PIC 16F877 > Timers > 2 servos

Ian Rogers · Dec 14, 2019

How do you control 8 servos with one CCPR1? I mean how is it loaded.. If the CCPR1 is servicing 8 different values.. Am I missing something?

Mike - K8LH · Dec 14, 2019

Servos are serviced sequentially. Each using 2.5-msecs. Each servo has a 20-msec period but they're staggered by 2.5-msec intervals.

Happy Holidays. Cheerful regards, Mike, K8LH

Code:

unsigned int servo[] = { 1500, 1500, 1500, 1500,
                         1500, 1500, 1500, 1500 };

Code:

void interrupt isr()               //
{ static unsigned char mask = 0;   // servo output pin mask
  static unsigned char n = 0;      // servo array index, 0..7

  if(CCP1IF)                       // if "special event" interrupt
  { CCP1IF = 0;                    // clear the interrupt flag
    if(LATB ^= mask)               // toggle the servo output pin
    { CCPR1 = servo[n];            // servo "on" time, 1000..2000 usecs
    }                              //
    else
    { CCPR1 = 2500 - servo[n];     // servo "off" time, 500..1500 usecs
      mask <<= 1; n++;             // setup for next servo
    }                              //
    if(mask == 0)                  // if last servo (20-msecs)
    { mask = 1; n = 0;             // reset mask and index
    }
  }
}

Pommie · Dec 14, 2019

Mike - K8LH said:
Please note that unlike the other "compare" modes, the special event trigger mode resets the CCPR1 register pair on a compare match.

This made me remember something, The compare match clears timer1 and so the above code is wrong. The lines setting CCP1 shouldn't be += just =. I.E CCP1 = servoPos[n] etc. The Special Events Trigger mode has (unfortunately) been removed from the newer chips.

Mike.
Edit, I see Mike has it correct above.

Mike - K8LH · Dec 14, 2019

Oops! I said it wrong, too, Mike. As you said, it's the timer that's cleared on the special event trigger match, not the CCPR1 register pair...

Mike - K8LH · Dec 14, 2019

Pommie said:
The Special Events Trigger mode has (unfortunately) been removed from the newer chips.

Some of the newer chips have PWM "pulse steering" which works quite nicely for servos. Unlike the "soft" servo methods, it's a hardware solution (no jitter) and since the PWM module is buffered you have plenty of time to service the PWM driver interrupt (you can turn off interrupts for short periods of time, if necessary, without affecting the servos). It's really a pretty nice hi-rez low-overhead servo solution...

Happy Holidays... Mike, K8LH

atferrari · Dec 15, 2019

Mike - K8LH said:
Please note that unlike the other "compare" modes, the special event trigger mode resets the CCPR1 register pair on a compare match.

In my 18F2321 (I know we are talking of a 16F) the compare function allows to reset the timer not the ECCP.

Ian Rogers · Dec 15, 2019

As per! I was wrong, thinking PWM.... Servos use PCM … It make sense to me now... I remember a couple of years back working on JPanhault's aero endeavours… We had all sorts of servo driving software... Believe it or not, this was the one we went with..

PWM was in my head because recently we've been messing with Arduino PWM outputs....

Thanks Mike... Oh and Mike....

atferrari · Dec 15, 2019

With PICs, to use PWM for servos, the "secret" so to speak, is to have an additional prescaler allowing to output a frequency, low enough, as required by them.

That is common (not sure if with all PICs) with ECCP modules. In some of the manuals (if not all) the example on how to reach 50 Hz is detailed with numerical values.

IIRC, 18F4431 is one.

Ian Rogers · Dec 15, 2019

We seem to have lost the Thread starter...
kawauso
I have made several "non" interrupt servo drivers, but it's difficult to maintain perfect timing (especially in C)

Interrupts are really the only way...

You need to just grab Mikes code and load it into a pic16f877a... It should work fine..

gophert · Dec 23, 2019

atferrari said:
With PICs, to use PWM for servos, the "secret" so to speak, is to have an additional prescaler allowing to output a frequency, low enough, as required by them.

That is common (not sure if with all PICs) with ECCP modules. In some of the manuals (if not all) the example on how to reach 50 Hz is detailed with numerical values.

IIRC, 18F4431 is one.

As I said in a very early post, you cannot get the prescalers to slow to 50Hz pwm. You also need to slow the CPU clock speed from 4MHz to 500kHz. That allows hardware to control the update.

Alternatively, a servo sets with a couple 50Hz pulses and holds x
For some time if you want to set other servos and then loop to check and update each servo

Pommie · Dec 23, 2019

gophert said:
As I said in a very early post, you cannot get the prescalers to slow to 50Hz pwm. You also need to slow the CPU clock speed from 4MHz to 500kHz. That allows hardware to control the update.

Alternatively, a servo sets with a couple 50Hz pulses and holds x
For some time if you want to set other servos and then loop to check and update each servo

That is not how you use the PWM module to drive servos. Did you even bother to read this thread?

Mike.

atferrari · Dec 23, 2019

gophert said:
As I said in a very early post, you cannot get the prescalers to slow to 50Hz pwm. You also need to slow the CPU clock speed from 4MHz to 500kHz. That allows hardware to control the update.

Alternatively, a servo sets with a couple 50Hz pulses and holds x
For some time if you want to set other servos and then loop to check and update each servo

Here you have it. As I said, at 40 MHz; nothing less.

Sorry it is the Power Control Module.

Pommie · Dec 23, 2019

atferrari said:
Here you have it. As I said, at 40 MHz; nothing less.

Sorry it is the Power Control Module.
View attachment 122178

Did you not read the thread either?

Mike.

Mike - K8LH · Dec 23, 2019

Any chance you or Gophert can provide an example? Does the method provide a usable pulse width resolution?

TIA... Mike

gophert · Dec 24, 2019

Mike - K8LH said:
Any chance you or Gophert can provide an example? Does the method provide a usable pulse width resolution?

TIA... Mike

As I understand hobby servos, the duty cycle doesn't matter as much as the on time so any frequency from 25 to 100Hz has worked ok for me (with some trimming).

Anyhow, 1 mSec is generally one extreme and 2 mSec is the other extreme with 1.5 mSec being neutral (midpoint).

To do this with PWM on a pic, specifically an 18F877, set parameters as follows to get 62Hz updates and 12% duty cycle (2 mSec) at 1MHz F(osc).

/*
* PWM registers configuration
* Fosc = 1000000 Hz
* Fpwm = 62.00 Hz
* Duty Cycle is about 12.2 % (2 mSec)
* Prescaler is 16
* Config pwm pin as output
*/
PR2 = 0b11111011 ;
T2CON = 0b00000111 ;
CCPR1L = 0b00011110 ;
CCP1CON = 0b00101100 ;

// If you want 1 mSec for other extreme, set...

CPR1L = 0b00001111 ;
CCP1CON = 0b00011100

This technique yields about 31 steps of resolution which is good enough for most flying/driving RC applications or the little gadgets people have asked me to make. Your milage may vary.

NOTE: you may have to adjust (trim) a bit to tune for your specific servo (add/subtract a few bits on the duty cycle). Also, remember that the two least significant bits for duty cycle are bits 5&4 of CCP1CON.

Using PWM allows the microcontrol hardware to maintain position and code can be used for monitoring sensors and other overhead.

If you are a purist and want 50Hz update frequency, you'll have to drop clock F(osc) to 500kHz and you'll have to settle for 16 steps of resolution.

If you have a non-hobby servo, check the datasheet for range of high/low times and adjust PWM as needed of this level of resolution fits your needs.

Pommie · Dec 24, 2019

Wow, 4 bits of resolution. Oh well.

Mike.

Mike - K8LH · Dec 24, 2019

Thank you for the explanation. Unfortunately, I don't think I can do much with only sixteen 64-usec steps across that 1000-us range.

gophert · Dec 24, 2019

Pommie said:
Wow, 4 bits of resolution. Oh well.

Mike.

MOD EDIT** If you can't keep it Civil then do not respond...

Ian Rogers · Dec 24, 2019

Mike... Remember this thread...

PIC CCP Module in Compare Mode

Microcontroller: PIC 16F1829 Language: MPASM I am using the CCP2 module to create a servo pulse. The interrupt is called at 20-ms intervals by TMR2. ;****************************************************************************** ; INTERRUPT SERVCE ROUTINE...

www.electro-tech-online.com

Ian Rogers · Dec 24, 2019

gophert said:
Do you constantly need some kind of validation? If you think everything in life is a competition, professional help is available and happiness is possible. Keep your local suicide prevention hotline number available - the holidays are difficult for people with your symptoms.

Let's not start a war!! Christmas an all!! There are some places where 16 levels would be fine... All's good..

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