Development: Why reducing the ADC and PWM resolution to 8 bits

This notes were written on 2019.06.17 by Casainho.

When I started this firmware, I had already developed a lot of it on the KT motor controllers OpenSource firmware. The TSDZ2 motor controller uses the same 8 bits 16MHz microcontroller STM8S105xx and so I just reused the most part of the code. The mainly big difference is the way the "low resolution FOC" algorithm is implemented because KT motor controllers has a specific part of the hardware to do it and on TSDZ2 it was to be done on a different way.

The STM8S105xx PWM channels that are in use have 16 bits resolution but I decided to use only 8 bits. The ADC channels have the resolution of 10 bits but I decided to use only 8 bits. The main advantages of using 8 bits are:

1. Faster processing speed
This microcontroller is very limited in terms of processing speed and to handle FOC we need to do it fast as possible. Also, we would like to increase the PWM frequency to make possible the motor to rotate faster with higher voltage and for that, we need more processing speed.

If not using the 8 bits resolution, the variables would be of 16 bits and the processing time would increase to a factor like x5 -- this would mean that probably FOC could not be implemented or would be worst resulting in the motor having less torque and the battery range would be lower.

2. Low pass filter of analog signals
This motor has wires with high peak currents and PWM high frequency, meaning the electric signals should be noisy.

On the analog signals measured by the ADC, the first 2 bits of the 10 bits are being discarded and this results in a fast way that filters the noisy analog signals.

3. Reducing programming memory usage
This microcontroller has very low memory size and so we need to save it if we want to implement advanced features like FOC.

The operations with 16 bits would increase to a factor like x4 the programming memory usage comparing to the 8 bits.