max characters to write in a characteristic

[ccssw]

I found that I can only write up to 216 characters in a characteristic, otherwise the callback is not run. Is it possible to enlarge it?

[h2zero]

That doesn't seem right, I'm able to write from a client device and receive them on the server. What is the MTU size?

[ccssw]

When I try to set the MTU size with NimBLEDevice::setMTU(BLE_ATT_MTU_MAX) I get:

D NimBLEDevice: ">> setLocalMTU: 527" assertion "mu->handle" failed: file ".pio/libdeps/noukee/NimBLE-Arduino/src/porting/npl/freertos/src/npl_os_freertos.c", line 185, function: npl_freertos_mutex_pend

[h2zero]

That's a very strange error. I have not had this issue nor seen any reports regarding this previously. I will do what I can to reproduce.

As for your initial issue, what is your setup are you connecting with a phone to the esp32 or another device? If from a phone you may need to use long write if the data size is greater than MTU-3.

[ccssw]

what is your setup are you connecting with a phone to the esp32 or another device?

Yes, it's a smart phone.

[h2zero]

Ok, so the limit you see is due most likely to the phone MTU. You cannot change it as the spec states that the lowest of the 2 MTU's is the one that is used.

In that situation you will need to use a long write procedure.

[h2zero]

@ccssw any updates on this?

[ccssw]

Requesting the MTU to 517 fixed the issue for now.

[chegewara]

Max MTU is 517 IIRC.

[h2zero]

Max MTU is 517 IIRC.

Max for NimBLE is 527 to accomodate signed writes, which we don't do (yet) so 517 works 😄

Going to close this as it seems to be resolved.