nRF Peripheral NUS sample, receiving message from nRF Toolbox UART service

RE: nRF Peripheral NUS sample, receiving message from nRF Toolbox UART service

Vidar Berg — Thu, 27 Mar 2025 13:07:12 GMT

There is a section named "Data length and MTU" in the devacademy course here https://academy.nordicsemi.com/courses/bluetooth-low-energy-fundamentals/lessons/lesson-3-bluetooth-le-connections/topic/connection-parameters/ which I would recommend reading.

To support longer MTUs you can add the following lines to your prj.conf file:

CONFIG_BT_BUF_ACL_RX_SIZE=251
CONFIG_BT_BUF_ACL_TX_SIZE=251
CONFIG_BT_L2CAP_TX_MTU=247

RE: nRF Peripheral NUS sample, receiving message from nRF Toolbox UART service

zachary_z — Thu, 27 Mar 2025 12:45:17 GMT

I tried to send more than 20 characters. I now tried under 12 characters and it worked!

Is there a way to increase the payload size?

RE: nRF Peripheral NUS sample, receiving message from nRF Toolbox UART service

Vidar Berg — Thu, 27 Mar 2025 12:41:15 GMT

What is the length of the string you're trying to send? The send function will return with -ENOMEM ('12') if you try to send more than the max. payload which is the ATT_MTU size - 3 bytes for the ATT header.

RE: nRF Peripheral NUS sample, receiving message from nRF Toolbox UART service

zachary_z — Thu, 27 Mar 2025 12:28:38 GMT

Thanks for this! The code mostly works.

However, I now have another problem. I can't seem to send a concatenated string with bt_nus_send. Everything else has been done correctly, but the send fails with error -12. I cannot find an error corresponding to the value -12. What could this be?

RE: nRF Peripheral NUS sample, receiving message from nRF Toolbox UART service

Vidar Berg — Tue, 25 Mar 2025 13:42:55 GMT

Hi,

If you want to modify the sample so it sends back a response when receiving data, you can try something like this:

peripheral_nus -> main.c

struct nus_rx_data {
	char message[CONFIG_BT_L2CAP_TX_MTU + 1];
};

/* 
 * Create message queue to pass NUS rx data to the main thread 
 */
K_MSGQ_DEFINE(nus_msgq, sizeof(struct nus_rx_data), 1, 1);

....

static void received(struct bt_conn *conn, const void *data, uint16_t len, void *ctx)
{
	int err;
	struct nus_rx_data rx_data;

	ARG_UNUSED(conn);
	ARG_UNUSED(ctx);

	memset(rx_data.message, 0x0, sizeof(rx_data.message));
	memcpy(rx_data.message, data, MIN(sizeof(rx_data.message) - 1, len));
	
	printk("%s() - Len: %d, Message: %s\n", __func__, len, rx_data.message);

	/* Forward received NUS RX data to main thread */
	err = k_msgq_put(&nus_msgq, &rx_data, K_NO_WAIT);
	if (err) {
		printk("k_msgq_put() failed. (err %d)\n", err);
	}
}


....

int main(void) 
{
    ...
    while (true) {

		/* Wait for NUS Command */
		k_msgq_get(&nus_msgq, &rx_data, K_FOREVER);

		//TODO: process received command?

		/* Send back response */
		err = bt_nus_send(NULL, rx_data.message, strlen(rx_data.message));
		printk("Data send - Result: %d\n", err);

		if (err < 0 && (err != -EAGAIN) && (err != -ENOTCONN)) {
			return err;
		}
	}

Best regards,

Vidar