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I am trying to write a 120 byte data through ble to raspberrypi from my android app(I increased the MTU to the required limit). But all I am able to transfer is first 20 bytes. When I tried to search on the internet, I see that android limits the size to 20 byte for ble transfers and I will have to send multiple 20 byte packets.

But why I tried to end the same data from nrfConnect android app, I see that the data is being transferred without any issue. Can you help me understand how nrfConnect is able to do it with you writing it as packets?

Through the data is getting truncated, i am getting GATT_SUCCESS response from raspberrypi

Just as info, I am able to send 52 bytes to our custom board with nrf52 chip from the same app

tharun
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1 Answers1

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Looks like android takes care of writing more than 20 bytes of data.

Below are the two cases that I had:

Case 1:

Android app trying to write data to custom program running in nrf52 chip.

Size of data : 50 bytes.

After increase the BLE MTU on the chip side, the write was successful without any changes from app side.(gatt.writeCharacteristic(characteristic))

Case 2:

Android app trying to write data to ble program running in raspberrypi 3

Size of data 120 bytes.

Issue:Even after increasing the BLE MTU in raspberry pi, when trying to write the data only 20 bytes are received, with successful write response.

Solution: After using gatt.requestMtu(120) and calling the write character in onMtuChanged() callback, was able to send the entire data.

tharun
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  • Are you using BlueZ or some other Bluetooth stack? If the write can't be done in one packet Android will split it into multiple parts using the Long Characteristic Write procedure. If the code running on the Pi is implemented correctly, it should combine the packets to one big value. – Emil Mar 12 '18 at 19:56