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PIC MICROCONTROLLER APPLICATION BASICS
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One last thing to notice about this circuit is the use of FSR to point to TRISA (for the switching between output and input in the A/D function). FSR is loaded with the address of TRISA (0x85), and using INDF, TRISA is accessed directly without having to change the default bank (the usual bsf STATUS, RP0 instruction). This is possible because the FSR register provides up to 8 bits of addressing, which eliminates the need for the RP0 bit when it is addressing registers. This method of using FSR may not be advisable in all applications because dedicating it to an individual function limits its usefulness elsewhere in the application.
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VLadder: RESISTOR LADDER OUTPUT
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While a single reference voltage produced by a voltage divider may be useful for some applications, a variable-voltage output is much more useful for many other applications. In this experiment, I want to show how multiple analog voltages (approximately 0.55 V apart) can be produced by the PIC microcontroller using the digital I/O pins with a voltage divider. The output voltage is determined from a voltage divider that has the formula Vout Vcc [Rn/(Rs Rn)]
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where Rn is the resistance between the tap and Vcc and Rs is the resistance between the tap and ground. A variable resistance voltage divider can be implemented on the PIC as a resistor ladder such as the Vref circuit of the 16C62x devices. The circuit used in this application is shown in Fig. 20.20, and the bill of materials is listed in Table 20.10. Depending
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16F84
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0.1 uF Tantalum Gnd
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4.7 K
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Analog Voltage Output
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10 K
4 16
2.2 K
RB2 _MCLR Osc 1 Osc 2 Vss
2.2 K
3.3 K
4 MHz
3.3K
4.7 K
10 K
Schematic for VLadder.
ANALOG INPUT/OUTPUT
TABLE 20.10 BILL OF MATERIALS FOR THE VLadder EXPERIMENT PART DESCRIPTION
PIC 0.1- F 10-k 4-MHz 4.7-k 1-k 2.2-k 3.3-k Misc.
PIC16F84 04/P 0.1- F tantalum 10 k, 1/4 W resistor 4 MHz with built-in capacitors 4.7 k, 1/4 W resistor 1 k, 1/4 W resistor 2.2 k, 1/4 W resistor 3.3 k, 1/4 W resistor Breadboard, wiring, +5-V power supply
on which PORTB output is active, the Rs resistance can be varied to change the output voltage of the circuit. The Rs resistance is varied by changing the PORTB I/O pin to active. All the port pins have been loaded with a 0, and when one of the them is put into output mode, the circuit is grounded at this point, with the resistance being all the resistors between it and the analog voltage output. Wiring this circuit is actually quite a challenge, and I found that when I redid it for the second edition, I had a number of dif culties getting it right (which will be discussed next). For the breadboard version of the application, the wiring is shown in Fig. 20.21.
+5 Volt P/S
1K 2.2 K 2.2 K 4.7 K 10 K
3.3 K 3.3 K 4.7 K 10 K 0.1 uF 4 MHz
Breadboard wiring for VLadder experiment.
PIC16F84
Analog Output
PIC MICROCONTROLLER APPLICATION BASICS
Getting the wiring right is not trivial, and in many ways, this was the most dif cult application for which to specify the wiring so as to keep it simple enough for you to follow and build your own circuit. When I created this experiment originally (for the rst edition), I rearranged the preceding formula to Rn (Vout/Vcc) Rs/[1 (Vout/Vcc)]
By choosing a value for Rs, (say, 10 k ), we can easily calculate the value of Rn for a given Vout. The reason why I am going to all this trouble to come up with a formula and calculate it out is to ensure that I can get reasonable linearity in Vout for different bit outputs. With the resistor ladder connected to port B, we can have nine different voltages. In selecting these voltages, I have tried to space them 0.55 V evenly apart. With the circuit shown in Fig. 20.21, you can get nine different voltages very simply. The rst is when all the output bits are turned off, which leaves only a pull-up connected to the analog voltage output for a maximum voltage output. If RB0 is in output mode and pulling the output line to ground, this is the low value for the application. Rs values can be added to the circuit by outputting a zero (low voltage) on a pin (terminating the resistor ladder at that point). The remaining seven voltages are selected by grounding intermediate resistors in the ladder using this method. One very important thing to remember is the minimal current (load) output drive characteristics of this circuit. A resistor ladder such as this is very poor at maintaining the output voltage if it has to source or drive current. If there is any type of tangible current ow (more than a few microamperes) outside the voltage divider, the output voltage will be changed from what you expect. The best way to avoid this is to buffer the resistor ladder output using an op-amp, as shown in Fig. 20.22. When I created this circuit originally, I calculated speci c values so that the output would be linear. The same value could be used for each resistor in the ladder, but if we were to plug these into the PIC microcontroller, we would nd that we would get the voltage output shown in Fig. 20.23. In the graph shown in this gure, I have plotted what I would consider to be the ideal voltage ramp.
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