The basic motor operation concepts described in that application note still apply today, therefore, I will not repeat them here. In the time since then, enhancements to two of the microcontroller peripherals have been made that dramatically improve the microcontroller capabilities for sensorless motor control applications.

This application note will describe those enhancements and how they are used to create a sensorless motor control solution with an 8 MHz PIC16FXXX device that has truly stellar performance with seamless operation from 100 RPM to over 90,000 RPM. Voltage to the motor from the motor supply is varied by pulse-width modulating the driver FETs. Only one side of the drive needs to be modulated: either the high side motor supply or the low side supply return. This requires that three of the microcontroller outputs to the FET drivers have PWM capability, while the other three can be general purpose output pins.

The microcontroller comparator is used to determine when the undriven phase voltage reaches the mid-point between the motor supply and motor return potentials. Since there are three phase voltages and one reference, this requires multiplexing each of the three phase outputs to one input of the comparator while the other comparator input remains on the fixed reference. We’ll now take a look at the new peripheral features to see how they meet the FET driver and comparator input requirements and how they are controlled. Two relatively minor enhancements are now incorporated in some PIC16FXXX devices that significantly improve the capabilities of the comparator and the enhanced capture, compare, and PWM (ECCP) peripherals. These enhancements are particularly useful for brushless motor control applications. The comparator now has four independently selectable inputs to the inverting input and the ECCP has up to four individually selectable outputs in single PWM mode.