On A0 I couldn’t even get analog voltage. However, on A1, A2 and so on I was able to successfully get a square wave by using pwm function. Maximum frequency is limited by ~47 kHz though (see Generating a 400 kHz square wave)
In case of SAMD21 pin description structure for A0 states that this is not a PWM pin. So, expectedly, pwm function will not work on A0. Is SAM232D supposed to be different in that regard?