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PIC MICROCONTROLLER APPLICATION BASICS
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Figure 20.16 The MPLAB IDE program memory dump showing the two instructions of the application at the end of program memory.
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Analog Input/Output
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When I took electromagnetics and transmission theory in university, there was nothing I wished for more than the world to consist of nothing but ones and zeros. Unfortunately, the world just doesn t work that way varying voltage levels are available inside the PIC microcontroller s Vdd and Vss voltage range to outside in terms of positive and negative voltages. In addition, the different speeds and waveforms of these voltages can make your life more dif cult as well. In the following experiments I want to demonstrate to you some practical examples of how analog I/O interfacing with the PIC microcontroller can be accomplished using the digital I/O functions of the PIC microcontroller to perform the basic tasks of converting an analog resistance to a digital value and a digital value to an anolog output voltage.
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ADCLess: MEASURING RESISTANCE VALUES WITHOUT AN ADC
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For measuring resistance values without an ADC, a simple RC network can be used with the PIC microcontroller, as shown in Fig. 20.17. This method has been taken from the Parallax Basic Stamp 2 and is quite easy to do and works quite well, although there are a few concerns that have to be worked through for this method to be used in an application.
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ANALOG INPUT/OUTPUT
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Vcc Vcc
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0.1 uF Tantalum
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0.1 uF Tantalum Gnd
RA0 220 10 K
10 K
4 16
_MCLR Osc 1 Osc 2 Vss
4 MHz
PORTB
Figure 20.17 Using the PIC microcontroller s digital I/O pins to read a resistor value.
To measure the resistance (assuming that the capacitor is of a known value), the PIC microcontroller rst charges the capacitor to 5 V (or its nominal output) using the I/O pin in output mode. Once this is done, the pin changes to input mode and waits for the capacitor to discharge through the potentiometer. Looking at this operation on an oscilloscope, the waveform produced by the circuit looks like Fig. 20.18. In this gure, the charge and discharge cycles can be seen clearly. From basic electronic theory, we know that the time required for the capacitor to charge is Time R C ln(Vend/Vstart)
where Vstart and Vend are the starting and ending voltages that we are interested in. For the PIC microcontroller, we would be interested in the capacitor voltage starting at Vdd (after being charged by the PIC microcontroller to 5 V) and then waiting for the capacitor to discharge to the input transition point (2.5 V in the PIC microcontroller). Because we know the capacitor value along with the voltages and the time it took for the capacitor to discharge, we can rearrange the preceding formula to nd R: R time/[C ln(Vend/Vstart)]
Therefore, by controlling the voltage applied to the network and knowing the value of the capacitor, we can determine the value of the resistor. The code used to test the analog I/O uses the following logic:
int PotRead() { int i; // Read the Resistance at the I/O Pin
PIC MICROCONTROLLER APPLICATION BASICS
Discharge Cycle
Charge Cycle
1) Ch 1
5 Volt
10 us
Figure 20.18 The charge/discharge waveform of charging the capacitor and measuring the discharge time.
TRIS.Pin = Output; // Set the Output Mode Pin = 1; // Output a 1 to Charge the Capacitor for (i = 0; i < 5 sec, i++ ); TRIS.Pin = Input; // Now, Time How Long it Takes for the TMR0 = 0; // the Capacitor to Discharge through while (Pin == 1); // the Potentiometer return TMR0; // // Return the TMR0 Value for the Discharge Time
end PotRead
This code is unique in that no RAM registers are used for the timing; this is done totally within the PIC microcontroller hardware. TMR0 does not have to be used; instead, a simple counter within the while (Pin == 1) loop could be incremented. The source for this experiment is ADCLess.asm and can be found in the code\ADCLess folder.
title ADCLess - Reading a Resistor Value without an ADC ; ; This Program copies the RCTIME instruction of the Parallax Stamp.
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