I’ve revised the previously posted code for PWM on the Orangutan controller to eliminate the possibility of glitches when speed=0 (motors off). I’ve pasted it in below. However, the issue of how to do PWM with this H-bridge, that is, to use “coast” or “brake” during the off portion of the cycle turned out to be quite interesting and surprising.
I’ve created a web page to discuss the implementation and the results of the performance analysis (speed linearity and current draw). I’ve also written new PWM routines to allow either “coast” or “brake” mode in the motor off state, as each has advantages. To do so requires interrupt programming, but the code is smaller.
You may download all code examples from the web page.
Cheers, Jim
Direct-mode (no interrupts) PWM example code. May be downloaded at above web site.
/*
** PWM routines for ORANGUTAN motor controller from Pololu
** atmega8/LB1836M motor controller, WinAVR
** by Jim Remington (sjames_remington _at_ yahoo.com)
** for more information, see [uoxray.uoregon.edu/orangutan](http://www.uoxray.uoregon.edu/orangutan)
*/
#include <avr/io.h>
/*
** Delays
*/
#define F_CPU 1000000UL
#include <util/delay.h>
#define Delay( x ) _delay_loop_2( x )
// above, x is uint16
/*
** PWM_Init - Set up the pwm ports (PORT B1=IN1, B2=IN3, D5=IN2, D6=IN4)
*/
void pwm_init( void )
{
DDRB |= (1<<1) | (1<<2);
DDRD |= (1<<5) | (1<<6);
TCCR1A = 1; // 8 bit pwm phase correct
TCCR1B = 1; // clock/1 use TCCR1B=2 (clock/8) for faster CPU clocks
OCR1A = 0;
OCR1B = 0;
}
/*
** pwm_set - Set the two PWM speeds. Each ranges from -255 to 255
** assumes left motor = PB.2 and PD.6, right = PB.1 and PD.5.
** SPEEDS> +/- 255 WILL LEAD TO UNEXPECTED BEHAVIOR! TIMER1 REGISTERS ARE 16 BITS.
If direction of travel is wrong, switch motor leads, but note asymmetry of PWM modes
in forward and reverse directions. Brake mode is more power-hungry but functions better at low speed.
It may be advantageous to reverse motor leads and the definition of forward and reverse speeds.
Truth Table for LB1836 controller from data sheet.
IN1/3 IN2/4 MODE
---- ----- ----
1 0 forward
1 1 brake (both outputs low), power hungry mode.
0 1 reverse
0 0 coast (both outputs off), power saving mode.
*/
void pwm_set( int left_speed, int right_speed )
{
if ( right_speed > 0 )
{
PORTD &= ~_BV(5); //IN2=0
TCCR1A |= _BV(COM1A1);
TCCR1A &= ~_BV(COM1A0); //set COM1A1=1, COM1A0=0. Result: OC1A=IN1=1 on upcount, =0 on compare match
OCR1A = right_speed; //(this should be forward on upcount, coast on compare match)
}
if (right_speed==0)
{
TCCR1A &= ~(_BV(COM1A1)|_BV(COM1A0)); //disconnect PWM channel A from PORTB
PORTD &= ~_BV(5); //IN2=0
PORTB &= ~_BV(1); //IN1=0 ; set coast mode (conserves power). Set both=1 for brake mode
}
if ( right_speed < 0 )
{
PORTD |= _BV(5); //IN2=1
TCCR1A |= (_BV(COM1A1)|_BV(COM1A0)); //set COM1A1=COM1A0=1. Result: OC1A=IN1=0 on upcount, =1 on compare match
OCR1A = -right_speed; //(this should be reverse on upcount, brake on compare match)
}
if ( left_speed > 0 )
{
PORTD &= ~_BV(6); //IN4=0
TCCR1A |= _BV(COM1B1);
TCCR1A &= ~_BV(COM1B0); //set COM1B1=1, COM1B0=0. Result: OC1B=IN3=1 on upcount, =0 on compare match
OCR1B = left_speed; //(this should be forward on upcount, coast on compare match)
}
if (left_speed==0)
{
TCCR1A &= ~(_BV(COM1B1)|_BV(COM1B0)); //disconnect PWM channel B from PORTB
PORTD &= ~_BV(6); //IN4=0
PORTB &= ~_BV(2); //IN3=0 ; set coast mode (conserves power). Set both=1 for brake mode
}
if ( left_speed < 0 )
{
PORTD |= _BV(6); //IN4=1
TCCR1A |= (_BV(COM1B1)|_BV(COM1B0)); //set COM1B1=COM1B0=1. Result: OC1B=IN1=0 on upcount, =1 on compare match
OCR1B = -left_speed; //(this should be reverse on upcount, brake on compare match)
}
}
/*
** MAIN. Exercise left and right motors.
*/
int main(void)
{
int i;
pwm_init();
while(1)
{
for(i=0;i<256;i++)
{
pwm_set(i,i);
Delay(5000); //20000 cycles
}
for(i=255;i>0;i--)
{
pwm_set(i,-i);
Delay(5000);
}
for(i=0;i<256;i++)
{
pwm_set(-i,-i);
Delay(5000); //20000 cycles
}
for(i=255;i>0;i--)
{
pwm_set(-i,i);
Delay(5000);
}
pwm_set(0,0);
for(i=1;i<20;i++) Delay(25000);
}
return 0;
}