I read in other topics that before starting any work with the Arduino Due, I most set the logic level jumper for 3.3v operation. Then, I connected the motor shield and the Arduino and I powered the processor through the Jack with 8v. I ran the DCMotorTest, I measured the SDA and SCL pins with a scope and I saw that the I2C was working.
Later, I measured the voltage in M1 and I got the PWM signal working fine (I did this in open-circuit); when I connected the motor, the output voltage from the PWM started to change and the duty cycle became constant, also, the Arduino stopped from running the code.
Why is this happening?