Modifier La Fréquence PWM – Arduino : L'essentiel

Un signal PWM est défini par son rapport cyclique, son amplitude et sa fréquence.

Par défaut, la fréquence du signal PWM délivrée par un Arduino est de 500Hz environ, une fréquence tout à fait adaptée s’il s’agit de modifier l’intensité d’une LED.

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• D3 et D11 : 490.20 Hz
• D5 et D6 : 976.56 Hz
• D9 et D10 : 490.20 Hz

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Mais il s’agit d’une fréquence audible, et si un résonateur est connecté au signal PWM (un moteur par exemple), cela engendrera un bruit désagréable.

Voici comment faire disparaitre le bruit gênant :

Utiliser la fonction setPwmFrequency()  définie ci-dessous :

/** * Divides a given PWM pin frequency by a divisor. * * The resulting frequency is equal to the base frequency divided by * the given divisor: * - Base frequencies: * o The base frequency for pins 3, 9, 10, and 11 is 31250 Hz. * o The base frequency for pins 5 and 6 is 62500 Hz. * - Divisors: * o The divisors available on pins 5, 6, 9 and 10 are: 1, 8, 64, * 256, and 1024. * o The divisors available on pins 3 and 11 are: 1, 8, 32, 64, * 128, 256, and 1024. * * PWM frequencies are tied together in pairs of pins. If one in a * pair is changed, the other is also changed to match: * - Pins 5 and 6 are paired on timer0 * - Pins 9 and 10 are paired on timer1 * - Pins 3 and 11 are paired on timer2 * * Note that this function will have side effects on anything else * that uses timers: * - Changes on pins 3, 5, 6, or 11 may cause the delay() and * millis() functions to stop working. Other timing-related * functions may also be affected. * - Changes on pins 9 or 10 will cause the Servo library to function * incorrectly. * * Thanks to macegr of the Arduino forums for his documentation of the * PWM frequency divisors. His post can be viewed at: * http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1235060559/0#4 */ void setPwmFrequency(int pin, int divisor) { byte mode; if(pin == 5 || pin == 6 || pin == 9 || pin == 10) { switch(divisor) { case 1: mode = 0x01; break; case 8: mode = 0x02; break; case 64: mode = 0x03; break; case 256: mode = 0x04; break; case 1024: mode = 0x05; break; default: return; } if(pin == 5 || pin == 6) { TCCR0B = TCCR0B & 0b11111000 | mode; } else { TCCR1B = TCCR1B & 0b11111000 | mode; } } else if(pin == 3 || pin == 11) { switch(divisor) { case 1: mode = 0x01; break; case 8: mode = 0x02; break; case 32: mode = 0x03; break; case 64: mode = 0x04; break; case 128: mode = 0x05; break; case 256: mode = 0x06; break; case 1024: mode = 0x7; break; default: return; } TCCR2B = TCCR2B & 0b11111000 | mode; } }

Sources :

• https://etechnophiles.com/change-frequency-pwm-pins-arduino-uno/
• http://playground.arduino.cc/Code/PwmFrequency
• http://arduino103.blogspot.fr/2013/02/ajuster-le-frequence-pwm-sur-un-arduino.html
• http://www.pobot.org/Modifier-la-frequence-d-un-PWM.html