Sending more than 20 bytes via notification

Question

I am making a simple device based on nrf5240 - I want to control the servo from a smartphone. I used the WART and Blinky examples as a starting point. The data exchange algorithm is this - the smartphone sends a command (writes data to the characteristic) and expects a response from nrf52840. The response is implemented through a notification (as in the UART example) and through another characteristic. 
 Everything works, but I wanted to send a 48-byte packet in response to the command, and I didn’t succeed. I can’t send more than 20 bytes through a notification. I saw similar questions on the forum, and if I understood correctly, then there is a limit on the size of the data sent through the notification. How to fix the code so that you can send more than 20 bytes through a notification? 
 
 SDK version: nRF5_SDK_16.0.0 
 Platform: nRF52840 
 SoftDevice: s140

Amanda Hsieh · Accepted Answer

Hi, 
 
 uchar said: but then I had to change the RAM allocation, otherwise I could not run the code - there was an error: the stack pointer was out of range. 
 When you change your MTU size you should also change the RAM allocation since the RAM usage for the stack will depend on certain things, like the GATT table size, MTU and TX-buffering settings, and similar. 
 If you modify NRF_SDH_BLE_GATT_MAX_MTU_SIZE you should adjust RAM start and RAM size for your project. 
 Please see this post . 
 uchar said: are there problems on older versions of the OS and smartphones? 
 ATT_MTU has to be negotiated between the two peers, and the minimum supported value for the two peers will set the limit. It seems fine in your use cases. 
 
 -Amanda H.

Sending more than 20 bytes via notification

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