analogWrite mosfet problem

Question

i have a custom atmega328p board which have connected to pin 9 the gate of a MOSFET. If i run a simple sketch like:

void setup() {
}
void loop() {
 analogWrite(9,70);
}

The motor connected to the mosfet works just fine, from 70pwm to 255pwm.

On the other sketch i'm receiving serial data from an android app via bluetooth. Ive 2 Servo motors and a DC motor connected to the mosfet. The 2 servos works like expected, the DC motor doesn't spin unless i use a 255pwm.

void loop() {
 if(Serial.available() > 0){ 
 state = Serial.read(); 
 }
 if (state == 'S') {
 servoONE.write(90);
 servoTWO.write(90);
 analogWrite(MOSFET,MOSFET_LOW);
 }
 else if (state == 'F') {
 analogWrite(9,70);
 servoONE.write(90);
 servoTWO.write(90);
 }
 else if (state == 'L') {
 servoONE.write(MAX_SERVO);
 servoTWO.write(MIN_SERVO);
 }
 else if (state == 'R') {
 servoONE.write(MIN_SERVO);
 servoTWO.write(MAX_SERVO);
 }
 else if (state == 'Q') {
 analogWrite(MOSFET,MOSFET_HIGH);
 servoONE.write(MAX_SERVO);
 servoTWO.write(MIN_SERVO);
 }
 else if (state == 'E') {
 analogWrite(MOSFET,MOSFET_HIGH);
 servoONE.write(MIN_SERVO);
 servoTWO.write(MAX_SERVO);
 }
}

Do you know why the same motor works on the first sketch with any pwm above 70 and on the second sketch it only works at 255?

Answer 1

You have a timer conflict.

Pin 9 is OC1A which is the Output Compare Channel A pin of Timer 1. However the Servo.h library also uses timer 1.

So you can't use servos and still use pins 9 (OC1A) and 10 (OC1B) as PWM.

However, you can still use pins 3, 5, 6 and 11 as PWM since they are on other timers.

• user3106888: Ok, thanks for the quick answer, i see the pin 11 is the MOSI pin and im using an ISP programmer to program my board. Will i be able to program my board if i use pin 11 as PWM ?
• Majenko: Of course. So long as what you have connected to it doesn't interfere with the ICSP signal - and the gate of a MOSFET shouldn't.
• user3106888: Tested right now using pin 3 instead of pin 9, it works!!! You saved me a lot of troubles :D