Max BLE concurrent connections

[ivanpondal]

Hello.

I'm trying to figure out which microcontroller is best for my use case. I need to have as many bluetooth low energy concurrent connections as possible and the device should be able to work both as a peripheral and central.

I've been reading some posts on the forum and have seen there appears to be a limit of 20 for SoftDevice, however with Zephyr the answers vary saying it depends on the available ram.

Given this information, would it be correct to assume I'd be able to get over 20 concurrent connections with a nRF52840 using Zephyr BLE controller? Would there be any benefit on using a microcontroller from the nRF53 series?

Thanks

[Amanda Hsieh]

Hi, 

would it be correct to assume I'd be able to get over 20 concurrent connections with a nRF52840 using Zephyr BLE controller?

We don't test that. 

Would there be any benefit on using a microcontroller from the nRF53 series?

No, it is limited to the CPUNET core flash size.

Regards,
Amanda H.

[ivanpondal]

I understand one must test if it is physically possible to have over 20 concurrent connections but from a software or microcontroller perspective, is there any hard limit? This is the post I had read NCS max concurrent peripheral connections.

I'm not sure I understand what you mean by limited by CPUNET core flash size.

[Amanda Hsieh]

ivanpondal said:

is there any hard limit? This is the post I had read NCS max concurrent peripheral connections.

When increasing a connection, the controller would also need more RAM to handle for that connection. Since the Zephyr BLE controller is not maintained by Nordic, I am not sure how much memory would be required for each connection and more than 20 connections. 

ivanpondal said:

I'm not sure I understand what you mean by limited by CPUNET core flash size.

The BLE controller is run on the nRF53 CPUNET core, so it is limited to the RAM on the CPUNET core.  

[ivanpondal]

Great, that makes sense.

Is there a way to get the currently available CPUNET RAM? That way I could run some tests and measure myself.

Thanks

[Amanda Hsieh]

It would show at the end of the build log such as 

-- Build files have been written to: C:/ncs/v2.7.0/nrf/samples/bluetooth/peripheral_uart/build/ipc_radio
-- Configuring done
-- Generating done
-- Build files have been written to: C:/ncs/v2.7.0/nrf/samples/bluetooth/peripheral_uart/build
-- west build: building application
[6/262] Generating include/generated/version.h
-- Zephyr version: 3.6.99 (C:/ncs/v2.7.0/zephyr), build: v3.6.99-ncs2
[262/262] Linking C executable zephyr\zephyr.elf
Memory region         Used Size  Region Size  %age Used
           FLASH:      155460 B      1008 KB     15.06%
             RAM:       33832 B       448 KB      7.37%
        IDT_LIST:          0 GB        32 KB      0.00%

[ivanpondal]

I meant during runtime, like to check how much gets used after performing a ble connection.

[Amanda Hsieh]

Unfortunately, no such function. It would only get the result after compiling according to the configurations. 

[ivanpondal]

OK, no problem. So this got me thinking, if the nRF52840 is single core; does that mean the ble stack gets to share the 256kb of ram with application code? I'm thinking out loud but couldn't that result in more connections in terms of memory limitation? If I read correctly, nRF53 has 64kb of ram on the CPUNET.

[Amanda Hsieh]

The nRF52840 is a single-core SoC with 256 KB of RAM that is shared between the BLE stack and the application code. nRF5340 network core typically handles the radio and protocol stack and it has 64 KB of RAM as you mentioned. See the nRF5340 memory configuration table.