Setting MTU Above 64 bytes Crashes ble_app_uart

[xeem]

Hi all,

I am trying to send a large stream of bytes(60kb) to an NRF52823 using bbowl. BBowl exposes the uart through a TCP socket connection using noble.

I modified the gatt_init code in SDK13 to support the largest MTU possible:

void gatt_init(void) {
    ret_code_t err_code;

    err_code = nrf_ble_gatt_init(&m_gatt, gatt_evt_handler);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_ble_gatt_att_mtu_periph_set(&m_gatt, 155);
    APP_ERROR_CHECK(err_code); }

I validated that the MTU is set properly and that packets are being sent by using:

sudo hcidump -i -x

Problems

1. The connect is very slow. It takes 2-5 minutes to transfer the full 60 kilobytes depending on the MTU.
2. If the MTU is set above 64 bytes on the raspberry pi the transfer times out and/or the NRF52DK reboots.

Questions

1. Could the NRF52DK be running out of memory due to the extremely fast MTU? Is there any way to verify? I didn't see anything in the J-Link logs.
2. Is there a setting I can change to achieve an MTU of 241 bytes instead of 158?
3. Do I need to increase the RX/TX buffer?
4. Would it help to enable DLE with the SDK 13 examples? Or is it already enabled?

Thanks so much for your help!

[FormerMember]

The Adafruit module, does it use a nRF51 chip? If so, it is normal that the long MTU feature doesn't work, because the nRF51/s130 doesn't support long MTU.

[xeem]

Good question! I'm using an NRF52DK so I think it's an NRF52832. In theory long MTU should be supported? Hcidump shows a long pairing MTU.

You bring up a good point about DLE though. Do I need to modify the ble_app_uart example to support it or does DLE work out of the box?

[FormerMember]

Yes, the chip on the nRF52-DK is a nRF52832. The example ble_app_att_mtu_throughput in the ble_central_and_peripheral/experimental folder in SDK13 shows how to use long MTU. Could you test this example as well?

I may have misunderstood, the nRF52-DK does it communicate with the Adafruit module, or a raspberry pi? For long MTU to work, both sides of the link have to support long MTU.

[xeem]

My problem here was that I set a low UART priority. Increasing the UART priority solved all problems.