#254 AvrHardwarePWM

All about hardware PWM and demonstrating the modes with the Arduino UNO/ATmega328.

Build

Notes

Arduino provides a great “out-of-the-box” PWM capability, but it does not provide a simply way to modify the PWM frequency. To do that you need to work directly with the processor registers.

This is my summary of hardware PWM control and example code. Secrets of Arduino PWM is a great article that goes into much more detail. It is the main source of information that I used, in addition to the Atmel datasheet.

Although the concepts are general to the AVR processors, the details here and the example sketch are specific to the Arduino UNO/ATmega328 and may require modification to work with other processors.

See the AvrHardwarePWM/ATtiny project for a similar treatment, specifically for the ATtiny85 processor.

Example Sketch

ATmega.ino exercises the PWM modes, primarily so they can be captured with an oscilloscope. The push-button attached to pin 8 is used to cycle through a few demonstration modes:

Mode 0a: Timer0, Fast PWM 977Hz
Mode 1a: Timer1, Fast PWM 10-bit 1955Hz
Mode 1b: Timer1, Phase Correct PWM, 8-bit 3906Hz
Mode 2a: Timer2, Fast PWM 62.5kHz
Mode 2b: Timer2, Fast PWM 977Hz

When the mode is changed, details of the new mode are sent to the Serial port, e.g

demo_console

Three Approaches to PWM

If you need a PWM signal generated onboard, there are three approaches:

use the standard library (pinMode/analogWrite). Works fine, but frequency is fixed.
“bit-banging” i.e. manually toggling a digital output to simulate a PWM signal. While more flexible than the standard library, it ties up the processor and requires care to get the timing just right.
directly control the PWM support built-in to the ATmega processor (“hardware PWM”)

The rest of the notes here concern the third approach - hardware PWM.

Hardware PWM - How it Works

There are basically four elements:

Timer Control Registers TCCRx define the waveform generation mode
Output Compare Registers OCRx define the upper compare limit (duty cycle)
Processor clock / prescaler > Timer Clock -> increments TCNTx register
Waveform generator produces the output in OCx according to TCCRx by comparing TCNTx with OCRx

pwm_how_it_works

Summary of the TCCR registers:

TCCR_summary

Which Timer to Use?

In the ATMega328P there are three timers that can be used to generate PWM signals:

Timer 0 is an 8-bit timer. But it is used for functions such as delay() and millis() - these will be affected if PWM frequency is changed.
Timer 1 is a 16-bit timer; higher-precision duty cycle, but frequency is limited.
Timer 2 is an 8-bit timer, and generally unencumbered (ready to use!)

Compare Register	Timer output	Arduino output	Chip pin	Pin name
OCR0A	OC0A	6	12	PD6
OCR0B	OC0B	5	11	PD5
OCR1A	OC1A	9	15	PB1
OCR1B	OC1B	10	16	PB2
OCR2A	OC2A	11	17	PB3
OCR2B	OC2B	3	5	PD3

Note:

regardless of other settings, PWM output will only be generated on a pin if the pinMode is set to OUTPUT.
the two outputs for each timer will normally have the same frequency, but can have different duty cycles.

Timer Prescalers

Each timer has a prescaler that generates the timer clock by dividing the clock source by a prescale factor. The clock source is normally the system clock, running at 16MHz on an Ardunio UNO. It is possible to use an external clock - see 17.3 in the datasheet.

The Timer 0 and Timer 1 prescaler is set in the TCCR0 ,TCCR1 registers (CS12, CS11, CS10).

For Timer 0 and Timer 1:

the prescaler is set in the TCCR0 ,TCCR1 registers (bits CS12, CS11, CS10)
the prescaler options are: 1, 8, 64, 256, or 1024.

The Timer 2:

the prescaler is set in the TCCR2 register (bits CS22, CS21, CS20).
For Timer 2, the prescaler options are: 1, 8, 32, 64, 128, 256, or 1024.

Waveform Generation Modes

Main modes of interest:

Fast PWM - counts up and resets on 1 extra clock pulse (hence prone to off-by-1 errors)
PWM, Phase correct - counts up and down, avoids the off-by-1 error by half the frequency

_BV Macro

In the example sketch I’m using the _BV macro to set register values. This avoids hard-coding bit-offsets.

More info in the AVR Libc Reference Manual.

Controlling Duty Cycle with analogWrite

See the analogWrite reference.

analogWrite(pin, dutyCycle)

Where:

dutyCycle is a value from 0 (always off) and 255 (always on)
pin is one of the PWM pins (3, 5, 6, 9, 10, or 11)

The digitalWrite() function turns off PWM output if called on a timer pin.

Performance

These results were measured with the ATmega.ino sketch and an oscilloscope.

Mode 0a: Timer0, Fast PWM 977Hz

0a> Switching to Timer0 on pin 5, 6
  Fast PWM  : WGM01 WGM00
  Clear OC0A/OC0B on Compare Match, set OC0A/OC0B at BOTTOM (COM0A1/COM0B1)
  Frequency : 16MHz/64/256 = 977Hz (CS01 CS00)
  Pin 6     : (127+1)/256 = 50% duty cycle (OCR0A)
  Pin 5     : (191+1)/256 = 75% duty cycle (OCR0B)
GTCCR: 0
TCCR0A : 10100011
TCCR0B : 11
OCR0A  : 1111111
OCR0B  : 10111111
TCCR1A : 1
TCCR1B : 10
OCR1A  : 1111111
OCR1B  : 10111111
TCCR2A : 11
TCCR2B : 100
OCR2A  : 1111111
OCR2B  : 10111111

mode_0a

Mode 1a: Timer1, Fast PWM 10-bit 1955Hz

1a> Switching to Timer1 on pin 9, 10
  Fast PWM, 10-bit : WGM12 WGM11 WGM10
  Clear OC1A/OC1B on Compare Match, set OC1A/OC1B at BOTTOM (COM1A1/COM1B1)
  Frequency : 16MHz/8/0x3FF = 1955Hz (CS11)
  Pin 9     : (510+1)/0x3FF = 50% duty cycle (OCR1A)
  Pin 10    : (766+1)/0x3FF = 75% duty cycle (OCR1B)
GTCCR: 0
TCCR0A : 11
TCCR0B : 11
OCR0A  : 0
OCR0B  : 0
TCCR1A : 10100011
TCCR1B : 1010
OCR1A  : 111111110
OCR1B  : 1011111110
TCCR2A : 11
TCCR2B : 100
OCR2A  : 1111111
OCR2B  : 10111111

mode_1a

Mode 1b: Timer1, Phase Correct PWM, 8-bit 3906Hz

1b> Switching to Timer1 on pin 9, 10
  Phase Correct PWM, 8-bit : WGM10
  Clear OC1A/OC1B on Compare Match, set OC1A/OC1B at BOTTOM (COM1A1/COM1B1)
  Frequency : 16MHz/8/0xFF/2 = 3906Hz (CS11)
  Pin 9     : (127)/0xFF = 50% duty cycle (OCR1A)
  Pin 10    : (191)/0xFF = 75% duty cycle (OCR1B)
GTCCR: 0
TCCR0A : 11
TCCR0B : 11
OCR0A  : 0
OCR0B  : 0
TCCR1A : 10100001
TCCR1B : 10
OCR1A  : 1111111
OCR1B  : 10111111
TCCR2A : 11
TCCR2B : 100
OCR2A  : 1111111
OCR2B  : 10111111

mode_1b

Mode 2a: Timer2, Fast PWM 62.5kHz

2a> Switching to Timer2 on pin 3, 11
  Fast PWM  : WGM21 WGM20
  Clear OC2A/OC2B on Compare Match (COM2A1/COM2B1)
  Frequency : 16MHz/1/256 = 62.5kHz (CS20)
  Pin 11    : (127+1)/256 = 50% duty cycle (OCR2A)
  Pin 3     : (191+1)/256 = 75% duty cycle (OCR2B)
GTCCR: 0
TCCR0A : 11
TCCR0B : 11
OCR0A  : 1111111
OCR0B  : 10111111
TCCR1A : 1
TCCR1B : 10
OCR1A  : 1111111
OCR1B  : 10111111
TCCR2A : 10100011
TCCR2B : 1
OCR2A  : 1111111
OCR2B  : 10111111

mode_2a

Mode 2b: Timer2, Fast PWM 977Hz

2b> Switching to Timer2 on pin 3, 11
  Fast PWM  : WGM21 WGM20
  Clear OC2A/OC2B on Compare Match (COM2A1/COM2B1)
  Frequency : 16MHz/64/256 = 977Hz (CS22)
  Pin 11    : (127+1)/256 = 50% duty cycle (OCR2A)
  Pin 3     : (191+1)/256 = 75% duty cycle (OCR2B)
GTCCR: 0
TCCR0A : 11
TCCR0B : 11
OCR0A  : 1111111
OCR0B  : 10111111
TCCR1A : 1
TCCR1B : 10
OCR1A  : 1111111
OCR1B  : 10111111
TCCR2A : 10100011
TCCR2B : 100
OCR2A  : 1111111
OCR2B  : 10111111

mode_2b