NRF52840 high throughput with custom service error changing mtu

[superdan1]

Hi,

I'm using the NRF52840 Preview DK and want to create my own BT stack with high throughput between NRF52 and other BT devices.

As base I'm using the ble_app_template of nordic examples, created a BT Profile with help of Bluetooth Developer Studio from Bluetooth.com (https://www.bluetooth.com/develop-with-bluetooth/developer-resources-tools/developer-kits/bluetooth-starter-kit)

Now I have a profile including: heartrate service, batterylevel service, device information service and one custum service with two characteristics. (1. read/write, 2. notify)

I used the ble_app_att_mtu_throughput example of nordic to expand my own project. All works fine, since I don't change my NRF_SDH_BLE_GATT_MAX_MTU_SIZE of 23, and notification payload of 20 bytes.

The Problem now:
I want to incese the mtu size and enable the datalength extension (like in throughput example), while using the same methods "gatt_mtu_set(...) , data_len_ext_set(...). But every time I increse the MTU size, an other error occur. 

Set the MTU size to 50: one characteristic of my custom service disappears. Set the MTU size to 80: 3 services disappears. (Disappearing in nrfConnect App while scanning) Set the MTU size to 247: programm crashing in main loop.

I my opinion it looks like a memory problem, but I don't really know how to fix it. I already changed the flash/start/size, ram/start/size like in the throughput example (I think the maximum: FLASH_START=0x22000, FLASH_SIZE=0xfffff, RAM_START=0x200030d0, RAM_SIZE=0x3CF30).

I'm using Segger Embedded Studio on macOS for developing.

Anyone got an idea?
Thanks alot!

[superdan1]

After spotted a thread about NRF_SDH_BLE_GATTS_ATTR_TAB_SIZE in DevZone (https://devzone.nordicsemi.com/f/nordic-q-a/30852/data-length-extension-causes-nrf_error_no_mem-on-services-init)
I found a solution for my problem.

Like I suspect it's a memory problem.

I increased the NRF_SDH_BLE_GATTS_ATTR_TAB_SIZE (randomly, no calculation) of 1408 to 8KB (8192) and then the application runs with MTU size of 247 without any errors.

Of course I increased the ram_start value and adjusted the ram_size value of the application too. My values are now: 

Physical ram size minus ram_start size = ram_size.

The ram_start size is shown while debbuging in ble_stack_init() method in line err_code = nrf_sdh_ble_enable(&ram_start); 
Just read the &ram_start variable.

RAM_START=0x20004800
RAM_SIZE=0x3b800

Apparently there's no fix way to calculate the NRF_SDH_BLE_GATTS_ATTR_TAB_SIZE, so just try out.

Daniel