PCM5101A: XSMT signal timing

Hi Nagata-san,

When the edge is slower than 20ns, it can still enter soft-mute, but there is a chance it will then enter an analog mute as well.  If the edge is sufficiently slow, the device will behave as though the Undervoltage Protection Mode has been entered.

Thanks,

Paul

The device XSMT pin can have 4 behaviors:

1. The XSMT pin is high, and the DAC will be in normal operation.
2. The XSMT is <2V and >1.2V, the device will enter soft-mute, where the volume ramps by -1dB per sample.  This will take up to 104samples to ramp to 0%FS.
3. The XSMT is <1.2V, the device enters analog mute, where the output becomes 0V immediately.
4. XSMT transitions from >2V to <1.2V in less than 20nS, in this case the output will do the same soft mute as condition 2, but will never enter analog mute.

I have attached some examples of this.  The customer’s solution would be something like the image “Condition 2 and 3”, where they might have some attenuation from the soft mute behavior, but XSMT will become <1.2V before the device fully attenuates, so it will have a sharp drop. 

If that is a problematic function and they cannot create a faster edge, then I recommend they implement some kind of RC function for the XSMT pin, where they can extend the time when XSMT is <2V and >1.2V.

This is condition 2 and 3.  You see that there is some attenuation before the XSMT pin becomes low enough to trigger the analog mute. 

Here you see the XSMT pin drop below the analog mute threshold, condition 3.

This is soft-mute (condition 4).  The XSMT edge is sufficiently fast that the device slowly attenuates the output.  Condition 2 would also look like this.

Note that these were measured on a system using 1.8V logic, so the thresholds scale from 2V and 1.2V to approximately 1V and 0.6V.