BLE multiple connection overlap

[Soonho]

I am a student currently studying BLE.

At present, I am working on a multi-connection setup where both the master and the slaves are using the nRF52832. Specifically, I have 1 master connected to 2 slaves.

Both slaves are transmitting data using the "write without response" method with a connection interval of 10ms.

Sometimes, the data from each slave is written at 10ms intervals, but other times, it is written at 20ms intervals. (the transmission interval of Slave 1 is sometimes 10ms, as shown on the left, and sometimes 20ms, as shown on the right.)

I hypothesize that when the transmission occurs at 10ms intervals, the connection events do not overlap.

However, when it occurs at 20ms intervals, it is because the connection events are overlapping.

Is this a possible phenomenon? Also, is my hypothesis correct?

I look forward to your response. Thank you.

[Kenneth]

Hello,

You can read more about how scheduling works here:
https://docs.nordicsemi.com/bundle/ncs-latest/page/nrfxlib/softdevice_controller/doc/scheduling.html 

Normally I would say that if it's the same central (master) for both links, and you are using for instance (on the central) BT_CTLR_SDC_CONN_EVENT_EXTEND_DEFAULT=n and BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT_OVERRIDE=3750 (possible try with 5000=5ms also), and the central is not doing anything else (e.g. scanning), then the central should have enough time to handle both links. But you may experience what you say if not yes.

Kenneth

[Soonho]

Kenneth 

Thank you for your response.
I’ll make sure to read the article you linked!

Here are some additional questions I have:

Q1. What is the role of BT_CTLR_SDC_CONN_EVENT_EXTEND_DEFAULT and BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT_OVERRIDE?
Q2. With two slaves, both having a connection interval of 10ms and a connection event length of 2.5ms, is it still possible to encounter a situation like mine?

Thank you in advance for your help!

[Kenneth]

Soonho said:

What is the role of BT_CTLR_SDC_CONN_EVENT_EXTEND_DEFAULT and BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT_OVERRIDE?

You can control how much time the scheduler should allow for each connection event.

Soonho said:

Q2. With two slaves, both having a connection interval of 10ms and a connection event length of 2.5ms, is it still possible to encounter a situation like mine?

No.

Kenneth

[Soonho]

 Kenneth 

Thank you so much for your consistent responses.

However, with the Adafruit nRF52832 I am using (product link: https://www.adafruit.com/product/3406), the transmission interval of Slave 1 & 2 is sometimes 10ms, as shown on the left, and sometimes 20ms, as shown on the right.


Additionally, I found the following content from the link you provided (https://docs.nordicsemi.com/bundle/ncs-latest/page/nrfxlib/softdevice_controller/doc/scheduling.html) and it seems to match my situation.

 

So, I understand that with two slaves, both having a connection interval of 10ms and a connection event length of 2.5ms, transmission occurs at 10ms intervals if the connection events do not overlap, but at 20ms intervals if they do overlap. Did I misunderstand this?

Looking forward to your reply.
Thank you

[Kenneth]

That may be yes, but can you try BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT_OVERRIDE=3750 and/or BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT_OVERRIDE=5000, instead of 2500? And also set BT_CTLR_SDC_CONN_EVENT_EXTEND_DEFAULT=n.

Kenneth

[Soonho]

Kenneth 

I am developing using Adafruit's nRF52832 board (https://www.adafruit.com/product/3406) in the Arduino environment, and I am using the following library:
https://github.com/adafruit/Adafruit_nRF52_Arduino?tab=readme-ov-file

I could not find BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT_OVERRIDE in this library. Did I perhaps overlook it?

Or do I need to develop using the nRF Connect SDK to configure BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT_OVERRIDE?

Thank you.

[Kenneth]

Hi,

If you are not able to set BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT_OVERRIDE then the softdevice may extend the event length if there is more data, so this may then override the configured event length and cause the problem you see. I guess you can (as you already suggest) set a connection interval of 20ms then, and just set: 
BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT_OVERRIDE=8750.

Kenneth