tone() minimum frequency is 1 Hz

teensy3/Tone.cpp

@@ -73,6 +73,7 @@ void tone(uint8_t pin, uint16_t frequency, uint32_t duration)
 	} else {
 		count = 0xFFFFFFFD;
 	}
+	if (frequency < 1) frequency = 1; // minimum is 1 Hz
 	usec = (float)500000.0 / (float)frequency;
 	config = portConfigRegister(pin);