Nuts and Volts - September 2008

GETTING STARTED WITHPICs THE LATEST IN PROGRAMMING MICROCONTROLLERS

■ BY CHUCK HELLEBUYCK

PULSE WIDTH MODULATION

PULSE WIDTH MODULATION (PWM) is a common term in today’s microcontroller (MCU) world. What PWM is and how you use it are the subjects of this column.

As I’ve covered in previous columns, a MCU typically senses digital signals such as switch inputs that are either on or off. A MCU can also sense analog signals by using an Analog-to-Digital Converter (ADC) to change the analog signal to digital. PWM is a way to take a digital output and vary the on — or high time — to create a variable output. If the PWM signal runs at a fixed frequency, then changing the high time of the signal will change the low time of the signal, as well. The amount of time the signal is high is called the pulse width. The period of the signal is defined as the time from one rising edge to the next rising edge of the square wave signal and is inversely proportional to the PWM frequency. The period can easily be calculated by using the formula period

■ FIGURE 1. Low pass filter. = 1/frequency. A 1 kHz frequency will give us a

1 millisecond period.

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Duty cycle is what we are really interested in when using PWM to control a circuit. Duty cycle is the amount of time the signal is high, relative to the period of the PWM signal. Duty cycle is measured in percentage of the whole period. For example, a 50% duty cycle will be high for half the period and low for half the period. If we wanted to create a different duty cycle — such as a 20% duty cycle — then we would make the pulse width last only 20% of the period.

Using PWM gets very interesting because there are many applications for it. Simplest of all and a great place to start is driving some kind of low-pass filter. By driving a simple resistor-capacitor low-pass filter circuit (see Figure

1) directly from the MCU’s PWM port, the variable pulse width will be averaged out by the capacitor’s charge and discharge rate through the resistor. Figure 2 shows this in detail, using a 50% duty cycle.

The top part of Figure 2 shows the square wave generated by the MCU. In a digital world, the high pulse can be considered on and the low pulse considered off. The circuit diagrams below the pulses show what the low-pass filter is doing. The arrow shows the direction of the current. The bottom portion of the figure shows the charging and discharging voltage of the capacitor. This depends upon the frequency of the signal, and the values you select for the resistor and _■_FIGURE _2. _PWM _signal.capacitor. The important thing to notice is the dashed line running through the charge/discharge signal. That dashed line is the average voltage across the capacitor. Even though the MCU is outputting five- or zero-volt signals only, you would see about 2.5 volts across the capacitor if you measured it with a voltmeter.