The PwmOut interface is used to control the frequency and mark-space ratio of a digital pulse train.

Hello World!¶

00001 #include "mbed.h"
00002 
00003 PwmOut led(LED1);
00004 
00005 int main() {
00006     // specify period first
00007     led.period(4.0f);      // 4 second period
00008     led.write(0.50f);      // 50% duty cycle, relative to period
00009     //led = 0.5f;          // shorthand for led.write()
00010     //led.pulsewidth(2);   // alternative to led.write, set duty cycle time in seconds
00011     while(1);
00012 }

API¶

API summary

Details¶

The default period is 0.020s, and the default pulsewidth is 0.

The PwmOut interface can express the pulse train in many ways depening on how it is to be used. The period and pulse width can be expressed directly in units of seconds, millisecond or microseconds. The pulsewidth can also be expressed as a percentage of the the period.

Implementation details¶

LPC1768¶

You can also specify the LED1-LED4 as PwmOut. Note that these are just different pinout options for the same underlying PWM hardware, so they are just alternative routing rather than extra PWM channels. So you can pin them out can't be used at the same time:

On the mbed LPC1768, the PwmOut hardware is limited to share the period value between all outputs. Therefore, if you change the period of one output, you change them all. The pulsewidth can be set independently for each output.

LPC11U24¶

Pin under the same "Timer" share the same period:

Code Examples¶

This code example sets the period in seconds and the duty cycle as a percentage of the period in floating point (0 to 1 range). The effect of this code snippet will be to blink LED2 over a 4 second cycle, 50% on, for a pattern of 2 seconds on, 2 seconds off.

#include "mbed.h"

PwmOut led(LED2);

int main() {
    // specify period first, then everything else
    led.period(4.0f);  // 4 second period
    led.write(0.50f);  // 50% duty cycle
    while(1);          // led flashing
}

The following example does the same thing. Instead of specifying the duty cycle as a relative percentage of the period it specifies it as an absolute value in seconds. In this case we have a 4 second period and a 2 second duty cycle, meaning the led will be on for 2 seconds and off for 2 seconds.

#include "mbed.h"

PwmOut led(LED2);

int main() {
    // specify period first, then everything else
    led.period(4.0f);  // 4 second period
    led.pulsewidth(2); // 2 second pulse (on)
    while(1);          // led flashing
}

This code example is for an RC Servo. In RC Servo's you set the position based on duty cycle or pulse width of the pwm signal. This example code uses a period of 0.020s and increases the pulse width by 0.0001s on each pass. This will cause an increase of .5% of the servo's range every .25s. In effect the servo will move 2% of its range per second, meaning after 50 seconds the servo will have gone from 0% to 100% of its range.

#include "mbed.h"

PwmOut servo(p21);

int main() {
    servo.period(0.020);          // servo requires a 20ms period
    while (1) {
        for(float offset=0.0; offset<0.001; offset+=0.0001) {
            servo.pulsewidth(0.001 + offset); // servo position determined by a pulsewidth between 1-2ms
            wait(0.25);
        }
    }
}