Thingy Sound Service | SDK13+ | MAX_MTU_SIZE

[Spirit]

Hello guys!

I'm trying to port the TSS service to one of my projects. This project is based on SDK13.1 while the Thingy is on SDK12.1

I have a lot of problems with a max MTU size. Somehow Thingy uses for a sound transfer a MTU of 276.

I tryed to use the same MTU size in a SDK13, but I'm blocked with

STATIC_ASSERT(NRF_BLE_GATT_MAX_MTU_SIZE <= 251);
STATIC_ASSERT(NRF_BLE_GATT_MAX_MTU_SIZE + LL_HEADER_LEN <= 255);

in nrf_ble_gatt.c.

Can anyone help me?

[Spirit]

Hi Sigurd!

But how it is then possible that Thingy uses a MTU of 276 for an audio decoder? Defined by:

#define NRF_BLE_MAX_MTU_SIZE            GATT_MTU_SIZE_DEFAULT*12 
#define GATT_MTU_SIZE_DEFAULT 23

[Sigurd]

Hi,

Seems like this is not a bug in the Thingy SDK after all. The SoftDevice actually support ATT MTU size up-to 512 bytes, but the on-air data packet length is 251 bytes. If the ATT packet cannot fit in a single on-air packet, it will be split up into multiple on-air packets. The current SDK version have therefore chosen to set a ATT MTU limit at 251 as well. This will avoid fragmentation of the ATT packet into several on-air data packets, and therefore often increase the data throughput. Note that the SDK will open up for longer ATT MTU sizes in the future.

[Spirit]

Thanks for the explanation :)

@Sigurd, do Nordic have any plans to port Thingys FW to a newer SDK14.0?

[Sigurd]

The current version of the Thingy SDK(1.1.0) uses SDK 12.1. In the next release, the Thingy SDK will be built based upon a newer SDK version. This will be either SDK 13 or newer.