MTU Exchange and default BLE_EVT_BUFFER size

[Eric Stutzenberger]

Hi,

In the BLE_UART example code posted here, I noticed a discrepancy with the SD 3.0 migration document, SDK12, and the example code.

In stock SDK 12.1.0, ble_stack_handler_types.h has BLE_EVT_BUFFER defined with a size of [(sizeof(ble_evt_t) + (GATT_MTU_SIZE_DEFAULT))] where GATT_MTU_SIZE_DEFAULT is 23. This does not work well with larger MTU sizes as the event buffer doesn't contain enough space for larger MTU sizes. I found this when trying to work with a characteristic that has a size of 128 bytes. When talking to an OS X based laptop, I was unable to send larger sizes and determined the cause seems to be related to the size of the above buffer.

In the example source, I see that BLE_EVT_BUFFER has a size of (sizeof(ble_evt_t) + (NRF_BLE_MAX_MTU_SIZE)) which is defined in the example's main.c file with

#if (NRF_SD_BLE_API_VERSION == 3)
#define NRF_BLE_MAX_MTU_SIZE            158   //DLE
#endif

I am wondering if I missed where this is a required change in the migration document or if it is not documented appropriately. Is the answer to change the defines in ble_stack_handler_types? Is there a formula for making sure BLE_EVT_BUFFER is large enough to handle the maximum supported MTU by my application?

Please clarify.

Thanks,

Eric

[Petter Myhre]

If you want to use the SoftDevice handler library with larger MTUs, the define has to be modified.

It wouldn't be in the SoftDevice migration document as the library is part of the SDK, but I agree that it could be better documented.

Why can't you use the formula in the example?

[Eric Stutzenberger]

Hi Petter,

I suppose you are correct in that the SoftDevice migration document isn't the appropriate place.

I guess I am wondering why there is a requirement to modify stock SDK files to use available SoftDevice functionality. I can of course implement something similar in my own firmware, I was just wondering if this is the official necessary modification to support larger MTU sizes. Perhaps for a future release, this modification could be documented and added to sdk_config.h or app_config.h?

[Petter Myhre]

I was wondering the same thing when I made the example. It shouldn't be a requirement, and the developers are aware of this now. Thank you for the feedback.