XSMT: a mute that ramps

Pulled up on the board

The XSMT pin goes straight to the chip, and a 10 kΩ resistor on the board connects it to 3V3. Nothing connected reads HIGH, and HIGH means play. A wire from a GPIO overrides the resistor whenever the GPIO drives it.

So XSMT is optional. Three wires and power are a working TK103.

LOW is a fade, not a switch

Driving XSMT LOW does not cut the output. The chip turns the level down in steps until it reaches silence, and driving it HIGH again turns it back up the same way. TI calls this soft mute and soft un-mute. Because the sound never jumps, there is no click at either end.

Muted while the clocks start

A sketch that owns XSMT can use it to start in silence. The TK101 book's live-monitor sketch mutes before it starts the DAC, and un-mutes once the clocks run:

pinMode(PIN_XSMT, OUTPUT);
digitalWrite(PIN_XSMT, LOW);          // muted while the clocks start
// ... begin() the microphone and the DAC ...
delay(50);
digitalWrite(PIN_XSMT, HIGH);         // un-mute: the DAC ramps up

The two sketches in this book set XSMT HIGH at the top of setup() and leave it there. With the wire off, the pull-up does the same.

When the sketch stops sending

The chip watches its clocks. If they stop or go wrong, it mutes by itself, without a pop, and turns its output stage off. If BCK and LRCK stay low for more than a second, it powers down. When the clocks come back it starts again on its own: there is nothing to reset and no pin to toggle.