66AK2G12: McASP TX Pin Toggling Randomly

Hi Dibin,

The McASP is highly programmable so we would need more information on the configuration but generally this is caused by the tri-stating of the TX line during inactive slots.  If nothing is driving the line and no pulling resistor is attached the TX line can drift up or down depending on the last valid state before the buffer is tri-stated. Are you observing this activity on an oscilloscope? If so, is the data line changing slowly or with fast transitions similar to those observed in an active channel?

Regards, Bill

Hi Bill,

Thank you very much for your reply . I am checking data on oscilloscope, data transfer fast like normal transfer (exactly one bit data) attaching the image for the same .  I have changed the tri-state setting to active low , In that case  issue is not observed .

If I change the tristate to active low , is there any possibility it will effect normal transfer ?

Hi Dibin,

If this is a dedicated point-to-point connection than active low should be fine.  If you have other devices connected to the McASP that may drive the bus during an inactive slot, the tri-state selection is necessary.  

Regards, Bill

Hi Bill,

It's a point to point communication.

Any pointers regarding the lines are going high?

Hi Dibin,

Is this causing an issue in your design?  It could take some time to determine what programming setting is causing the McASP to end the slot with the data line in the high state.  

Regards, Bill

Hi Bill,

The toggling is causing a noise in the analog output in our board . This issue is coming continuously when McASP is in idle state (  twice or thrice in a minute).

Regards,

Dibin

Hi Dibin,

Do you believe that the high bit is being clocked into your audio device or is this just a noise issue?  Is the bit high for every slot transmitted or just for some of the slots? Does the high bit fall outside the defined slot?

In the short term, can you use the active low setting as a workaround?

Regards, Bill

Hi Bill,

The high bit is clocked to audio output , as I can see noise every time the bit high is observed .

The issue is occurring very random. we havnt  tested  active low state configuration for different combinations .

I will update once the testing is completed.

Regards,

Dibin

Hi Dibin,

If the audio device is adding noise based on the bit going high I am assuming that it is the last active bit in the channel.  Have you attempted to send other data patterns to see if the last bit is corrupted or have you always sent the all zero pattern?  What is the blue signal in the capture above?  What is the audio device that is connected?

Regards, Bill

Hi Bill,

I havnt tried sending other data pattern , as we are not initiating any data transfer from application space . I am assuming since we havnt initiated any data transfer  yet there should not be any active bit .

The blue line is work clock (AFSX).

Thanks & Regards

Dibin

Hi Dibin,

It may be a issue with programming.  How do you have the McASP transmit channel programmed?  You didn't mention to what the McASP is connected. Can you provide the timing diagram for that receive interface of that device?

Regards, Bill