NRF maximum gatt mtu size

[pranavsk1994]

Hi,

I am trying to develop an application which recieves data over SPI and I want to send this data over ble, I am using NUS service. I am recieving 5 bytes over SPI at a time, and SPI sampling rate is around 4 Mhz.

I am sending this data to ble buffer, till I get NRF_ERROR_RESOURCES err_code. Also when I get BLE_GATTS_EVT_HVN_TX_COMPLETE notification I push remaining data in BLE queue. I repeat this procedure.

I am able to maximum MTU size set in config file to 158.

I have used gatt_mtu_set(),data_len_ext_set,conn_evt_len_ext_set from nrf's demo project for throughput.

But when I set maximum mtu size to 251 and try to use it, nrf chip doesn't advertise and there is no ble transfer?

From different ques on nordic developer zone I came to know that max size is 247 bytes. Do we need to change something on app side for this?

[Petter Myhre]

Do you really need a bigger ATT MTU? If you are sending 5 bytes at the time?

Anyways, which SDK and SoftDevice version are you using? Which example? Sounds like you are getting an error code somewhere and the chip resets. Please try to turn off optimizations, define DEBUG, and set a breakpoint in app_error_handler(). If you hit the breakpoint, extract the error code, line number and file name.

[pranavsk1994]

So the data I recieve over SPI from peripheral device is sampled at really fast rate. I am not sending only 5 bytes over ble. I am pushing data recieved over SPI to ble queue till I get an err_code message of NRF_ERROR RESOURCES. I want tp increase the ATT_MTU size which is currentky set to 158 in my config file. But if I increase it to 247 which is maximum as mentioned in documents the code doesn't seem to work. It doesn't even advertise.

[Petter Myhre]

Did you try my suggestion?

[pranavsk1994]

I am using nrf 13.0.0 SDK, with nrf52840 chip, and S140 softdevice. I didn't understand how to use the DEBUG option. But I solved the error I was not initializing the MAX size in ble stack. Now I am able to set att mtu to 247 bytes. But the throughput is coming out around 106 Kbps. But in the following link its mentioned you can get a throughput of 424 kbps for GATT size of 247, max conn interval time of 400ms. infocenter.nordicsemi.com/index.jsp

I don't understand if I am missing something here

[Petter Myhre]

You can have a look at the ble_app_att_mtu_throughput example, see if you see any differences. This configures for maximum throughput. What event length are you using? If the connection interval is 400 ms, the event length should also be 400 ms. 424.6 kbps is the maximum measured throughput in 1Mbps with simultaneous send Notification and receive Write command. Only sending notifications one way you can achieve 771.1 kbps.

It also depends what you are connecting to, what kind of central device is it? You will typically only achieve the numbers above when you have our devices in both ends of the link.