Implementing Pulse Width Modulation Using the PIC | ECE 480, Study notes of Principles of Theater Design

Download Implementing Pulse Width Modulation Using the PIC | ECE 480 and more Study notes Principles of Theater Design in PDF only on Docsity! 1 Implementing Pulse Width Modulation using the PIC18F4520 Bryan Thomas A35345102 October 28 th , 2007 Keywords: Pulse Width Modulation, Voltage Levels, Duty Cycle, Analog Control Table of Contents: Abstract…………………Page 1 Introduction……………..Page 1 PWM Implementation……Page1 PWM Application…………Page 2 Summary………………….Page 3 Useful Figures…………….Page 3 References………………….Page 6 Abstract The technique of pulse width modulation (PWM) is used to regulate the output voltage in a circuit. By changing the duty cycle, the average output power can be adjusted even though the output pulse remains the same throughout the different duty cycles. The uses of PWM are many. For our project, PWM will be implemented to create variable frequencies to operate our speaker at. One of the requirements of the sponsor is that the frequency of the sound coming from the base be different than the frequency of the sound coming from the ball. Keeping this in mind, we have decided to make the frequency of the base be under 1 kHz. Introduction Pulse width modulation is a technique used to manipulate the average power seen at a load. By using the formula*: The average value of a waveform can be calculated. By changing the length of time in which the signal is considered “high”, also known as the duty cycle, the area under the signal can be manipulated. Without changing the period, changing the area under the signal will thereby change the average value of the signal during one period. The microcontroller that we are using, the PIC18F4520, comes with an onboard PWM module. Using this module and the appropriate registers, we will be able to manipulate our signal going to the speaker using PWM. PWM Implementation To properly implement PWM into our design, a reference of the PIC18F4520 datasheet had to be made**. Upon 2 viewing the block diagram of the PWM module: It can be seen that the PWM relies upon the comparison between the CCPR1 registers (High/Low), TMR2 (Timer 2), and the PR2 register. CCPR1L and CCPR1H are the two registers used to set the duty cycle. Additionally, the PWM duty cycle can also be manipulated by setting the Timer 2 prescaler bits. The formula, specific to the PIC18F4520, for the duty cycle is as follows: Another aspect of the PWM module which must be taken into consideration is the period of the signal. Similarly to the duty cycle calculation, the calculation for signal period relies on the PR2 register, Timer 2 prescaler value, and the period of the oscillator (Tosc). The formula for the PWM period is as follows: Using the PWM period, taking the inverse will give you the frequency of your signal. In order to properly utilize the PWM module on the PIC18F4520, the following steps should be taken: 1) Configure the period of the PWM by writing the appropriate value to the PR2 register 2) Configure the duty cycle of the PWM by writing the appropriate value to the CCP1CON<5:4> bits as well as the CCPR1L register (and possibly the CCPR1H register if necessary) 3) Set the CCP1 pin as an output by manipulating the appropriate TRIS register 4) Set the Timer 2 prescale value 5) Enable Timer 2 by writing to the T2CON register 6) Write to the CCP1 register to enable PWM operation PWM Application PWM can be used in a variety of settings. Looking at Figure 3, we can see a generic load being driven by PWM output from a PIC microcontroller. In our specific case, we will be driving the Piezo buzzer with one of the four PWM outputs on the 18F4520. As you can see by looking at Figure 6, we have chosen to drive the Piezo element using port P1C. In most applications, the PIC will probably not be able to source enough current to power a load. When this is the case, a transistor can be placed in between the PIC output pin and the load in order to boost the current. For our setup, the 18F4520 can source up to 5 Figure 5: PWM Output pin locations on the PIC 18F4520 Figure 6: Our specific application of driving a Piezo buzzer with PWM 6 References 1) PWM – Wikipedia (*): http://en.wikipedia.org/wiki/Pulse-width_modulation 2) PIC18F4520 Datasheet (**): http://ww1.microchip.com/downloads/en/DeviceDoc/39637c.pdf 3) Piezo Buzzer Image Courtesy: http://www.piezoswitch.com/about_switches.html