BLE characteristic write fails silently when value is more than 20 bytes

[Oli]

I'm using Nrf connect for Mobile to test my BLE firmware. I noticed that when trying to write more than 20 bytes to a characteristic, nothing gets logged which means my write callback function isn't called. Either the nrf connect app isn't sending anything above 20 bytes or the BLE stack is just silently ignoring the message for some reason. According to BLE spec, characteristic value can be up to 512 bytes so that's a bit unexpected. Am I doing something wrong? I attached my prj.conf and some code.

# Enable logging system to USB
CONFIG_LOG=y

# <BLE>
CONFIG_BT=y
CONFIG_BT_PERIPHERAL=y
CONFIG_BT_DEVICE_NAME="bale module"
# Allow multiple devices to connect at once
CONFIG_BT_MAX_CONN=6
CONFIG_BT_BUF_ACL_RX_COUNT=7
# Enable Security Management Protocol
# CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE=2048
# CONFIG_BT_GATT_DYNAMIC_DB=y
# </BLE>

# Needed for LEDs
CONFIG_DK_LIBRARY=y
CONFIG_BT_HCI_ERR_TO_STR=y

# <CRYPTO>
# Enable nordic security backend and PSA APIs
CONFIG_NRF_SECURITY=y
CONFIG_MBEDTLS_PSA_CRYPTO_C=y

CONFIG_PSA_WANT_ALG_ECDH=y
CONFIG_PSA_WANT_KEY_TYPE_ECC_KEY_PAIR_GENERATE=y
CONFIG_PSA_WANT_KEY_TYPE_ECC_KEY_PAIR_IMPORT=y
CONFIG_PSA_WANT_KEY_TYPE_ECC_KEY_PAIR_EXPORT=y
CONFIG_PSA_WANT_ECC_SECP_R1_256=y

# For key generation
CONFIG_PSA_WANT_GENERATE_RANDOM=y

CONFIG_SYS_CLOCK_TICKS_PER_SEC=1000
CONFIG_MBEDTLS_ENABLE_HEAP=y
CONFIG_MBEDTLS_HEAP_SIZE=8192

CONFIG_MAIN_STACK_SIZE=4096
CONFIG_HEAP_MEM_POOL_SIZE=4096
# </CRYPTO>

BT_GATT_SERVICE_DEFINE(rcs_svc,
	BT_GATT_PRIMARY_SERVICE(BT_UUID_RCS_SERVICE),
	BT_GATT_CHARACTERISTIC(BT_UUID_RCS_TX,
		BT_GATT_CHRC_NOTIFY,
		BT_GATT_PERM_READ,
		NULL,
		NULL,
		NULL),
	BT_GATT_CCC(NULL, BT_GATT_PERM_READ | BT_GATT_PERM_WRITE),
	BT_GATT_CHARACTERISTIC(BT_UUID_RCS_RX,
		BT_GATT_CHRC_WRITE,
		BT_GATT_PERM_READ | BT_GATT_PERM_WRITE,
		NULL,
		on_receive,
		NULL),
	);

// RX attribute was written to
static ssize_t on_receive(struct bt_conn* conn,
	const struct bt_gatt_attr* attr,
	const void* buf,
	uint16_t len,
	uint16_t offset,
	uint8_t flags)
{
	char addr[BT_ADDR_LE_STR_LEN];
	bt_addr_le_to_str(bt_conn_get_dst(conn), addr, sizeof(addr));
	LOG_DBG("[%s] Received data, handle %d, conn %p",
		addr, attr->handle, (void*)conn);
	return len;
}

[Oli]

I see some suggestions to use `NRF_SDH_BLE_GATT_MAX_MTU_SIZE` in other threads, but if I set it in my prj.conf, I am getting this error: ignoring malformed line 'NRF_SDH_BLE_GATT_MAX_MTU_SIZE=128' and using a `#define NRF_SDH_BLE_GATT_MAX_MTU_SIZE 128` doesn't do anything.

[Oli]

I also tried with following configs in my prj.conf but is still failing. The Nrf Connect for Mobile app logs show "Error 6 (0x6): GATT REQ NOT SUPPORTED" when I try to  write the value.

CONFIG_BT_L2CAP_TX_MTU=247
CONFIG_BT_BUF_ACL_TX_SIZE=251
CONFIG_BT_BUF_ACL_RX_SIZE=251

[Oli]

Actually, I think those latest settings fixed the issue but I have to manually request the MTU change from the NrfConnect mobile app's "Request MTU" function before writing to the characteristic. Would be nice to be able to avoid doing that but not sure if it's possible.

[Maria Gilje]

Hello,

Good to see that you found a resolution. Exercise 2 from our Bluetooth Low Energy Fundamentals course may also shed some light on this.

As you found out, the peripheral can request an MTU change to a larger value, but it is ultimately up to the central to decide the final value.

Best regards,

Maria