The PIC18F4550 has four PWM output channels and they are P1A, P1B, P1C and P1D. All of them are capable of generating PWM waves at a time. In this project only one of the PWM channels are using. The P1A is the PWM channel in this particular project. This channel is used to generate the PWM waves which are then applied to a filter circuit to generate the sine wave which is described in a previous project on PIC Sine Wave Generation. In this project a driver circuit is designed to generate the sound of that audible sine wave in a Loud Speaker.
A simple example of the of waves generated at P1A pin is shown below;
The period of the wave is the sum of the ‘ON time + OFF time’. Duty-cycle is the percentage of time period for which the logic1 voltage exists in a cycle (ON time), starting from the beginning of the cycle.
The PWM is that kind of a wave in which the ON time and OFF time can vary in a cycle but the sum of ‘ON time + OFF time’ remains constant for every cycle.
Period = ON time + OFF time
Duty-cycle = ON time / (ON time + OFF time) = ON time / Period
Increasing the Duty-cycle will increase the voltage at the filter device’s output and decreasing the Duty-cycle will decrease the voltage as well.
In this particular project the sine wave samples are generated periodically by re-writing the value of the CCPR1 register to vary the Duty-cycle. It is done by generating interrupts periodically with another timer module timer0 and changing the CCPR1 value when the code is inside the timer0’s ISR. It is done as shown in the following figure;
The voltage generated by the PWM wave in the interval between two interrupts will be a constant value and this time period can be called ‘sampling period’.
The values of voltage that appears at each sampling period are simply called ‘samples’. The more the number of PWM cycles in a Sampling time, more stable the output voltage will be an example of the sine wave samples is shown following figure in which 10 samples are used to resemble a sine wave. These values when applied to a filter circuit can generate the sine wave at its output by smoothing the step size. The brown line shows the actual sine wave constructed by the filter circuit.
The values that should be assigned to the CCPR1 register to generate such consecutive samples are actually taken from a look-up table. The look-up table with 50 samples which is used in this particular project to generate the sine wave is shown in the following;
Sample no. | Value | Sample no. | Value | Sample no. | Value | Sample no. | Value | Sample no. | value |
1 | 52 | 11 | 85 | 21 | 69 | 31 | 28 | 41 | 16 |
2 | 57 | 12 | 86 | 22 | 65 | 32 | 25 | 42 | 18 |
3 | 62 | 13 | 86 | 23 | 61 | 33 | 22 | 43 | 20 |
4 | 66 | 14 | 86 | 24 | 56 | 34 | 19 | 44 | 23 |
5 | 70 | 15 | 85 | 25 | 52 | 35 | 17 | 45 | 26 |
6 | 74 | 16 | 83 | 26 | 48 | 36 | 15 | 46 | 39 |
7 | 77 | 17 | 81 | 27 | 44 | 37 | 14 | 47 | 34 |
8 | 80 | 18 | 78 | 28 | 39 | 38 | 14 | 48 | 38 |
9 | 82 | 19 | 75 | 29 | 35 | 39 | 14 | 49 | 43 |
10 | 84 | 20 | 72 | 30 | 31 | 40 | 15 | 50 | 48 |
Time period calculations:
In this section the calculations for the sampling time, PWM Period, PWM Duty-Cycle, frequency of the sine-wave etc. are calculated and the details of the calculations are available in a previous project on PIC Sine Wave Generation
Sampling time:
In this project the TMR0 is set to zero and the timer0 is configured as an 8 bit timer with pre-scale value 1:2 which gives a sampling time;
Sampling time = 40us
PWM period:
In this particular project the PR2 is written to a very small value so as to generate small time periods and hence to get more number of PWM cycles per sampling period.
PR2 = 22
The PWM Period = 2us
Number of PWM cycles per Sample = 20
Thus the timer0 will generate an interrupt after every 20 PWM cycles.
The frequency of the sine wave = 500 Hz
PWM Duty-cycle:
The Duty-cycle in this particular project is varied according to the look-up table whenever a timer0 interrupt occurs. The maximum Duty-cycle (100%) is 2us only since it is the value of the PWM period, hence
The CCPR1 can be written with any value between 0 and 100
The filter design:
The following figure shows a microcontroller generating PWM wave which is then used to generate the corresponding analog voltage with the help of a filter circuit
In this particular project the filter circuit is actually an integrator made with a single capacitor. The filter simply integrates the duty cycle of each PWM cycles and hence averages out the voltage in a PWM wave.The integrator is a circuit which has a resistor and a capacitor in series connected across the input and the ground and the analog voltage is obtained across the capacitor as shown in the following figure;
RC >= PWM Period
If the value for the C is taken as 0.1uF and the value of R is taken as 20 ohms, which will make an integrator circuit, required to generate the voltage equivalent of the PWM wave having a Period of 2us.
The driver circuit design:
The devices like LED can be directly driven by the PWM pin of the microcontroller, but when it comes to high power consuming devices like Loud Speaker or DC Motor etc. a specially designed driver circuit is necessary due to the following reasons.
{C}{C}{C}{C}{C}· The microcontroller is not able to source the required current
{C}{C}{C}{C}{C}· High current flow to the load attached to the PWM pin can cause internal drop of voltage inside the I/O pin and hence the PWM voltage level varies.
{C}{C}{C}{C}{C}· The filter circuit may not be able to generate the required voltage in such situations.
{C}{C}{C}{C}{C}· Making the microcontroller to source that much current may damage the microcontroller permanently.
{C}{C}{C}{C}{C}· In short the Load should get enough current and voltage without affecting the functioning of the filter circuit or the microcontroller.
The driver circuit itself consumes some current and hence the current flowing through the driver circuit from the PWM pin should also be limited. This can be done by connecting a high value series resistance with the driver circuit.
The following block diagram shows the arrangement of the filter circuit, driver circuit and the load.
In this particular project the Darlington pair using two NPN transistors is used. The Darlington Pair can source very high current to the load device like relays, motors, loud speaker etc. however consuming very small amount of current from the input device. Hence this circuit is ideal for this kind of applications.
The resistor R determines how much current that should be flow into the Darlington pair. The resistor in this case is selected to be 100K so as to nullify the loading effect on the PWM filter or PWM pin by the driver circuit and the Load. The circuit using the NPN transistor as Darlington pair with an input current limiting resistor is shown in the following diagram.
The filter takes the PWM input from the microcontroller PWM pin and there it converts the modulated waveform to its equivalent voltage. In this particular project the output of the filter circuit is a sine wave. The sine wave is then send to the driver circuit consist of Darlington pair through a 100K resistor so as not to load the filter circuit by the driver. Now the driver source enough current to the loud speaker corresponding to the low current sine wave it is receiving.
The sine wave is in audible range and hence the loud speaker normally 8 ohm can generate the sound equivalent to the sine wave frequency.The circuit:
The circuit operates on well regulated 5V supply with a 4MHz external crystal oscillator which forms 48MHz CPU clock with the help of an internal PLL. The standard 0.1uF capacitor is used as the filter circuit (RC integrator) to generate the voltage equivalent of the PWM. The capacitor is connected across the PWM pin (PIN 17) of the microcontroller and the ground. A Darlington pair is used to drive the loud speaker which can generate the sound equivalent of the sine wave frequency. The Darlington pair itself is connected to the filter circuit through a 100K resistor so as not to load the filter by the Darlington pair. The probe from the CRO can be connected across the ground and the PWM pin of the microcontroller incase if need to watch the sine wave.
Tips for better results:
{C}{C}{C}{C}{C}· 1.{C}{C}{C}{C}{C}Always use a well regulated power supply, since very small ripples in the power supply can induce noise in the PWM wave generated by the microcontroller and hence affects their voltage generation at the filter circuit.
{C}{C}{C}{C}{C}· 2.{C}{C}{C}{C}{C}Use driver circuits as per the load requirement so as not to load or distort the PWM waveform
{C}{C}{C}{C}{C}· 3.{C}{C}{C}{C}{C}The period should not be less than which the load is not able to respond when using no filter circuits
{C}{C}{C}{C}{C}· 4.{C}{C}{C}{C}{C}The Duty-cycle should be select in such a way that it will generate the voltage in the range at which the load device can operate.
{C}{C}{C}{C}{C}· 5.{C}{C}{C}{C}{C}The filter circuit’s design should be precise so as to give maximum performance at the generated PWM Period and Duty-cycle.
{C}{C}{C}{C}{C}· 6.{C}{C}{C}{C}{C}Always operate the microcontroller at its maximum possible frequency so as to get maximum number of PWM cycles at any Sampling period, which will give a more stable voltage with the filter circuit or load.
{C}{C}{C}{C}{C}· 7.{C}{C}{C}{C}{C}Use more number of samples per sine wave (50 to 200 samples per sine wave cycle) to get better results
{C}{C}{C}{C}{C}· 8.{C}{C}{C}{C}{C}Use general purpose or audio frequency transistors having high current ratings for Darlington pair.
{C}{C}{C}{C}{C}· 9.{C}{C}{C}{C}{C}The loudness can be varied by varying the resistance which connects the filter circuit with the Darlington pair.
{C}{C}{C}{C}{C}· 10.{C}{C}{C}{C}{C}A variable resistor can be used as volume controller as decreasing the resistance will increase the volume and vice versa