Motorshield AnalogWrite functioning badly

[pirana2014]

Hi,
Using arduino Uno, arduino motorshield, and a simple dc motor (not important because I change the motor I test on constantly), and a bench power system 10v.

I run simple code which I will attach, and I have the problem where with one call of the AnalogWrite(), the motor moves in two increments no matter what. When I call for the motor to go 'up', it goes up briefly, stops then continues going up for the specified delay time. Same happens when i call it to go down.

I have tried putting a PWM of 0 at the start of the code to initialise it, and it still does the same thing. When i put a multimeter over the motorshield, and a 50 percent duty cycle, whenever I run the code i can see the motor channel A driving pins briefly go up to 5V, drop to 0 then go back to 5v.

Any idea why or how I can prevent it as this 'jitter' is not something I can account for during my project.

const int directionPin = 12;
const int pwmPin = 3;
const int brakePin = 9;


void setup() {

  pinMode(directionPin, OUTPUT);
  pinMode(pwmPin, OUTPUT);
  pinMode(brakePin, OUTPUT);
  analogWrite(pwmPin, 0);

  // Perform initial actions (for example, go down)
  digitalWrite(directionPin, HIGH); // Set direction to go up
  analogWrite(pwmPin, 125);
  delay(1000);
  analogWrite(pwmPin, 0);

}

void loop() {
}

void goUp() {
  digitalWrite(directionPin, LOW); // Set direction to go up
  analogWrite(pwmPin, 255); 
}

void goDown() {
  digitalWrite(directionPin, HIGH); // Set direction to go down
  analogWrite(pwmPin, 255);          // Start the motor
}

void brake() {
  analogWrite(pwmPin, 0);
      // Stop the motor
}

[UKHeliBob]

Your sketch does not call any of the functions that control the motor. Please post one that does

It would also help if you posted a schematic of your project

How are the motors powered ?

[ps0709]

Try to call the functions (goUP and goDown) inside the loop, currently it is not doing that, that's why the motors stop after a while

[pirana2014]

I decided to avoid the functions for debugging purposes, that is why i didn't include it in this snippet of code. There is no need to use the function as I am directly calling AnalogWrite.

Motor is connected directly to the arduino motorshield.
Arduino motorshield powered by bench power supply.

[pirana2014]

That is not the problem, thank you for your response though. The problem is that the first time I run the code and call analogwrite, I notice there is a 'bounce'. As in I call analogwrite 255 dutycycle once and then set the analogwrite PWM to 0, but on the oscilloscope I can see 2 changes like as below

This bounce happens only on the first movement of the motor as I run the code, however I have tried manually entering PWM values into the register as well and it still does the same thing.

[jim-p]

Try using digitalWrite (pwmPin, LOW) instead of analogWrite(pwmPin, 0)
Sometimes PWM 0 doesn't go all the way to zero.