Driving a piezo connected across 2 pins to increse the output volume

[adrianblakey]

In order to increase the volume of a piezo connected to a Pi Pico W, it's connected across 2 pins - 20, 21. So one pin has to be driven high while the other is low then swapped around for the 2nd half of the oscillation period.
How do I arrange the code to drive both pins with pwm?

This works - but it's normal volume:

        p20 = Pin(20, Pin.OUT)
        p21 = Pin(21, Pin.OUT)
        p20.high()
        p21.low()
        buzzerUp = PWM(Pin(21))
        buzzerDown = PWM(Pin(20))
        buzzerUp.duty_u16(2 ** 15) # setting this to 
        buzzerDown.duty_u16(0) # setting this to  2 ** 15 results in no sound
        buzzerUp.freq(500)
        buzzerDown.freq(500)

[davefes]

Is the piezo not polarity-sensitive? Use the PWM output to drive a transistor, say off 5V 12V whatever the voltage rating of the piezo is.

[peterhinch]

@robert-hh That code takes no account of phase. Any delay between setting p1 and p2 will result in a phase shift. At higher frequencies the two pins might end up being in phase with no output at all...

[GitHubsSilverBullet]

Pins 20 and 21 are attached to the same PWM slice, thus sharing a common TOP counter. They have no chance not to be in sync if both CC are identical.

[robert-hh]

@peterhinch I entered the sample code manually. And the output was perfectly in sync, since both pins used the same slice, as @GitHubsSilverBullet told.

[peterhinch]

Apologies. Interesting - I hadn't appreciated that aspect of the RP2.