NCS peripheral uart increase data length

[anSolo]

Hello,

I use central_uart (nrf52840dk) and peripheral_uart (nrf52833dk) examples, NRF Connect SDK v1.6.0

I want to send more than 20 bytes of data,

So, I increase UART_BUF_SIZE 20 to 50, and when I send data from central device to peripheral It works well.

But I have no success sending more than 20 bytes from peripheral to central,

<wrn> bt_att: No ATT channel for MTU 25
<wrn> bt_gatt: No buffer available to send notification
<wrn> peripheral_uart: Failed to send data over BLE connection

Is it possible to send more than 20 bytes from both sides?

Thank you,

Anton

[Karl Ylvisaker]

Hello,

Is it possible to send more than 20 bytes from both sides?

It is, but you will need to increase the MTU size to be able to do so in a single transfer. The UART_BUF_SIZE is the size of the UART buffer, and does not have to do with the BLE MTU size, or the NUS service configuration.
Please see this answer by my colleague Heidi for more information on how to do this in NCS v1.6.0

The summer holidays have begun here in Norway, and DevZone is therefore operating with reduced staff for the time being. Sorry for any inconvenience this might cause.

Best regards,
Karl 

[anSolo]

Thank you,
I've set 
CONFIG_BT_L2CAP_TX_MTU=128
CONFIG_BT_BUF_ACL_RX_SIZE=128
for both projects and it works

What is the maximum possible value for these params?

[Karl Ylvisaker]

anSolo said:

Thank you,
I've set 
CONFIG_BT_L2CAP_TX_MTU=128
CONFIG_BT_BUF_ACL_RX_SIZE=128
for both projects and it works

No problem at all, I am happy to hear that you got it working as intended!  

anSolo said:

What is the maximum possible value for these params?

The maximum LL PDU size is 251 bytes. However, you will have to make sure that your connection events are long enough for the device to handle your payload size.
Packets that are larger than the LL PDU size will be split over several connection events.
You could use the Online Power Profiler for BLE to visualize how the different parameters and configurations will play out.

Best regards,
Karl