Page 258 - ARM Based Microcontroller Projects Using MBED

244 8. INTERMEDIATE LEVEL PROJECTS

8.33.6 Suggestions for Additional Work
Modify the program given in Fig. 8.99 by moving the code inside a timer ISR.

8.34 USING THE PWM

In many microcontroller-based applications, we may want to drive loads which may re-
quire large currents or voltages. Some examples are large motors, actuators, heaters, etc.
The output current and voltage capabilities of microcontrollers are very limited where the
maximum output current capacity is around 20mA and the maximum output voltage is
+3.3 or +5V depending on the type of microcontroller used. PWM is used to drive loads that
require large currents or voltages.
Basically, PWM is a positive going square waveform as shown in Fig. 8.116. The waveform
has two parts: On time where the pulse if present, and the OFF time where there is no pulse.
By varying the ratio of the ON time to OFF time, we can effectively vary the average DC volt-
age applied to the load. The PWM waveform usually drives the load through a switching
power transistor or a transducer.

FIG. 8.115 Generating an arbitrary waveform.