I am helping some Jr High students with a robot for science olympiad. We are using some potentiometers to control the duty cycle of a HPWM output for the drive motor. The HPWM runs an N channel mosfet that switches the 0V signal to the motor. It works fine until the analog value becomes zero, setting the duty cycle to zero in the code. When that happens the motors appear to run at full speed. We don't have an oscilloscope to check the actual output from the chip but we do have an LED connected in parallel with the mosfet. Its brightness correlates perfectly with the motor speed, even when the duty cycle is zero. It is as bright as it gets at the same time the motor is running full speed.

Does anyone know how the HPWM works with a zero duty cycle? It doesn't appear to be caused by some reaction in the mosfet because we see it in the LED. I thought about putting a pull down resistor on the output to make sure it goes off but it has a 1K current limiting resistor connected to the LED.

Look forward to anyone's help and suggestions.