Slow Transfer - Calling ble_nus_string_send multiple times to send a large array

[kian79]

Hi all,

I am using PCA10040 with nRF5_SDK_14.2.0 and using the ble_app_uart example.

Basically I have a large array of size 18750 to send out and having read on the forum, I understand that I need to split up the data into smaller chunks and call ble_nus_string_send multiple times. Most people manage to get it to work by splitting into chuck of 20 bytes.

What I have tried so far is that I can provide a 150 byte array into the ble_nus_string_send function without any problem and its receiving well on a mobile phone. I also made a few changes:

#define MIN_CONN_INTERVAL MSEC_TO_UNITS(7.5, UNIT_1_25_MS) /**< Minimum acceptable connection interval (20 ms), Connection interval uses 1.25 ms units. */
#define MAX_CONN_INTERVAL MSEC_TO_UNITS(7.5, UNIT_1_25_MS) /**< Maximum acceptable connection interval (75 ms), Connection interval uses 1.25 ms units. */

Inside the gatt_init function, I also increase the max MUT size to 247:

err_code = nrf_ble_gatt_att_mtu_periph_set(&m_gatt, 247);

This is what I did in the main function:

// Enter main loop.
    for (;;)
    {
        UNUSED_RETURN_VALUE(NRF_LOG_PROCESS());
        power_manage();
			
				if(start_transfer)
				{
					start_transfer = false;
					uint16_t length = 150;
					uint8_t j = 0;
					uint16_t k = 0;
					for(j=0; j<125; j++)
					{
						do
						{	
							err_code = ble_nus_string_send(&m_nus, mybuffer+k, &length);
							if ( (err_code != NRF_ERROR_INVALID_STATE) && (err_code != NRF_ERROR_BUSY) )
							{
								APP_ERROR_CHECK(err_code);
							}
						} while (err_code == NRF_ERROR_BUSY);
						k = k + 150;
						while(!tx_complete);
						tx_complete = false;
					}
					
				}
    }

Each call to ble_nus_string_send, I am sending out 150 bytes, and I go this in a for loop that iterates for 125 times, so essentially I will transfer all 18750 bytes. After each call to ble_nus_string send, I am checking a flag tx_complete , which is set to true when a BLE_GATTS_EVT_HVN_TX_COMPLETE event happens. 

case BLE_GATTS_EVT_HVN_TX_COMPLETE:
	tx_complete = true;
	break;

This is working well except that the transfer seems to be still fairly slow about 19-20 seconds to send everything over. Is there anything I am doing wrong? How can I make this go much more faster?

[kian79]

Hi all,

Looks like my problem isn't entirely solved. I just found another issue. Here is my RTT log:

0> <warning> nrf_sdh_ble: RAM starts at 0x20002A68, can be adjusted to 0x200025D0.
0> <warning> nrf_sdh_ble: RAM size can be adjusted to 0xDA30.
0> <info> app: UART Start!
0> <debug> ble_gatt: Requesting to update ATT MTU to 64 bytes on connection 0x0.
0> <debug> ble_gatt: Requesting to update data length to 251 on connection 0x0.
0> <error> ble_gatt: sd_ble_gap_data_length_update() (request) on connection 0x0 returned unexpected value 0x13.
0> <info> app: Connected
0> <debug> ble_gatt: ATT MTU updated to 64 bytes on connection 0x0 (response).
0> <info> app: Data len is set to 0x3D(61)
0> <debug> app: ATT MTU exchange completed. central 0xF7 peripheral 0x40
0> <debug> ble_gatt: Peer on connection 0x0 requested a data length of 123 bytes.
0> <debug> ble_gatt: Updating data length to 123 bytes on connection 0x0.
0> <debug> ble_gatt: Data length updated to 123 on connection 0x0.
0> <debug> ble_gatt: max_rx_octets: 123
0> <debug> ble_gatt: max_tx_octets: 123

I presume that my Data Length has been increased to 123 bytes from the RTT log. Hence I am calling ble_nus_string_send() with a length of 100.

				if(start_BLE_xfer)
				{
					NRF_LOG_INFO("Start Transferring");
					start_BLE_xfer = false;
					// start transferring BLE data
					uint8_t x;
					uint16_t y = 0;
					uint16_t length = 100;
					for(x=0; x< 188; x++)
					{
						do
            {
							err_code = ble_nus_string_send(&m_nus, buffer1 + y, &length);
              if ( (err_code != NRF_ERROR_INVALID_STATE) && (err_code != NRF_ERROR_BUSY) )
              {
								APP_ERROR_CHECK(err_code);
              }
            } while (err_code == NRF_ERROR_BUSY);	
						y = y+100;
						
						//NRF_LOG_INFO("Send OK");
						while(!tx_complete);
						tx_complete = false;
					}
					//NRF_LOG_INFO("Finish transferring");
				}

I have a buffer of size 1880 and I am transferring 100 bytes per call to ble_nus_string_send. On the receiving side I have an android phone running nrf ToolBox and connected using the UART app. Looking at the log on the mobile app, I see that I am only getting 61 bytes in each notification and the remaining 39 bytes is lost. 

Eg.

first notification: buffer1[0] to buffer1[60] is received, buffer1[61] to buffer1[99] is lost.

second notification: buffer1[100] to buffer1[160] is received, buffer1[161] to buffer1[199] is lost.

I also tried to set the MUT to 100: 

err_code = nrf_ble_gatt_att_mtu_periph_set(&m_gatt, 100);

This is the RTT log:

0> <warning> nrf_sdh_ble: RAM starts at 0x20002A68, can be adjusted to 0x200025D0.
0> <warning> nrf_sdh_ble: RAM size can be adjusted to 0xDA30.
0> <info> app: UART Start!
0> <debug> ble_gatt: Requesting to update ATT MTU to 100 bytes on connection 0x0.
0> <debug> ble_gatt: Requesting to update data length to 251 on connection 0x0.
0> <error> ble_gatt: sd_ble_gap_data_length_update() (request) on connection 0x0 returned unexpected value 0x13.
0> <info> app: Connected
0> <debug> ble_gatt: ATT MTU updated to 100 bytes on connection 0x0 (response).
0> <info> app: Data len is set to 0x61(97)
0> <debug> app: ATT MTU exchange completed. central 0xF7 peripheral 0x64
0> <debug> ble_gatt: Peer on connection 0x0 requested a data length of 123 bytes.
0> <debug> ble_gatt: Updating data length to 123 bytes on connection 0x0.
0> <debug> ble_gatt: Data length updated to 123 on connection 0x0.

Now on the android app, I am receiving 97 bytes and losing 3 bytes for each notification call.

I don't understand why. Can anyone help advise? And how can I solve this problem?

[hmolesworth]

Looks suspiciously like the old bug (feature) in gatt_init(), as 61 = 64-3. I saw this a while back, perhaps there is similar code on the central end

was:
    err_code = nrf_ble_gatt_att_mtu_periph_set(&m_gatt, 64);
should be:
    err_code = nrf_ble_gatt_att_mtu_periph_set(&m_gatt, NRF_SDH_BLE_GATT_MAX_MTU_SIZE);

[kian79]

Hi hmolesworth,

I made the changes you suggested and it worked! Thanks a lot! I wonder why the default example had the MUT size set to 64 and not to NRF_SDH_BLE_GATT_MAX_MTU_SIZE. That would have not caused so much confusion. 

Thanks a lot for the advise. Appreciate it!