1. Introduction
On the agenda was the task of developing a communication protocol for the nrf52832 microcontroller with two half-bridge Chinese strain gauges.
The task turned out to be not easy, as I faced the lack of any intelligible information. It is more likely that the "root of evil" is in the Nordic Semiconductor SDK itself - this is constant version updates, some redundancy and intricacies of functionality. I had to write everything from scratch.
I think this topic is quite relevant, given that this chip has a BLE stack and a whole set of goodies in the power saving mode. But I will not go deep into the technical part, since many articles have been written on this topic.
2. Description of the project
Hardware:
- Adafruit Feather nRF52 Bluefruit LE (what happened to be at hand)
- ADC HX711
- Chinese load cells 2 pcs. (50x2 kg)
- Programmer ST-LINK V2
Software:
- IDE VSCODE
- NRF SDK 16
- OpenOCD
- Programmer ST-LINK V2
Everything is in one project, all you have to do is hack the Makefile (specify the location of your SDK).
3. Description of the code
We will use the GPIOTE module to work with peripherals based on the binding of tasks and events, as well as the PPI module to transfer data from one peripheral to another without the participation of a processor.
ret_code_t err_code;
err_code = nrf_drv_gpiote_out_init(PD_SCK, &config);//настраеваем на выход
nrf_drv_gpiote_out_config_t config = GPIOTE_CONFIG_OUT_TASK_TOGGLE(false);//будем передергивать пин для импульса
err_code = nrf_drv_gpiote_out_init(PD_SCK, &config);//настраеваем на выход
We set the PD_SCL synchronization line to the output to generate pulses with a duration of 10 μs.
nrf_drv_gpiote_in_config_t gpiote_config = GPIOTE_CONFIG_IN_SENSE_HITOLO(false);// переход уровня с высокого на низкий
nrf_gpio_cfg_input(DOUT, NRF_GPIO_PIN_NOPULL);// на вход без подтяжки
err_code = nrf_drv_gpiote_in_init(DOUT, &gpiote_config, gpiote_evt_handler); static void gpiote_evt_handler(nrf_drv_gpiote_pin_t pin, nrf_gpiote_polarity_t action)
{
nrf_drv_gpiote_in_event_disable(DOUT);//отключаем прерывание
nrf_drv_timer_enable(&m_timer0);//включаем таймер
}
We configure the DOUT data line to read the HX711 ready state, if there is a low level, a handler is triggered in which we disable the interrupt and start a timer to generate clock pulses at the PD_SCL output.
err_code = nrf_drv_ppi_channel_alloc(&m_ppi_channel1);
APP_ERROR_CHECK(err_code);
err_code = nrf_drv_ppi_channel_assign(m_ppi_channel1, nrf_drv_timer_event_address_get(&m_timer0, NRF_TIMER_EVENT_COMPARE0), nrf_drv_gpiote_out_task_addr_get(PD_SCK));// подключаем таймер к выходу
APP_ERROR_CHECK(err_code);
err_code = nrf_drv_ppi_channel_enable(m_ppi_channel1);// включаем канал
APP_ERROR_CHECK(err_code);
nrf_drv_gpiote_out_task_enable(PD_SCK); // enable gpiote
After that, we initialize the PPI module and switch our timer to the PD_SCL output to generate pulses with a duration of 10 μs when a comparison event occurs, and also turn on the GPIOTE module.
nrf_drv_timer_config_t timer_cfg = NRF_DRV_TIMER_DEFAULT_CONFIG;// по умолчанию
timer_cfg.frequency = NRF_TIMER_FREQ_1MHz;// тактируем на частоте 1Мгц
ret_code_t err_code = nrf_drv_timer_init(&m_timer0, &timer_cfg, timer0_event_handler);
APP_ERROR_CHECK(err_code);
nrf_drv_timer_extended_compare(&m_timer0,
NRF_TIMER_CC_CHANNEL0,
nrf_drv_timer_us_to_ticks(&m_timer0,
10),
NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK,
true);// срабатывает по сравнениюWe initialize the zero timer and its handler.
if(m_counter%2 != 0 && m_counter<=48){
buffer <<= 1;// переменная считанных даных
c_counter++;// счетчик положительных импульсов
if(nrf_gpio_pin_read(DOUT))buffer++;//считываем состояние входа
}
The most interesting thing happens in the timer handler. The pulse period is 20 μs. We are interested in odd pulses (on the rising edge) and provided that their number is not more than 24, and events - 48. For each odd event, DOUT is read
It follows from the datasheet that the number of pulses must be at least 25, which corresponds to a gain of 128 (in the code I used 25 pulses), this is equivalent to 50 timer events, which indicates the end of the data frame.
++m_counter;// счетчик событий
if(m_counter==50){
nrf_drv_timer_disable(&m_timer0);// отключаем таймер
m_simple_timer_state = SIMPLE_TIMER_STATE_STOPPED;//
buffer = buffer ^ 0x800000;
hx711_stop();//jотключаем hx711
}
After that, we turn off the timer and process the data (according to the datasheet) and put the HX711 into low power mode.
static void repeated_timer_handler(void * p_context)
{
nrf_drv_gpiote_out_toggle(LED_2);
if(m_simple_timer_state == SIMPLE_TIMER_STATE_STOPPED){
hx711_start();// включаем hx711
nrf_drv_gpiote_out_toggle(LED_1);
m_simple_timer_state = SIMPLE_TIMER_STATE_STARTED;
}
}
/**@brief Create timers.
*/
static void create_timers()
{
ret_code_t err_code;
// Create timers
err_code = app_timer_create(&m_repeated_timer_id,
APP_TIMER_MODE_REPEATED,
repeated_timer_handler);
APP_ERROR_CHECK(err_code);
}
We are waiting for events from the RTC timer with an interval of 10 s (this one is up to you) in the handler, we start HX711, causing an interrupt on the DOUT line.
There is one more thing, the logs are output via UART (baud rate 115200, TX - 6 pins, RX - 8 pins) all settings are in sdk_config.h
Conclusions
Thank you all for your attention, I hope this article will be useful and will reduce the valuable time for developers to find a solution. I want to say that the technical approach that Nordic uses in their platforms is quite interesting in terms of energy efficiency.
PS
The project is still under development, so if this topic is of interest, in the next article I will try to describe the algorithm for calibrating weight sensors, as well as connecting the BLE stack.
Materials
Source: habr.com