Arduino Nano PWM frequency

Question

I have been using an Arduino Nano as a PWM controller for a heating element.

I am currently using D11 (PB3). I could change, although all pins with PWM are in use, and would require a re-write and re-wire.

This produces a signal at ~490Hz, but I would like to experiment with changing this to a higher frequesncy.

I gather it its possible to vary the frequency by changing the divisor from its default 64.

My sketch uses Uses http://playground.arduino.cc/Main/SevenSegmentLibrary and calls millis() - will changing PWM divisor impact on either of these?

NOTE I have read the setPwmFrequency documentation, but find "disrupts the normal operation" unhelpful - I am actually after some explanation of the concrete effect. I only use millis() to time button presses and flash a LED, which are not time critical

Please keep in mind that changing the PWM frequency changes the Atmega's timers and disrupts the normal operation of many functions that rely on time (delay(), millis(), Servo library).

I have been doing some further study. http://playground.arduino.cc/Code/PwmFrequency states Changes on pins 3, 5, 6, or 11 may cause the delay() and millis() functions to stop working.

Other references say 3, 11 use timer 2 so do they affect millis() or not as other references claim timer 0 is used for these.

Answer 1

I don't know where this misinformation comes from... The answer is no, reconfiguring Timer 2 does not affect millis() nor delay(), as these functions rely on Timer 0. AFAIK the Arduino core does not use Timer 2 at all. But note that there may be other Arduino or third party libraries that rely on Timer 2.

You can see here the source code of millis() and delay().

As MITU RAJ correctly states in his answer, the PWM signal on pin 11 is provided by Timer 2, so you can use it safely.

Answer 2

Depends on the pin you used for the PWM.

Arduino UNO/NANO specs say following things:

The base frequency for pins 3, 9, 10, and 11 is 31250 Hz.

The base frequency for pins 5 and 6 is 62500 Hz.

The divisors available on pins 5, 6, 9 and 10 are: 1, 8, 64, 256, and 1024.

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.

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.

Answer 3

The answer is NO, changing the PWM divider for pin 11 will not affect the millis() function (nor delay()).

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Explanation:
The Arduino Nano uses the ATMega328P microcontroller, same as the Arduino Uno.

Pin 11's PWM is controlled by Timer 2. By default Timer 2 has a prescale of 64 (ie. setting 0x04). The base frequency of Timer 2 is 31372.55 Hz resulting in a frequency of 490.2 Hz (ie. 31372.55/64 = 490.1961).

On the other hand, the microsecond timer (ie. the one used by millis(), micros(), delay() and delayMicroseconds()) is controlled by Timer 0.

So changing the prescaler of Timer 2 will not affect millis().

It will however change the PWM frequency of Pin 3.

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Back to your question:
The possible PWM frequencies of Pin 11 (higher than 488 Hz) are:

31373 / 32 = 980.4 Hz
31373 / 8 = 3921.6 Hz
31373 / 1 = 31373 Hz

Where 32, 8, 1 are the prescaler.
Which equates to a setting value of 0x03, 0x02 & 0x01 respectively.

Add this line of code:

TCCR2B = TCCR2B & 0b11111000 | setting;

Where setting is the value of the setting for the respective prescaler.

============================================  
|| Frequency [Hz] || Prescaler || Setting ||  
============================================  
|| 31373.55       || 1         || 0x01    ||  
|| 3921.57        || 8         || 0x02    ||  
|| 980.39         || 32        || 0x03    ||  
|| 490.20         || 64        || 0x04    ||  
|| 245.10         || 128       || 0x05    ||  
|| 122.55         || 256       || 0x06    ||  
|| 30.64          || 1024      || 0x07    ||  
============================================  

Source code: https://github.com/arduino/ArduinoCore-avr/blob/master/cores/arduino/wiring.c