16-channel 12-bit I2C PWM Servo Driver and initial servo pos

[msousa]

I've embellished the servo example as seen below. This is so I might experiment with just one servo. With everything hooked up (servo brd connected to arduino, power to 16-channel I2C brd, one servo connected to the brd), when I apply power from the PC to the arduino (through the USB cable) the servo moves almost 45 degrees. This is before I upload the program below. Any idea why this might happen?

Thanks...

/***************************************************************
Control of the servos on the UAS
***************************************************************/
#include <Wire.h>
#include <Adafruit_PWMServoDriver.h>

// called this way, it uses the default address 0x40
Adafruit_PWMServoDriver pwm = Adafruit_PWMServoDriver();

// Depending on your servo make, the pulse width min and max may vary, you 
// want these to be as small/large as possible without hitting the hard stop
// for max range. You'll have to tweak them as necessary to match the servos you
// have!
#define SERVOMIN  200 // this is the 'minimum' pulse length count (out of 4096)
#define SERVOMAX  300 // this is the 'maximum' pulse length count (out of 4096)

// our servo # counter
uint8_t servonum = 0;
uint8_t servo1 = 0;

void setup() {
  Serial.begin(9600);
  delay(3000);
  Serial.println("16 channel Servo test!");

  pwm.begin();
  
  pwm.setPWMFreq(60);  // Analog servos run at ~60 Hz updates
}

void loop()
{
  pwm.setPWM(0,0,225);
  delay(2000); // two seconds
  pwm.setPWM(0,0,475);
  return;
}

[adafruit_support_mike]

Servos are controlled by a continuous stream of pulses. Any time the pulse stream changes, the servo will move to the appropriate position.

In this case, it sounds like the servo is moving to a 'home' position when it senses the first electrical connection from the shield.

[msousa]

Thanks adafruit_support_mike. With the initial motion I've reset the arm on the servo and that does appear to be home. When I start up the arduino and servo driver it appears that it runs before I do an upload. Is it possible that the I2C pulses are still running on the pwm_servo_driver? Is there a certain sequence of powering up the system ( right now, it's 1st arduino, 2nd pwm_servo_driver, 3rd servo power to V+)?

Thanks...

[adafruit_support_bill]

If you wait to power the servos until the driver board is initialized, the servos should seek to that initial position on power-up.

[helio.b.teixeira]

Hi,

Is there any possibility to turn off the power of one specific channel? I noticed that (in Python), servo only moves when we call setPWM(), even both the servo power supply is connected...What would happen if we set both start and end of "Pin High" at 0 like this -> setPWM([channel], 0, 0)

[adafruit_support_bill]

What would happen if we set both start and end of "Pin High" at 0 like this -> setPWM([channel], 0, 0)

That won't turn off the power to the servo, but it will stop sending a pulse. Most servos will stop active control at that point, so the motor will be idle.

[helio.b.teixeira]

Thank you very much,

I noticed on other posts that you refer an OE (Output Enable) pin. How can i set it to low?

[adafruit_support_bill]

The Output Enable pin is active LOW and is pulled low by default. You can drive it high with any GPIO pin to suppress output to all channels simultaneously.

[helio.b.teixeira]

Can I find any additional tutorial/info on this?
(I'm only used to Adafruit's Python library).

[adafruit_support_bill]

There is no tutorial specifically for that. The data sheet describes how it works: http://www.adafruit.com/datasheets/PCA9685.pdf

In any case, it is not complicated. Just connect it to a GPIO pin. Set the pin HIGH to disable outputs. Set it LOW to enable them.

[helio.b.teixeira]

I've found this on the data sheet:

Screen Shot 2014-11-06 at 01.48.53.png (75.04 KiB) Viewed 696 times

I'm using a RaspberryPi so the voltage GPIO (3.3V) is within the allowed voltages, right?
Will I need a resistor?

[adafruit_support_bill]

Yes. 3.3v is fine. When controlling it from the Pi, the i2c bus voltage is 3.3v.