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This page describes all the nitty gritty of getting your Arduino set up to output decent audio on its PWM via Timer1. For more in-depth information on what the settings mean, and what values are right for your application, please check out our PWM Tutorial on the main site. Timer1 outputs its data on pins 9 and 10 on the Arduino (pin 9/10 are OC1A/B and PORTB1/2 on the ATmega328p). For all of the following, it will be assumed that pin 9 is used for Single PWM, and is the high byte for Dual PWM. Pin 10 is not used for Single PWM, and is the low byte for Dual PWM. You can also set up for Dual PWM, and output 2 different signals, one on each output. Other timers can be similarly setup to run PWM, but we use Timer1 because it is a 16 bit timer, so it can do greater bit depth at lower frequencies (if desired). The register settings for other timers is similar, but check the ATmega328p datasheet to be make sure its all correct. Also, be careful when using Timer0 on the Arduino, as it is already in use for the delay() and millis() functions, and its overflow interrupt can not be reused.

The following charts all assume a microcontroller clock frequency of 16MHz (standard on the Arduino). If you are using a different oscillator, just multiply the frequencies listed by Fcpu/16MHz, where Fcpu is your oscillator frequency. For example, if you were running at 8MHz, you would multiply by 1/2, and if you were running at 20MHz, you would multiply by 5/4. Everything else stays the same. There are only 3 values to set in order to configure your PWM. they are: PWM_FREQ - This is the frequency value taken from the table below. PWM_MODE - Set to 1 for Fast mode, and 0 for Phase Correct mode. PWM_QTY - Set to 1 for Single PWM, and 2 for Dual PWM. Fast - PWM_MODE 1 Phase Correct - PWM_MODE 0 Frequency PWM_FREQ Bit Depth PWM_FREQ Bit Depth Fcpu = 16MHz Value Single Dual Value Single Dual 250kHz 0x003F 6 bit 12 bit 0x001F 5 bit 10 bit 125kHz 0x007F 7 bit 14 bit 0x003F 6 bit 12 bit 62.5kHz 0x00FF 8 bit 16 bit 0x007F 7 bit 14 bit 31.3kHz 0x01FF 9 bit N/A 0x00FF 8 bit 16 bit 15.6kHz 0x03FF 10 bit N/A 0x01FF 9 bit N/A 7.81kHz 0x07FF 11 bit N/A 0x03FF 10 bit N/A 3.91kHz 0x0FFF 12 bit N/A 0x07FF 11 bit N/A 1.95kHz 0x1FFF 13 bit N/A 0x0FFF 12 bit N/A 976Hz 0x3FFF 14 bit N/A 0x1FFF 13 bit N/A 488Hz 0x7FFF 15 bit N/A 0x3FFF 14 bit N/A 244Hz 0xFFFF 16 bit N/A 0x7FFF 15 bit N/A 122Hz N/A N/A N/A 0xFFFF 16 bit N/A

Dual PWMs just mix an upper and lower value together, to make one analog value. This mixing occurs through 2 resistors, one of which is 2^n times the other, where n is the bit depth. For 2 - 8 bit PWMs (16b), use 1:256, or 3.9k and 1M (as shown below). For 2 - 7 bit PWMs (14b), use 1:128, or 3.9k and 499k, etc. The 3.9k is used so that the output resistance of the microcontroller is much less than the total resistance, reducing errors. And be sure to use 1% resistors for 14b and higher PWMs.

This skecth uses the above chart to set the Arduino PWM to 31.25kHz, Dual PWM, Phase Correct, at 16 bit depth. It takes in audio on ADC0, and plays it out the PWM, so you can hear the sort of quality you can expect from 10 bit data (10 bits is the depth of the ADC). For ATmega programmers, the Arduino sketch is written in C, and is essentially usable if copied into your programming interface of choice. The following low-pass filter and summing circuit should be used, along with a the bias circuit for the ADC. pwm_schem_sm.jpg ADC to PWM Arduino Sketch > 8);]]> > 8;]]> > 8; ]]> > 8;]]>