[CIS][SDK3.2.1] nRF5340 with SDK 3.2.1 periodically misses CIS events from Pixel 10 Phone - clicks in audio

[Alex B]

I have a nRF5340 module with an application built using SDK 3.2.1.

The nRF5340 is acting as a CIS Peripheral.

It has an ACL connection and a CIS connection with a Pixel 10 Phone acting as a Central and sending audio over the CIS.

CIS configuration:

• ISO Interval = 10 ms
• NSE = 2
• BN C to P = 1
• BN P to C = 0
• MAX PDU C to P = 40
• PHY C to P = 2M
• PHY P to C = 1M

Sniffer capture of the CIS_REQ packet:

The Audio received on the nRF5340 has some clicks in it.

These are some captures from the sniffer (Teledyne LeCroy Sodera) which showed that the nRF5340 periodically misses some CIS events form the Pixel 10 phone.

The nRF53 also acts as Central for a Peripheral device, on an ACL connection, with a very high Peripheral Latency, around 80.

Please see below and example of the nRF5340 not responding to a CIS event from the PIxel 10, and the Pixel 10 repeating the payload in both subevents of the CIS.

I noticed that the missed CIS events seem to happen immediately after an ACL event with the other device.

Is it possible tat the nRF5340 prioritizes the ACL over the CIS and that is why it is missing the CIS event?

Technically, it has enough time to figure out that the Peripheral did not answer and catch at least the retransmission from the second CIS subevent which is ~1.6 ms from the end of the ACL packet.

The clicks were not audible in the previous SDK we used, 3.1.1. Has the behavior changed since then?

If this is the cause of the missed CIS is it possible to configure the device to always prioritize the CIS over ACL (maybe with the exception of special situations like connection update instants)?

[Alex B]

Thanks for the quick reply, Amanda Hsieh .

My impression after doing a lot of tests is that the nRF5340 does take into consideration the timing of the ACL + CIS with the Pixel 10 (where it is acting as Peripheral) when setting the timing of the ACL with the other device (where it is acting as Central). I have not seen situations where the ACL with the other device was established right on top of the ACL or the CIS with the Pixel 10.

The issues I see are in a situation where the CIS event with the Pixel 10 is very close after the ACL with the Peripheral.

When initially established, the ACL with the Peripheral device did not cause any issues with the ACL + CIS with the Pixel 10. The issues started after a particular ACL event with the Peripheral, approximately 6 seconds after the connection is established. Up to that ACL event the nRF5340 responds to CIS events from the Pixel 10 if the ACL has only one PDU from each side. If the ACL exchange is longer, obviously the nRF5340 does not have enough time to handle the CIS. Immediately after that event the nRF5340 fails to respond to all CIS events from the Pixel 10 immediately following an ACL event with the Peripheral, irrespective of the ACL length with the Peripheral. It does not respond even if the Peirpheral sends no PDU during the ACL event and it has more than enough time to handle the CIS.

This is obviously a conflict between the Peripheral ACL and the CIS. But it starts manifesting itself some time after the ACL is established. This behavior starts immediately after an ACL event with the Peripheral where not even the nRF5340 sends an ACL packet. This is why I am suspecting Window Widening issues, based on my experience as a Controller developer.

Anyway, this is not reproducible every time and the ACL Peripheral must not be continuously connected. It can be connected later, only when it is needed.

We will think about the idea to disconnect and reconnect the Peripheral until it does not bother the CIS. But it's hard to figure out from the software when this happens. Maybe we can use the audio underrun warnings we get in the log when this issue starts manifesting itself.

[Alex B]

Amanda Hsieh I did some more tests in the last few days and I found out you were right about the scheduling.

Unfortunately, I stumbled again on a situation where the audio has clicks.

And I could confirm by looking at the over-the-air traces that the RC ACL was scheduled right on top of the CIS.

The Peripheral does high duty cycle directed advertising and the nRF5340 connects to it while the CIS+ACL with the Phone are being handled.

The Peripheral ACL has an interval of 90 ms and causes the CIS with the Pixel 10 to be dropped (10 ms ISO interval).

And sometimes both the CIS and ACL with the Pixel 10 are dropped. The ACL with the Pixel 10 has a connection interval of 20 ms.