PWM from wheel speeds

The circuit again

• DPDT switch is a relay, switched by "backwards" bit.
• SPST switch is a FET, switch by "pulse width modulation" (PWM) bit
• Average volage across motor is proportional to PWM "on" time.

The code

• TMR0 is free-running from 0..255, incrementing every 64uS
• Main program is just a loop (pulse widths handled in interrupts)
• "backwards" bit energizes relay (slow)
• FET at bottom completes circuit (fast)
• First cut: if (speed > TMR0) pin=ON else pin=OFF
  • Problem: speed = 255 and TMR0 = 255 means pin OFF
  • 99.6% duty cycle -- not bad, but 100% would be better.
  • FET consumes power when switching, nice to leave it fully on if possible
• Second cut: if (speed >= TMR0) pin=ON else pin=OFF
  • Even worse: speed = 0 and TMR0 = 0 means pin ON
  • Wheels can't be fully stopped, robot twitches
  • Motors draw even more power than switching FET
• Solution:
  if (speed == 0) pin=OFF
  else if (speed >= TMR0) pin=ON
  else pin=OFF
• Slight discontinuity at speed=1, who cares
• No careful timing needed, TMR0 does all the work
  • Just do the comparison "often enough"
  • Lose resolution, not correctness, if checking slows
  • Self-correcting for "missed" checks
• TMR0 overflows every 16mS, so PWM frequency is 61Hz. Slow but acceptable.

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