
    return false;
  *lastSampleNum = sampleCount;
  // We read from the opposite buffer than what's currently being written to
  // this is to avoid race condition where we read while it's being written
  *result = convert_sample(hx711_results[*lastSampleNum % NUM_HX711_BUFFERED]);
  return true;
}

HX711.h

#ifndef HX711_h
#define HX711_h
#include "stdbool.h"
#include "stdint.h"

// From HX711 datasheet
typedef enum {
  GAIN_A_128 = 25,
  GAIN_A_64  = 27,
  GAIN_B_32  = 26,
} HX711_GAIN;

void hx711_init(void);
void hx711_power_on(void);
void hx711_power_off(void);
bool hx711_try_read_result(int32_t* result, uint16_t* lastSampleNum);
#endif /* HX711_h */