NimBLE MTU Size Issue

[ZainNoman]

I cant seem to increase the MTU size in NimBLE. the longest data i can receive is 271 byte. Ive slightly altered the spp_server example. i am printing the length and received data in the function

ble_svc_gatt_handler

. I have incresed the

CONFIG_BT_NIMBLE_ATT_PREFERRED_MTU

to 512. I am using nRF Connect as gatt client and I have set the MTU Size from there as well as can be seen in the snapshot of the logs attached.

[irahul]

Is exchange MTU procedure being done ? If yes, then what is the value being negotiated in it ?

[ZainNoman]

Yes, Im setting the MTU Size to 512 using the NRF connect app. Then I send a 512 byte (or a little smaller) text string. Only 271 bytes are received

[irahul]

You can check on console log ( idf.py monitor) for device side OR the nrf logger in nrfapp side. I agree that you entered 517 as value, but was that set ? If the chip side sends smaller MTU, then the final negotiated value will not be 517 , but the minimum of the size mentioned by both side.

For e.g. something like below on esp device side
I (26793) NimBLE: mtu update event; conn_handle=0 cid=4 mtu=256

Or something like below on nrfapp side
MTU changed to : 256

[ZainNoman]

These were the logs i got. they do show that the MTU size was set. I was able to set to 512 only after I had altered the menuconfig

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