nRF52840 BLE Transfer speed issues

[masterLee]

Hi all,

I have a custom nRF52840 board that transmits 32-byte packets to mobile device and I want to achieve maximum transfer speed.

The code is based on the ble_app_uart project

The BLE settings in my project are as follow:

//sdk_config
NRF_SDH_BLE_GAP_DATA_LENGTH 251
NRF_SDH_BLE_GATT_MAX_MTU_SIZE 247
NRF_SDH_BLE_GAP_EVENT_LENGTH 6

//main
MIN_CONN_INTERVAL   MSEC_TO_UNITS(8, UNIT_1_25_MS)
MAX_CONN_INTERVAL   MSEC_TO_UNITS(8, UNIT_1_25_MS)
 
ble_stack_init();
gap_params_init();
gatt_init();
services_init();
advertising_init();
sd_ble_gap_tx_power_set(BLE_GAP_TX_POWER_ROLE_ADV,m_advertising.adv_handle,8); // Max  ADV Power = 8dB
sd_ble_gap_tx_power_set(BLE_GAP_TX_POWER_ROLE_CONN,m_advertising.adv_handle,8); // Max CONN Power = 8dB

The transmission data rate from the custom device to the mobile is 8kB/sec +/- ~400B

I want to reach maximum transfer speed but I don't know what the upper limit is.

Based on table 2 of the following link, I see that the max transfer speed is ~165kB/sec. So I try to figure out what I am doing wrong and my device's transfer speed is so low.

(https://infocenter.nordicsemi.com/index.jsp?topic=%2Fsds_s140%2FSDS%2Fs1xx%2Fble_data_throughput%2Fble_data_throughput.html)

From the debug session I get the following Information where the connection is established with DLE (247Byte / BLE_packet)

So I assume that something else keeps the transmission speed from my board to the mobile so low.

Also, I have a few questions :

1. How can I increase NRF_SDH_BLE_GATT_MAX_MTU_SIZE from 247 to 512? Because it is a 1-byte value and 512 needs 2-bytes  will need to mess with the SD libraries
2. Is it possible to increase the NRF_SDH_BLE_GAP_EVENT_LENGTH  higher than 6? Because when I set it to 7 or 8 the whole system stops.

If there is something else that I have not taken into account please correct me.

Thank you for your time

Regards

[masterLee]

Hi,

No the peripheral is connected to a single central device (tablet). The tablet app is configured such a way that it can connect up to 5 peripherals. The good thing is that the speed is increasedto 50kBps by changing  the GAP_EVENT_LENGTH from 5 to 10. Further increments did not increase the speed. 

Also, I noticed that although I set the PHY to 2M (inside ble_evt_handler) the tablet keeps it at 1M all the time.

Any suggestions?

[Sigurd]

Regarding 2MPHY, did you e.g. add something like this to the BLE_GAP_EVT_CONNECTED event ?

            ble_gap_phys_t const phys =
            {
                .rx_phys = BLE_GAP_PHY_2MBPS,
                .tx_phys = BLE_GAP_PHY_2MBPS,
            };
            err_code = sd_ble_gap_phy_update(p_ble_evt->evt.gap_evt.conn_handle, &phys);
            APP_ERROR_CHECK(err_code);

If yes, then you should be able to see a LL_PHY_REQ packet from the slave, and a LL_PHY_UPDATE_IND packet sent from the master in the sniffer log, indicating if it accepts the 2MPHY request or not.

[masterLee]

Yes I set both at 2Mpbs in the EVT_CONNECTED event.

Ok, I will check it out tomorrow morning first thing.

Any suggestions, on how to add more 244-byte packets in one connection interval? Because as you can see in the sniffer log, for the duration of the connection interval only one 244-byte package is being sent to the peer and the rest of the interval is idle...

[masterLee]

Good News at  last!! ! I reached 100kBps +/- 20kBps for a single connection between one peripheral device and the tablet.

To sum up, In order to reach maximum throughput with BLE 5 PHY_2Mbps I performed the following changes (Slave Device = Custom nRF52840 Board & Master Device = Samsung Tab S6 with BLE 5)

• In the ble_nus_data_send function() I use most of the length of the ATT package (244)
• Set the GAP_EVENT_LENGTH = 10
• Set both connection interval MIN and MAX = 10

Sigurt, thank you for your valuable time and help on this issue!

[Sigurd]

masterLee said:

Good News at  last

 Great! Happy to help