Pololu Baby Orangutan B - Manual (Page 12)

9. Motor Driver Truth Table
input output
PD5, PD3 PD6, PB3 M1A, M2A M1B, M2B
motor effect
H
H L L brake
L H L H “forward”
H L H L “reverse”
L L OFF (high-impedance) coast
Motor 1 is controlled by pins PD5 and PD6, and motor 2 is controlled by PD3 and PB3. These pins are connected
to the ATmega48/168’s four eight-bit hardware PWM outputs, which allows you to achieve variable motor speeds
through hardware timers rather than software. This frees the CPU to perform other tasks while motor speed is
automatically maintained.
The suggested procedure for using hardware PWM outputs to control the motors is as follows:
1. Make the four motor control pins outputs and drive them high; this drives all four motor outputs low.
2. Configure Timer0 and Timer2 to use a prescaler of 8, which results in a PWM frequency of 20 MHz/8/
256 = 9.8 kHz. Set these timers for inverted PWM mode output on both OCxA and OCxB, meaning that these
PWM pins are set on timer compare match and cleared on timer overflow. This results in negative PWM pulses
with duty cycles determined by registers OCR0A, OCR0B, OCR2A, and OCR2B.
3. You can command motor 1 to drive “forward” at a speed ranging from 0 – 255 by setting OCR0B = speed
and holding fixed OCR0A = 0. You can command motor 1 to drive “reverse” at a speed ranging from 0 – 255
by setting OCR0A = speed and OCR0B = 0. During the period where the two input pins have opposite values,
the motor drives at full speed. During the period where the two inputs have the same value (high), the motor
brakes. Cycling between drive and brake and high frequency results in variable motor speed that changes as a
function of PWM duty cycle. Analogous results can be obtained for motor 2 using OCR2A and OCR2B. (Note
that the concept of “forward” is arbitrary as simply flipping the motor leads results in rotation in the opposite
direction.)
Using these PWM settings, OCR0B = 255 is equivalent to holding PD5 low while OCR0A = 0 is equivalent to
holding PD6 high. As you can see from the truth table above, in this state M1B connects to your battery’s positive
terminal and M1A connects to ground. Decreasing OCR0B to something less than 255 decreases the percentage of
time PD5 is low, causing M1B to alternate between VIN and GND (and hence causing motor 1 to alternate between
drive and brake). Similarly, OCR2B = 255 is equivalent to holding PD3 low while OCR2A = 0 is equivalent to
holding PB3 high. In this state, M2B connects to your battery’s positive terminal and M2A connects to ground.
Pololu Baby Orangutan B User's Guide © 2001–2012 Pololu Corporation
9. Motor Driver Truth Table Page 12 of 12