GATT_UART_MAX_DATA_LEN limited to 247 on PCA10056 board, nrf52840, s140, and SDK 15.3

[Dylan Nano]

Hi--

On dev kit PCA100056, SoftDevice s140, SDK 15.3, in project ble_app_uart_pca10056_s140, we are trying to operate the UART TX/RX characteristic with outgoing payloads of 512 bytes.  ATT MTU in s140 is apparently limited to 247 bytes.  Setting the link tx/rx max data length to 512 with variable data length should support 512 payloads (even in multiple lower level messages).  However, with NRF connect v3.1.0 (PC) and nrfConnect 4.22.3 (Android), even though the server requests ATT MTU of 512, the peripheral sets to 247, -- and -- sets the max characteristic data length to 247.  I can't seem to get it to accept 512, regardless of hardcoding the characteristic values.  

Can you help?  

Thanks,

Dylan

[Sigurd]

Dylan Nano said:

ATT MTU in s140 is apparently limited to 247 bytes.

You can set it to 512 on S140, that's not an issue. But, the BLE connectivity firmware that nRF Connect for desktop(PC) uses today has set the limit to 247.