Data transfer from sensor connected with I2C over Bluetooth NUS.

[Esbriven]

Hi everyone, 

I am working with the nRF SDK v2.0.4 for VsCode and with the nRF52840 DK. 

I want to develop an application that transfer in real time the data collected by a sensor. I managed to implement the I2C logic and print the value collected by the sensor to the serial terminal but I have trouble understanding the logic of the NUS protocol.

I followed the course Bluetooth Low Energy Fundamentals – Nordic Developer Academy (nordicsemi.com) but it didn't helped me much. 

I also took a look at the peripheral_uart example provide by Zephyr, but I don't really see where in the code I could send the data. Those data are stored into 6 uint6_t variables but managing to send only one of them would already be a huge step for me :). I'd also like to know if the data transmission is limited to a certain type of data, like could I send text and values in the same time ? Or if there is a way to send directly a file containing the said data. 

Thanks in advance, 

Best regards.

This the code used in the peripheral_uart example : 

/*
 * Copyright (c) 2018 Nordic Semiconductor ASA
 *
 * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
 */

/** @file
 *  @brief Nordic UART Bridge Service (NUS) sample
 */
#include "uart_async_adapter.h"

#include <zephyr/types.h>
#include <zephyr/kernel.h>
#include <zephyr/drivers/uart.h>
#include <zephyr/usb/usb_device.h>

#include <zephyr/device.h>
#include <zephyr/devicetree.h>
#include <soc.h>

#include <zephyr/bluetooth/bluetooth.h>
#include <zephyr/bluetooth/uuid.h>
#include <zephyr/bluetooth/gatt.h>
#include <zephyr/bluetooth/hci.h>

#include <bluetooth/services/nus.h>

#include <dk_buttons_and_leds.h>

#include <zephyr/settings/settings.h>

#include <stdio.h>

#include <zephyr/logging/log.h>

LOG_MODULE_REGISTER(Lesson4_Exercise3, LOG_LEVEL_INF);

#define STACKSIZE CONFIG_BT_NUS_THREAD_STACK_SIZE
#define PRIORITY 7

#define DEVICE_NAME CONFIG_BT_DEVICE_NAME
#define DEVICE_NAME_LEN	(sizeof(DEVICE_NAME) - 1)

#define RUN_STATUS_LED DK_LED1
#define RUN_LED_BLINK_INTERVAL 1000

#define CON_STATUS_LED DK_LED2

#define KEY_PASSKEY_ACCEPT DK_BTN1_MSK
#define KEY_PASSKEY_REJECT DK_BTN2_MSK

#define UART_WAIT_FOR_BUF_DELAY K_MSEC(50)
#define UART_WAIT_FOR_RX CONFIG_BT_NUS_UART_RX_WAIT_TIME

static K_SEM_DEFINE(ble_init_ok, 0, 1);

static struct bt_conn *current_conn;
static struct bt_conn *auth_conn;

static const struct device *uart = DEVICE_DT_GET(DT_CHOSEN(nordic_nus_uart));
static struct k_work_delayable uart_work;

/* STEP 6.2 - Declare the struct of the data item of the FIFOs */
struct uart_data_t {
	void *fifo_reserved;
	uint8_t data[CONFIG_BT_NUS_UART_BUFFER_SIZE];
	uint16_t len;
};

/* STEP 6.1 - Declare the FIFOs */
static K_FIFO_DEFINE(fifo_uart_tx_data); 
static K_FIFO_DEFINE(fifo_uart_rx_data);

static const struct bt_data ad[] = {
	BT_DATA_BYTES(BT_DATA_FLAGS, (BT_LE_AD_GENERAL | BT_LE_AD_NO_BREDR)),
	BT_DATA(BT_DATA_NAME_COMPLETE, DEVICE_NAME, DEVICE_NAME_LEN),
};

static const struct bt_data sd[] = {
	BT_DATA_BYTES(BT_DATA_UUID128_ALL, BT_UUID_NUS_VAL),
};

#if CONFIG_BT_NUS_UART_ASYNC_ADAPTER
UART_ASYNC_ADAPTER_INST_DEFINE(async_adapter);
#else
static const struct device *const async_adapter;
#endif

static void uart_cb(const struct device *dev, struct uart_event *evt, void *user_data)
{
	ARG_UNUSED(dev);

	static size_t aborted_len;
	struct uart_data_t *buf;
	static uint8_t *aborted_buf;
	static bool disable_req;

	switch (evt->type) {
	case UART_TX_DONE:
		LOG_DBG("UART_TX_DONE");
		if ((evt->data.tx.len == 0) ||
		    (!evt->data.tx.buf)) {
			return;
		}

		if (aborted_buf) {
			buf = CONTAINER_OF(aborted_buf, struct uart_data_t,
					   data);
			aborted_buf = NULL;
			aborted_len = 0;
		} else {
			buf = CONTAINER_OF(evt->data.tx.buf, struct uart_data_t,
					   data);
		}

		k_free(buf);

		buf = k_fifo_get(&fifo_uart_tx_data, K_NO_WAIT);
		if (!buf) {
			return;
		}

		if (uart_tx(uart, buf->data, buf->len, SYS_FOREVER_MS)) {
			LOG_WRN("Failed to send data over UART");
		}

		break;

	case UART_RX_RDY:
		LOG_DBG("UART_RX_RDY");
		buf = CONTAINER_OF(evt->data.rx.buf, struct uart_data_t, data);
		buf->len += evt->data.rx.len;

		if (disable_req) {
			return;
		}

		if ((evt->data.rx.buf[buf->len - 1] == '\n') ||
		    (evt->data.rx.buf[buf->len - 1] == '\r')) {
			disable_req = true;
			uart_rx_disable(uart);
		}

		break;

	case UART_RX_DISABLED:
		LOG_DBG("UART_RX_DISABLED");
		disable_req = false;

		buf = k_malloc(sizeof(*buf));
		if (buf) {
			buf->len = 0;
		} else {
			LOG_WRN("Not able to allocate UART receive buffer");
			k_work_reschedule(&uart_work, UART_WAIT_FOR_BUF_DELAY);
			return;
		}

		uart_rx_enable(uart, buf->data, sizeof(buf->data),
			       UART_WAIT_FOR_RX);

		break;

	case UART_RX_BUF_REQUEST:
		LOG_DBG("UART_RX_BUF_REQUEST");
		buf = k_malloc(sizeof(*buf));
		if (buf) {
			buf->len = 0;
			uart_rx_buf_rsp(uart, buf->data, sizeof(buf->data));
		} else {
			LOG_WRN("Not able to allocate UART receive buffer");
		}

		break;

	case UART_RX_BUF_RELEASED:
		LOG_DBG("UART_RX_BUF_RELEASED");
		buf = CONTAINER_OF(evt->data.rx_buf.buf, struct uart_data_t,
				   data);

		if (buf->len > 0) {
			/* STEP 9.1 -  Push the data received from the UART peripheral into the fifo_uart_rx_data FIFO */
			k_fifo_put(&fifo_uart_rx_data, buf);


		} else {
			k_free(buf);
		}

		break;

	case UART_TX_ABORTED:
		LOG_DBG("UART_TX_ABORTED");
		if (!aborted_buf) {
			aborted_buf = (uint8_t *)evt->data.tx.buf;
		}

		aborted_len += evt->data.tx.len;
		buf = CONTAINER_OF(aborted_buf, struct uart_data_t,
				   data);

		uart_tx(uart, &buf->data[aborted_len],
			buf->len - aborted_len, SYS_FOREVER_MS);

		break;

	default:
		break;
	}
}

static void uart_work_handler(struct k_work *item)
{
	struct uart_data_t *buf;

	buf = k_malloc(sizeof(*buf));
	if (buf) {
		buf->len = 0;
	} else {
		LOG_WRN("Not able to allocate UART receive buffer");
		k_work_reschedule(&uart_work, UART_WAIT_FOR_BUF_DELAY);
		return;
	}

	uart_rx_enable(uart, buf->data, sizeof(buf->data), UART_WAIT_FOR_RX);
}

static bool uart_test_async_api(const struct device *dev)
{
	const struct uart_driver_api *api =
			(const struct uart_driver_api *)dev->api;

	return (api->callback_set != NULL);
}

static int uart_init(void)
{
	int err;
	int pos;
	struct uart_data_t *rx;
	struct uart_data_t *tx;

	if (!device_is_ready(uart)) {
		return -ENODEV;
	}

	if (IS_ENABLED(CONFIG_USB_DEVICE_STACK)) {
		err = usb_enable(NULL);
		if (err && (err != -EALREADY)) {
			LOG_ERR("Failed to enable USB");
			return err;
		}
	}

	rx = k_malloc(sizeof(*rx));
	if (rx) {
		rx->len = 0;
	} else {
		return -ENOMEM;
	}

	k_work_init_delayable(&uart_work, uart_work_handler);


	if (IS_ENABLED(CONFIG_BT_NUS_UART_ASYNC_ADAPTER) && !uart_test_async_api(uart)) {
		/* Implement API adapter */
		uart_async_adapter_init(async_adapter, uart);
		uart = async_adapter;
	}

	err = uart_callback_set(uart, uart_cb, NULL);
	if (err) {
		LOG_ERR("Cannot initialize UART callback");
		return err;
	}

	if (IS_ENABLED(CONFIG_UART_LINE_CTRL)) {
		LOG_INF("Wait for DTR");
		while (true) {
			uint32_t dtr = 0;

			uart_line_ctrl_get(uart, UART_LINE_CTRL_DTR, &dtr);
			if (dtr) {
				break;
			}
			/* Give CPU resources to low priority threads. */
			k_sleep(K_MSEC(100));
		}
		LOG_INF("DTR set");
		err = uart_line_ctrl_set(uart, UART_LINE_CTRL_DCD, 1);
		if (err) {
			LOG_WRN("Failed to set DCD, ret code %d", err);
		}
		err = uart_line_ctrl_set(uart, UART_LINE_CTRL_DSR, 1);
		if (err) {
			LOG_WRN("Failed to set DSR, ret code %d", err);
		}
	}

	tx = k_malloc(sizeof(*tx));

	if (tx) {
		pos = snprintf(tx->data, sizeof(tx->data),
			       "Starting Nordic UART service example\r\n");

		if ((pos < 0) || (pos >= sizeof(tx->data))) {
			k_free(tx);
			LOG_ERR("snprintf returned %d", pos);
			return -ENOMEM;
		}

		tx->len = pos;
	} else {
		return -ENOMEM;
	}
	// Send a welcome message over UART 
	err = uart_tx(uart, tx->data, tx->len, SYS_FOREVER_MS);
	if (err) {
		LOG_ERR("Cannot display welcome message (err: %d)", err);
		return err;
	}
	// Enable start receiving data over UART
	return uart_rx_enable(uart, rx->data, sizeof(rx->data), 50);
}

static void connected(struct bt_conn *conn, uint8_t err)
{
	char addr[BT_ADDR_LE_STR_LEN];

	if (err) {
		LOG_ERR("Connection failed (err %u)", err);
		return;
	}

	bt_addr_le_to_str(bt_conn_get_dst(conn), addr, sizeof(addr));
	LOG_INF("Connected %s", addr);

	current_conn = bt_conn_ref(conn);

	dk_set_led_on(CON_STATUS_LED);
}

static void disconnected(struct bt_conn *conn, uint8_t reason)
{
	char addr[BT_ADDR_LE_STR_LEN];

	bt_addr_le_to_str(bt_conn_get_dst(conn), addr, sizeof(addr));

	LOG_INF("Disconnected: %s (reason %u)", addr, reason);

	if (auth_conn) {
		bt_conn_unref(auth_conn);
		auth_conn = NULL;
	}

	if (current_conn) {
		bt_conn_unref(current_conn);
		current_conn = NULL;
		dk_set_led_off(CON_STATUS_LED);
	}
}

#ifdef CONFIG_BT_NUS_SECURITY_ENABLED
static void security_changed(struct bt_conn *conn, bt_security_t level,
			     enum bt_security_err err)
{
	char addr[BT_ADDR_LE_STR_LEN];

	bt_addr_le_to_str(bt_conn_get_dst(conn), addr, sizeof(addr));

	if (!err) {
		LOG_INF("Security changed: %s level %u", addr, level);
	} else {
		LOG_WRN("Security failed: %s level %u err %d", addr,
			level, err);
	}
}
#endif

BT_CONN_CB_DEFINE(conn_callbacks) = {
	.connected    = connected,
	.disconnected = disconnected,
#ifdef CONFIG_BT_NUS_SECURITY_ENABLED
	.security_changed = security_changed,
#endif
};

#if defined(CONFIG_BT_NUS_SECURITY_ENABLED)
static void auth_passkey_display(struct bt_conn *conn, unsigned int passkey)
{
	char addr[BT_ADDR_LE_STR_LEN];

	bt_addr_le_to_str(bt_conn_get_dst(conn), addr, sizeof(addr));

	LOG_INF("Passkey for %s: %06u", addr, passkey);
}

static void auth_passkey_confirm(struct bt_conn *conn, unsigned int passkey)
{
	char addr[BT_ADDR_LE_STR_LEN];

	auth_conn = bt_conn_ref(conn);

	bt_addr_le_to_str(bt_conn_get_dst(conn), addr, sizeof(addr));

	LOG_INF("Passkey for %s: %06u", addr, passkey);
	LOG_INF("Press Button 1 to confirm, Button 2 to reject.");
}


static void auth_cancel(struct bt_conn *conn)
{
	char addr[BT_ADDR_LE_STR_LEN];

	bt_addr_le_to_str(bt_conn_get_dst(conn), addr, sizeof(addr));

	LOG_INF("Pairing cancelled: %s", addr);
}


static void pairing_complete(struct bt_conn *conn, bool bonded)
{
	char addr[BT_ADDR_LE_STR_LEN];

	bt_addr_le_to_str(bt_conn_get_dst(conn), addr, sizeof(addr));

	LOG_INF("Pairing completed: %s, bonded: %d", addr, bonded);
}


static void pairing_failed(struct bt_conn *conn, enum bt_security_err reason)
{
	char addr[BT_ADDR_LE_STR_LEN];

	bt_addr_le_to_str(bt_conn_get_dst(conn), addr, sizeof(addr));

	LOG_INF("Pairing failed conn: %s, reason %d", addr, reason);
}


static struct bt_conn_auth_cb conn_auth_callbacks = {
	.passkey_display = auth_passkey_display,
	.passkey_confirm = auth_passkey_confirm,
	.cancel = auth_cancel,
};

static struct bt_conn_auth_info_cb conn_auth_info_callbacks = {
	.pairing_complete = pairing_complete,
	.pairing_failed = pairing_failed
};
#else
static struct bt_conn_auth_cb conn_auth_callbacks;
static struct bt_conn_auth_info_cb conn_auth_info_callbacks;
#endif

static void bt_receive_cb(struct bt_conn *conn, const uint8_t *const data,
			  uint16_t len)
{
	int err;
	char addr[BT_ADDR_LE_STR_LEN] = {0};

	bt_addr_le_to_str(bt_conn_get_dst(conn), addr, ARRAY_SIZE(addr));

	LOG_INF("Received data from: %s", addr);

	for (uint16_t pos = 0; pos != len;) {
		struct uart_data_t *tx = k_malloc(sizeof(*tx));

		if (!tx) {
			LOG_WRN("Not able to allocate UART send data buffer");
			return;
		}

		/* Keep the last byte of TX buffer for potential LF char. */
		size_t tx_data_size = sizeof(tx->data) - 1;

		if ((len - pos) > tx_data_size) {
			tx->len = tx_data_size;
		} else {
			tx->len = (len - pos);
		}

		memcpy(tx->data, &data[pos], tx->len);

		pos += tx->len;

		/* Append the LF character when the CR character triggered
		 * transmission from the peer.
		 */
		if ((pos == len) && (data[len - 1] == '\r')) {
			tx->data[tx->len] = '\n';
			tx->len++;
		}
        /* STEP 8.3 - Forward the data received over Bluetooth LE to the UART peripheral */
		err = uart_tx(uart, tx->data, tx->len, SYS_FOREVER_MS);
		if (err) {
			k_fifo_put(&fifo_uart_tx_data, tx);
		}
	}
}
/* STEP 8.1 - Create a variable of type bt_nus_cb and initialize it*/
static struct bt_nus_cb nus_cb = {
	.received = bt_receive_cb,
};

void error(void)
{
	dk_set_leds_state(DK_ALL_LEDS_MSK, DK_NO_LEDS_MSK);

	while (true) {
		/* Spin for ever */
		k_sleep(K_MSEC(1000));
	}
}

#ifdef CONFIG_BT_NUS_SECURITY_ENABLED
static void num_comp_reply(bool accept)
{
	if (accept) {
		bt_conn_auth_passkey_confirm(auth_conn);
		LOG_INF("Numeric Match, conn %p", (void *)auth_conn);
	} else {
		bt_conn_auth_cancel(auth_conn);
		LOG_INF("Numeric Reject, conn %p", (void *)auth_conn);
	}

	bt_conn_unref(auth_conn);
	auth_conn = NULL;
}

void button_changed(uint32_t button_state, uint32_t has_changed)
{
	uint32_t buttons = button_state & has_changed;

	if (auth_conn) {
		if (buttons & KEY_PASSKEY_ACCEPT) {
			num_comp_reply(true);
		}

		if (buttons & KEY_PASSKEY_REJECT) {
			num_comp_reply(false);
		}
	}
}
#endif /* CONFIG_BT_NUS_SECURITY_ENABLED */

static void configure_gpio(void)
{
	int err;

#ifdef CONFIG_BT_NUS_SECURITY_ENABLED
	err = dk_buttons_init(button_changed);
	if (err) {
		LOG_ERR("Cannot init buttons (err: %d)", err);
	}
#endif /* CONFIG_BT_NUS_SECURITY_ENABLED */

	err = dk_leds_init();
	if (err) {
		LOG_ERR("Cannot init LEDs (err: %d)", err);
	}
}

void main(void)
{
	int blink_status = 0;
	int err = 0;

	configure_gpio();
	/* STEP 7 - Initialize the UART Peripheral  */
	err = uart_init();
	if (err) {
		error();
	}

	if (IS_ENABLED(CONFIG_BT_NUS_SECURITY_ENABLED)) {
		err = bt_conn_auth_cb_register(&conn_auth_callbacks);
		if (err) {
			LOG_ERR("Failed to register authorization callbacks.\n");
			return;
		}

		err = bt_conn_auth_info_cb_register(&conn_auth_info_callbacks);
		if (err) {
			LOG_ERR("Failed to register authorization info callbacks.\n");
			return;
		}
	}

	err = bt_enable(NULL);
	if (err) {
		error();
	}

	LOG_INF("Bluetooth initialized");

	k_sem_give(&ble_init_ok);

	if (IS_ENABLED(CONFIG_SETTINGS)) {
		settings_load();
	}
/* STEP 8.2 - Pass your application callback function to the NUS service */
	err = bt_nus_init(&nus_cb);
	if (err) {
		LOG_ERR("Failed to initialize UART service (err: %d)", err);
		return;
	}

	err = bt_le_adv_start(BT_LE_ADV_CONN, ad, ARRAY_SIZE(ad), sd,
			      ARRAY_SIZE(sd));
	if (err) {
		LOG_ERR("Advertising failed to start (err %d)", err);
		return;
	}

	for (;;) {
		dk_set_led(RUN_STATUS_LED, (++blink_status) % 2);
		k_sleep(K_MSEC(RUN_LED_BLINK_INTERVAL));
	}
}
/* STEP 9.3 - Define the thread function  */
void ble_write_thread(void)
{
	/* Don't go any further until BLE is initialized */
	k_sem_take(&ble_init_ok, K_FOREVER);

	for (;;) {
		/* Wait indefinitely for data from the UART peripheral */
		struct uart_data_t *buf = k_fifo_get(&fifo_uart_rx_data,
						     K_FOREVER);
        /* Send data over Bluetooth LE to remote device(s) */
		//const uint8_t test = 12;
		if (bt_nus_send(NULL, buf->data, buf->len)) {
		//if (bt_nus_send(NULL, &test, sizeof(test))){
			LOG_WRN("Failed to send data over BLE connection");
		}

		k_free(buf);
	}
}
/* STEP 9.2 - Create a dedicated thread for sending the data over Bluetooth LE. */
K_THREAD_DEFINE(ble_write_thread_id, STACKSIZE, ble_write_thread, NULL, NULL,
		NULL, PRIORITY, 0, 0);

[Hung Bui]

Hi Esbriven, 

Have you gone through Lesson 4 exercise 2 ? In the exercise we show you how to send (simulated) sensor data using bt_gatt_notify(). 

It's similar to what the NUS service in peripheral_uart does. In the code you can find bt_nus_send() is called when there is some data in the UART buffer. 
In your case you just need to call bt_gatt_notify() with the size of the data you want to send. 
Note that you need to enable notification on the peer side (the phone) and if you are simply sending raw data it may not display as  normal text as it's not ASCII you need to look into the raw data that you receive on the phone. 

[Esbriven]

Hi, 

I have followed the exercise 2 and I managed to send data collected by my sensor but they are indeed in hex format. I've read that since it's binary data that are transmitted, the display has to be managed from the central (my phone with nRF Connect for mobile).

Another thing that I don't understand in the exercise 2 is how the message "button released" or "button pressed" are transmitted. I've notice that my phone get a notify with a value of 0x00  for "button released" and 0x01 for "button pressed" on top of the ASCII message. I searched in the different header files of zephyr to find a mention of both " button released" and "pressed"  but i couldn't find any. Is it that nRF_connect (my phone) automatically recognized the value 0x00 as "button released" with the specific UUID related to the button or is it defined somewhere ?

Thank you for you response.

best regards. 

[Hung Bui]

Hi Esbriven, 

The "Button pressed" and "button releases" is just a feature in the app that it detect the LBS service automatically and translate 1 and 0 to "pressed" and "released". 

When you use nRF Connect for mobile, and you declare your own service, it will display the hex value. For example: 

[Esbriven]

OK that's what i thought about the button. 

I think managed to understand the bt_gatt_notify() function i'm able to send the data collected from my sensor, but they are in a hex format. 

So now if I want to display the data on the central in a proper format (i.e float with some text like : AccX = %f g) what should I do ? If I understand properly the NUS protocol (with bt_nus_send() function) it recognize the data as ASCII and display it as such in the receiving end right ? Do I have to convert my data as a string prior the sending if I want to use this method ? 

By the way I also have a nRF52840 dongle which could be programmed to convert the hex data send with the custom LBS explained in lesson 3 ex2 into a more readable format but I'm not sure if it's doable. 

Thanks again for the response :) 

[Hung Bui]

Hi Esbriven, 

I don't see much point converting the data to human readable format then send over BLE. 

Instead you send hex value over BLE and then convert to human readable format on the peer device, for example the app on the phone or the dongle. 

On the peer you may want to look for "convert float to string"  in the program language that you use on the peer. 

For example in C: https://www.geeksforgeeks.org/gcvt-convert-float-value-string-c/