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Topic: Seeed Studio motor shield v2 and a servo, HOW? (Read 1 time) previous topic - next topic


It has come to my attention that there is a fundamental problem with the Seeed Studio motor shield when trying to control a servo along with it. I have learned that this is because Servo.H disables PWM on pins 9 and 10 which are used by the motor shield.

I've been searching for a solution and honestly I'm about to throw this shield away. Does anybody have a solution for this? I just need to be able to control two motors and a servo which moves an HC-SR04 but because of how servo.h is written, it won't work with the shield.

I have not seen this addressed by anybody over at Seeed either.


I'm trying to find where in this code the PWM for pins 9 and 10 are disabled because I was hoping there might be a way to make a modified version that would work with this stupid shield but I'm not seeing it. Anybody have an idea?

Code: [Select]

  Servo.h - Interrupt driven Servo library for Arduino using 16 bit timers- Version 2
  Copyright (c) 2009 Michael Margolis.  All right reserved.

  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.

  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public
  License along with this library; if not, write to the Free Software
  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

  A servo is activated by creating an instance of the Servo class passing the desired pin to the attach() method.
  The servos are pulsed in the background using the value most recently written using the write() method

  Note that analogWrite of PWM on pins associated with the timer are disabled when the first servo is attached.
  Timers are seized as needed in groups of 12 servos - 24 servos use two timers, 48 servos will use four.
  The sequence used to sieze timers is defined in timers.h

  The methods are:

   Servo - Class for manipulating servo motors connected to Arduino pins.

   attach(pin )  - Attaches a servo motor to an i/o pin.
   attach(pin, min, max  ) - Attaches to a pin setting min and max values in microseconds
   default min is 544, max is 2400 
   write()     - Sets the servo angle in degrees.  (invalid angle that is valid as pulse in microseconds is treated as microseconds)
   writeMicroseconds() - Sets the servo pulse width in microseconds
   read()      - Gets the last written servo pulse width as an angle between 0 and 180.
   readMicroseconds()   - Gets the last written servo pulse width in microseconds. (was read_us() in first release)
   attached()  - Returns true if there is a servo attached.
   detach()    - Stops an attached servos from pulsing its i/o pin.

#ifndef Servo_h
#define Servo_h

#include <inttypes.h>

 * Defines for 16 bit timers used with  Servo library
 * If _useTimerX is defined then TimerX is a 16 bit timer on the curent board
 * timer16_Sequence_t enumerates the sequence that the timers should be allocated
 * _Nbr_16timers indicates how many 16 bit timers are available.

// Say which 16 bit timers can be used and in what order
#if defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
#define _useTimer5
#define _useTimer1
#define _useTimer3
#define _useTimer4
typedef enum { _timer5, _timer1, _timer3, _timer4, _Nbr_16timers } timer16_Sequence_t ;

#elif defined(__AVR_ATmega32U4__) 
#define _useTimer3
#define _useTimer1
typedef enum { _timer3, _timer1, _Nbr_16timers } timer16_Sequence_t ;

#elif defined(__AVR_AT90USB646__) || defined(__AVR_AT90USB1286__)
#define _useTimer3
#define _useTimer1
typedef enum { _timer3, _timer1, _Nbr_16timers } timer16_Sequence_t ;

#elif defined(__AVR_ATmega128__) ||defined(__AVR_ATmega1281__)||defined(__AVR_ATmega2561__)
#define _useTimer3
#define _useTimer1
typedef enum { _timer3, _timer1, _Nbr_16timers } timer16_Sequence_t ;

#else  // everything else
#define _useTimer1
typedef enum { _timer1, _Nbr_16timers } timer16_Sequence_t ;                 

#define Servo_VERSION           2      // software version of this library

#define MIN_PULSE_WIDTH       544     // the shortest pulse sent to a servo 
#define MAX_PULSE_WIDTH      2400     // the longest pulse sent to a servo
#define DEFAULT_PULSE_WIDTH  1500     // default pulse width when servo is attached
#define REFRESH_INTERVAL    20000     // minumim time to refresh servos in microseconds

#define SERVOS_PER_TIMER       12     // the maximum number of servos controlled by one timer
#define MAX_SERVOS   (_Nbr_16timers  * SERVOS_PER_TIMER)

#define INVALID_SERVO         255     // flag indicating an invalid servo index

typedef struct  {
  uint8_t nbr        :6 ;             // a pin number from 0 to 63
  uint8_t isActive   :1 ;             // true if this channel is enabled, pin not pulsed if false
} ServoPin_t   ; 

typedef struct {
  ServoPin_t Pin;
  unsigned int ticks;
} servo_t;

class Servo
  uint8_t attach(int pin);           // attach the given pin to the next free channel, sets pinMode, returns channel number or 0 if failure
  uint8_t attach(int pin, int min, int max); // as above but also sets min and max values for writes.
  void detach();
  void write(int value);             // if value is < 200 its treated as an angle, otherwise as pulse width in microseconds
  void writeMicroseconds(int value); // Write pulse width in microseconds
  int read();                        // returns current pulse width as an angle between 0 and 180 degrees
  int readMicroseconds();            // returns current pulse width in microseconds for this servo (was read_us() in first release)
  bool attached();                   // return true if this servo is attached, otherwise false
   uint8_t servoIndex;               // index into the channel data for this servo
   int8_t min;                       // minimum is this value times 4 added to MIN_PULSE_WIDTH   
   int8_t max;                       // maximum is this value times 4 added to MAX_PULSE_WIDTH   



Actually re-reading and not skipping the notes, it looks like PWM is being shut down in timers.h


At any rate, I've given up on trying to use this shield with servos and motors, mostly because I can't find a header that makes easy use of servos along with the shield. I'll be putting it on another robot chassis that has three fixed position rangefinders. One directly in the front and two others around 30 degrees off to the left and the right. The nice thing about this setup is I don't have to stop to look around, I can just poll all three sensors on a regular interval and have the robot make decisions on which way to turn based on what numbers the three sensors are returning.

I will have to use a different power source though because currently I have a LiPo connected to a power distribution and charging circuit. The output is 5.2v at 500 miliamps. Running everything through the motor driver shield, I don't have enough power to run the motors. I'm planning to swap this out for a battery holder that takes four 18650 cells for around 17 volts. Though, going through the regulators on the shield and the arduino board, I'm not sure how efficient it will actually be, and this battery holder doesn't have a protection circuit, nor do the cells themselves.

I've got a few options for power for the robot and will be able to "make it work" but if anybody does have a good solution for using servos with the Seeed Studios MotorShield V2, I would still like to see them.


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