Negotiating max mtu size

[Daniel Reisfeld]

In previous questions I inquired about setting the MTU size beyond 27 and thank to the answers I, indeed, manage to compile and run my program with larger MTU size definitions (by changing the value of NRF_SDH_BLE_GATT_MAX_MTU_SIZE and RAM allocation).

However, when coming to negotiation with the client (an Android phone) I failed to understand how to get the maximal supported size (and only found out empirically that I cannot send packets beyond 20 bytes).

In a previous answer I understood that calling "sd_ble_gap_data_length_update(uint16_t conn_handle, ble_gap_data_length_params_t const *p_dl_params, ble_gap_data_length_limitation_t *p_dl_limitation)" with a null p_dl_params and a non-null p_dl_params will result in the maximal available values in the p_dl_limitation. However, I get all zeros in the returned structure.

Please advise how can I get the phone mtu size limit.

Thanks,

Daniel

[Edvin]

Hello Daniel,

What SDK are you using? And what chip?

Have you tried using the nrf_ble_gatt_att_mtu_periph_set() function? Depending on your SDK, if you enable logging the status of this call is usually printed in the NRF_LOG module.

Best regards,

Edvin

[Daniel Reisfeld]

Edvin,

I'm using SDK v15 and nrf52840.

I'll check it - thanks!

Daniel 

[Edvin]

The easiest would be to use the desired MTU in the gatt_init() function. But you can also request it later.

Please see the attached project. First it sets the mtu in gatt_init(), and then it is requested changed on the connection event. Both are marked with the comment:

"// UPDATE MTU HERE"

ble_app_uart_mtu_ext.zip

The project is from SDK15.0.0, and in Keil µVision, but I have only turned off optimization, so you should be able to use any IDE you like.

Best regards,

Edvin

[Daniel Reisfeld]

Thanks Edvin! I understand it now, Daniel