Unlike the PWM pins, DAC0 and DAC1 are Digital to Analog converters, and act as true analog outputs. The Arduino Due supports analogWrite() on pins 2 through 13, plus pins DAC0 and DAC1. Older Arduino boards with an ATmega8 only support analogWrite() on pins 9, 10, and 11. On the Arduino Mega, it works on pins 2 through 13. On most Arduino boards (those with the ATmega168 or ATmega328), this function works on pins 3, 5, 6, 9, 10, and 11. The frequency of the PWM signal is approximately 490 Hz. After a call to analogWrite(), the pin will generate a steady square wave of the specified duty cycle until the next call to analogWrite() (or a call to digitalRead() or digitalWrite() on the same pin). Can be used to light a LED at varying brightnesses or drive a motor at various speeds. Writes an analog value ( PWM wave) to a pin. Here is the on-line documentation repeated for your convenience: The analogWrite() function takes a great deal of work off your shoulders. PWM pins will have a tilde (~) next to their Arduino pin number. You can tell which pins are PWM-capable just by looking at the board. The Arduino system pre-configures these for you. Each of these has an A and B component, so it’s possible to generate six PWM signals. The Uno achieves PWM through the use of its three internal counters. Examples would include driving a resistive heating element to control temperature or driving an LED to control brightness. For some loads, we wouldn’t even have to filter the result. If we sped this up and the pulses were milliseconds or microseconds in width and repeated over and over, we could low pass filter the pulse train and achieve smoothly varying results. In other words the greater the duty cycle, the higher the average voltage level. Similarly, if that five volt pulse stayed high for two seconds, then over the course of the ten second period the average value would be one volt. That is, the average value of either pulse over the course of ten seconds is one half volt. The same area would be achieved by a five volt pulse that stayed high for just one second out of ten. Suppose we have a one volt pulse signal that lasts for a period of five seconds, goes low for five seconds and then repeats. The key to understanding pulse width modulation is to consider the “area under the curve”. Some loads may require further processing of the PWM signal (such as filtering). Note that not all loads will operate properly with a simple PWM signal. This scheme is employed on the Uno and many other Arduino boards. The second method relies on pulse width modulation. The DAC (along with a low-pass reconstruction filter) reconstructs these data into a smooth signal. We then write each byte to the port at the desired rate. How then can we get the controller to produce analog signals? One method is to use an entire bank of port pins, say all eight bits of port B, and feed these directly to an external parallel-input DAC. The ATmega 328P on the Uno is one of those that don’t. While some more advanced microcontrollers contain a digital to analog converter (DAC) to produce a continuously variable analog output signal, most do not.
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