USB Audio left right output randomly reversed at startup

[Claudio_ML]

Hello Guys,

I've made a USB Composite CDC + AUDIO Device (bm) starting from SDK 2.11 and recently updated to 2.13.1 on LPC5526 custom board.

Everything is working properly except that sometime, in a random way, USB audio channels (stereo) at LPC5526 I2S outputs are reversed (left on right and viceversa) and stay like this till next power cycle.

At next startup situation could go back to normal one or stay reversed for one or two power cycles

Anyone got the same issue?

[xiangjun_rong]

Hi,

For the FlexComm module of LPC552x, which is configured as I2S interface, the I2S protocol do not identify the channel, in other words, there is only one FIFOWR register which can write to both right channel (assume that the WS signal is high) and left channel(assume that the WS signal is high), so it is possible that the right channel data is written to  left channel or the left channel data is written to  right channel. When you play the stream, it is possible that the two channels switch because of error handling.

BR

XiangJun Rong

BR 

[Claudio_ML]

Hi XiangJun Rong,

any known fix for this issue? Left and Right position on streaming should be always the same, infact it's first time that I have a similar issue on MCU or DSP

Claudio

[xiangjun_rong]

Hi,

I have asked the AE team whether a bug fix is possible.

I will post the reply here once I get it

BR

XiangJun Rong

[Claudio_ML]

Hi XiangJun Rong,

any feedback from AE team?

Claudio

[xiangjun_rong]

Hi,

The AE team will discuss with SDK driver team and check if it is possible to check the channel when the FIFO_WR is written.

Thank you for pointing the issue.

BR

XiangJun Rong

[Claudio_ML]

Hi XiangJun Rong,

any update?

Claudio

[xiangjun_rong]

Hi,Claudio

Unfortunately, this is a hardware bug of I2S for LPC family, the FIFOWR register can write both right and left channels, but there is not a bit in status register to reflect the channel status.

There is not a solution for the bug at the time being.

BR

XiangJun Rong

[TwoBias]

Hi,

these are bad news, i will run in this problem soon, too.

A device should not switches channels randomly, if so it is not usable.

Is it possible to switch to other controller to prevent this problem, maybe LPC5569 has the missing flag build in?

@claudio how often happens this to your board? Could it be fixed by reseting your audiostream source during plug-in so LPC Controller is synced with the stream?

Greetings

[Claudio_ML]

Hi TwoBias,

as I wrote it's random, you can have 10 times right in a row as well as 4-5 times wrong.

We haven't a reliable statistics but i feel like 70-80% right, 20-30% wrong

I didn't tried to reset stream, in our application USB audio is used for measurement purpose (we have Audio IN and OUT) so we patched the bug with a procedure that switch channels if reverted, but your idea it could be a good workaround to test

Thank you

[TwoBias]

Hi Claudio_ML,

thanks for the statistic.

I did a quick test with development kit and sdk example composite_hid_audio.

I start a stream with left audio channel data, plug in the development kit and check the headphone output, unplug and replug.

At the moment i seems to be fine, i will do more tests, maybe i am lucky and get the right direction 10 times in a row.

Bye