Best approach to interface HX711 with Nordic nrf52832?

[anuragshukla]

I am working with HX711 and I am trying to interface with nrf52832. Should I proceed with SAADC example or UART or Twi Sensor

[Vidar Berg]

Hi, 

Please refer to the datasheet for the hx711, it does not use a standardized serial interface such as UART or TWI, which means that it has to be implemented in software (i.e., bit banging).

We don't have an official reference example for this ADC, but I have played with it in my spare time, see attached code.  Documentation for this code is lacking, and I haven't done any thorough testing of it, but hopefully it can serve as a starting point. You can find other examples as well if you do a google search for hx711 and nrf52/51. Just keep in mind that the implementation should account for higher priority interrupts such as radio events that may occur during ADC readout.  

Fullscreen hx711.c Download

/******************************************************************************/
/*                                                                            */
/******************************************************************************/

#include <stdint.h>
#include <string.h>
#include "hx711.h"
#include "nrf_drv_gpiote.h"
#include "app_util_platform.h"
#include "boards.h"
#include "nrf_delay.h"

#define NRF_LOG_MODULE_NAME hx711

#if HX711_CONFIG_LOG_ENABLED
    #define NRF_LOG_LEVEL           HX711_CONFIG_LOG_LEVEL
    #define NRF_LOG_INFO_COLOR      HX711_CONFIG_INFO_COLOR
    #define NRF_LOG_DEBUG_COLOR     HX711_CONFIG_DEBUG_COLOR
#else // HX711_CONFIG_LOG_ENABLED
    #define NRF_LOG_LEVEL           0
#endif  // HX711_CONFIG_LOG_ENABLED
#include "nrf_log.h"
NRF_LOG_MODULE_REGISTER();

/* Defines frequency and duty cycle for clock signal - default: 1 MHz 50%*/  
#define TIMER_COUNTERTOP          32
#define TIMER_COMPARE             16

// Pinout
#define PD_SCK                    11    
#define DOUT                      7    
#define VDD                       13  // Note: Depending on design, GPIO may not be able to source enough current for hx711 and loadcell.    

/* HX711 ADC resolution in bits */
#define ADC_RES                   24

typedef  struct
{
   uint32_t   value;   // buffer for ADC read
   uint8_t    count;   // number of bitshifts. Read complete when count == ADC_RES 
}hx711_sample_t;

static volatile hx711_sample_t                    m_sample;
static hx711_event_handler_t                      m_evt_handler;
static uint32_t                                   m_mode;
static bool                                       m_continous_sampling;


static int convert_sample(uint32_t sample)
{
  return (int)(sample << 8) >> 8;
}


/* Clocks out HX711 result - if readout fails consistently, try to increase the clock period and/or enable compiler optimization */
static void hx711_sample()
{
    NRF_TIMER2->TASKS_CLEAR = 1;
    m_sample.count = 0;
    m_sample.value = 0;
    NRF_TIMER1->TASKS_START = 1; // Starts clock signal on PD_SCK

    for (uint32_t i=0; i < ADC_RES; i++)
    {
        do
        {
            /* NRF_TIMER->CC[1] contains number of clock cycles.*/
            NRF_TIMER2->TASKS_CAPTURE[1] = 1;
            if (NRF_TIMER2->CC[1] >= ADC_RES) goto EXIT; // Readout not in sync with PD_CLK. Abort and notify error.
        }
        while(NRF_TIMER1->EVENTS_COMPARE[0] == 0);
        NRF_TIMER1->EVENTS_COMPARE[0] = 0;
        m_sample.value |= (nrf_gpio_pin_read(DOUT) << (23 - i));
        m_sample.count++;
    }
    EXIT:

    if (!m_continous_sampling)
    {
        NRF_GPIOTE->TASKS_SET[1] = 1; // HX711 power-down
    }

    NRF_LOG_INFO("Number of bits : %d", m_sample.count);
    NRF_LOG_INFO("ADC val: 0x%x", m_sample.value);

    if (m_sample.count == ADC_RES)
    {
        // Send notification with raw ADC value
        m_evt_handler(DATA_READY, convert_sample(m_sample.value));
    }
    else
    {
       // Notify readout error
       m_evt_handler(DATA_ERROR, m_sample.value);
    }

}


static void gpiote_evt_handler(nrf_drv_gpiote_pin_t pin, nrf_gpiote_polarity_t action)
{
    nrf_drv_gpiote_in_event_disable(DOUT);
    hx711_sample();
    
    if (m_continous_sampling)
    {
       nrf_drv_gpiote_in_event_enable(DOUT, true); 
    }   
}


void hx711_init(hx711_mode_t mode, hx711_event_handler_t evt_handler)
{

  ret_code_t                  ret_code;
  nrf_drv_gpiote_in_config_t  gpiote_config = GPIOTE_CONFIG_IN_SENSE_HITOLO(false);

  m_evt_handler = evt_handler;
  m_mode = mode;

  nrf_gpio_cfg_output(PD_SCK);
  nrf_gpio_pin_set(PD_SCK);
  
  nrf_gpio_cfg(VDD, 
               NRF_GPIO_PIN_DIR_OUTPUT, 
               NRF_GPIO_PIN_INPUT_DISCONNECT, 
               NRF_GPIO_PIN_PULLUP, 
               NRF_GPIO_PIN_S0H1, 
               NRF_GPIO_PIN_NOSENSE);
  nrf_gpio_pin_set(VDD);

  nrf_gpio_cfg_input(DOUT, NRF_GPIO_PIN_NOPULL);

  if (!nrf_drv_gpiote_is_init())
  {
    ret_code = nrf_drv_gpiote_init();
    APP_ERROR_CHECK(ret_code);
  }

  ret_code = nrf_drv_gpiote_in_init(DOUT, &gpiote_config, gpiote_evt_handler);
  APP_ERROR_CHECK(ret_code);

  /* Set up timers, gpiote, and ppi for clock signal generation*/
  NRF_TIMER1->CC[0]     = 1;
  NRF_TIMER1->CC[1]     = TIMER_COMPARE;
  NRF_TIMER1->CC[2]     = TIMER_COUNTERTOP;
  NRF_TIMER1->SHORTS    = (uint32_t) (1 << 2);    //COMPARE2_CLEAR
  NRF_TIMER1->PRESCALER = 0;

  NRF_TIMER2->CC[0]     = m_mode;
  NRF_TIMER2->MODE      = 2;

  NRF_GPIOTE->CONFIG[1] = (uint32_t) (3 | (PD_SCK << 8) | (1 << 16) | (1 << 20));

  NRF_PPI->CH[0].EEP   = (uint32_t) &NRF_TIMER1->EVENTS_COMPARE[0];
  NRF_PPI->CH[0].TEP   = (uint32_t) &NRF_GPIOTE->TASKS_SET[1];
  NRF_PPI->CH[1].EEP   = (uint32_t) &NRF_TIMER1->EVENTS_COMPARE[1];
  NRF_PPI->CH[1].TEP   = (uint32_t) &NRF_GPIOTE->TASKS_CLR[1];
  NRF_PPI->FORK[1].TEP = (uint32_t) &NRF_TIMER2->TASKS_COUNT; // Increment on falling edge
  NRF_PPI->CH[2].EEP   = (uint32_t) &NRF_TIMER2->EVENTS_COMPARE[0];
  NRF_PPI->CH[2].TEP   = (uint32_t) &NRF_TIMER1->TASKS_SHUTDOWN;
  NRF_PPI->CHEN = 7;
}


void hx711_power_down()
{
   nrf_gpio_cfg_default(VDD);
   nrf_gpio_cfg_default(PD_SCK);
   nrf_gpio_cfg_default(DOUT);
}

void hx711_start(bool single)
{
    
    m_continous_sampling = !single;

    NRF_LOG_INFO("Start sampling");
    
    NRF_GPIOTE->TASKS_CLR[1] = 1;
    // Generates interrupt when new sampling is available. 
    nrf_drv_gpiote_in_event_enable(DOUT, true);
}

void hx711_stop()
{
    nrf_drv_gpiote_in_event_disable(DOUT);
    NRF_GPIOTE->TASKS_SET[1] = 1; // Must be kept high for >60 us to power down HX711 
}

hx711.h

Update 9/11-18: 

I've included an example in this thread: https://devzone.nordicsemi.com/f/nordic-q-a/40271/timer-issue-with-hx711---stopped-by-vector-catch-error  

[Droidik]

Vidar Berg
I just wanted to thank you for the provided sample code.
"Just keep in mind that the implementation should account for higher priority interrupts such as radio events that may occur during ADC readout."

I'm receiving DATA_ERROR in the callback every time. I think it's because of the BLE interrupts. What's the best solution to this problem in general?

[Droidik]

Solved by turning on the optimizations. Interesting.

[Vidar Berg]

Droidik,  the hx711_sample() function needs to be compiled with optimization to work when PD_SCK frequency is 1MHz. I suspect it would not have been necessary if I had rewritten this line "m_sample.value |= (nrf_gpio_pin_read(DOUT) << (23 - i));" to something more efficient. 

[Droidik]

Vidar Berg, once again thank you for your amazing work and support.