Software PWM ( Servo driver)

int16 high_time=0;

void read_port_b()
{
while(true)
{
high_time = ( input_b() * 10L ) + 1000L; // 1000us to 2000us
}
}

void main()
{

setup_oscillator(OSC_4MHZ);
setup_timer_0(RTCC_INTERNAL|RTCC_DIV_128);

set_tris_a(0x00);
set_tris_b(0xFF);


enable_interrupts(INT_TIMER0);
enable_interrupts(GLOBAL);
disable_interrupts(INT_RB);

output_a(0xFF); // Test LED {
delay_ms(1000L);
output_a(0x00);
delay_ms(1000L);
output_a(0xFF);
delay_ms(1000L);
output_a(0x00); // }

set_timer0(100); // initial duty
output_a(0xFF);
delay_us(high_time);
output_a(0x00);

read_port_b();
}



#INT_TIMER0
void Timer0_ISR()
{
disable_interrupts(GLOBAL);
set_timer0(100);
output_a(0xFF);
delay_us(high_time);
output_a(0x00);
clear_interrupt(INT_TIMER0);
enable_interrupts(GLOBAL);
}

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jpanhalt said:

Are you using a digital or analog servo? Some digital servos are known to chatter when put into a set position. Swap in an analog servo and see if the "vibration" still happens. John

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void read_port_b()
{
while(true)
{
portVal = (int16) input_b();
disable_interrupts(INT_TIMER1);
high_time = ( portVal * 10L ) + 1000L;
enable_interrupts(INT_TIMER1);
}
}

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jpanhalt said:

I have no experience with using a PC to control a servo. My suggestion related to the digital servo was based on experience with cheap servo controllers. For example, the early versions based on the 555 chip worked fine with analog servos, but digital ones tended to chatter. When I have used a 12F5xx PIC as a servo controller, both analog and digital worked fine. Your experience using just the PIC is consistent with that observation.

Unfortunately, I can't help you on the PC interface. To confirm that is really the problem, get a general purpose analog servo. If it doesn't chatter, then at least you know where to look for the problem. Those general purpose ("standard") servos typically came packaged with transmitter/receiver sets. Hitec HS300, Futaba S148, and Airtronics 94102 are examples. They are inexpensive when new (<$15) and even cheaper when used. If you were near Cleveland, the only thing I would ask is "how many?" Most modelers would probably give you one, or your local hobby shop might let you borrow a display unit.

John

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