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When I discuss the PIC microcontroller s processing capabilities with regard to audio, I tend to be quite disparaging. The reason for this is the lack of hardware multipliers in low-end and mid-range PIC microcontrollers and the inability of all the devices to natively handle greater than 8 bits in a oating-point format. The PIC microcontroller processor has been optimized for responding to digital inputs and cannot implement the real-time processing routines needed for complex analog I/O. Despite this, you can implement some surprisingly sophisticated audio output that goes beyond simple beeps and boops using a circuit such as the one shown in Fig. 17.23.
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Vcc 8 Ohm Speaker 15 Ohm Piezo Buzzer 0.47 uF
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Figure 17.23 The PIC microcontroller can drive a simple piezo speaker.
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250 us 250 us
Figure 17.24 you expect.
The voltage waveform across the speaker is probably not what
This passes dc waveforms through the capacitor (which lters out the kickback spikes) to the speaker or piezo buzzer. When a tone is output, your ear will hear a reasonably good tone, but if you were to look at the actual signal on an oscilloscope, you would see the waveform shown in Fig. 17.24 both from the PIC microcontroller s I/O pin and from the piezo buzzer itself. The PIC microcontroller pin, capacitor, and speaker are actually quite a complex analog circuit. Note that the voltage output on the PIC microcontroller s I/O pin is changed from a straight waveform. This is due to the inductive effects of the piezo buffer. The important thing to note in this gure is that the upward spikes appear at the correct period for the output signal. Timing the output signal generally is accomplished by toggling an output pin at a set period within the TMR0 interrupt handler or using the CCP module to produce a PWM tone. To generate a 1-kHz signal in a PIC microcontroller running at 4 MHz, you can use the following code (which does not use the prescaler) for TMR0 and the PIC microcontroller s interrupt capability:
org int: movwf bcf movlw movwf 4 _w ; Save Context Registers INTCON, TOIF ; Reset the Interrupt 256 - (250 - 4) TMRO ; Reset TMR0 for another 500
secs
PIC MCU INPUT AND OUTPUT DEVICE INTERFACING
btfsc goto bsf goto bcf swapf swapf ret e
SPKR $ + 2 SPKER $ + 2 SPKER _w, f _w, w
; ; ; ;
Toggle the Speaker Speaker Output High Speaker Output Low Restore Context Registers
There are two points to notice about this interrupt handler: The rst is that I don t bother saving the STATUS register s contents because neither the zero, carry, nor digit carry ags are changed by any of the instructions used in the handler. The second point to notice is the reload value of TMR0 to generate a 1-kHz output in a 4-MHz PIC microcontroller (an instruction clock period of 1 s); I have to delay 500 cycles for the wave s high and low. Because TMR0 has a divide by two counts on its input, I have to wait a total of 250 ticks. When I record TMR0, note that I also take into account the cycles taken to get to the reload (which is 7 or 8), divide them by 2, and take them away from the reload value. For this handler, the reload value may be off by one cycle depending on how the mainline executes, for a worst-case error of 0.2 percent, or 2,000 ppm. This level of accuracy is approximately the same as what you would get for a ceramic resonator; and the change in the frequency should not cause a noticeable warbling (changes in the frequency) of the output. When developing applications that output audio signals, I try to keep the tone within the range of 500 Hz to 2 kHz. This is well within the range of human hearing and is quite easy to create the software for. When you look at the Christmas tree in Chap. 21, you can see how this is done for creating simple tunes on the PIC microcontroller.
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