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PIC MCU INPUT AND OUTPUT DEVICE INTERFACING
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Data = 0; for (i = 0; i < 6; i++) { Clock = 1; Clock = 0; } // rof Data = 1; Clock = 1; Clock = 0; Data = RS; Clock = 1; Clock = 0; for (i = 0; i < 4; i++) { if ((Nybble & 0x008) != 0) Data = 1; else Data = 0; Clock = 1; Clock = 0; Nybble = Nybble << 1; } // rof Clock = 1; Clock = 0; } // End LCDNybble
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Clear the 174 Repeat for six bits Write the 0 s into the 174
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Output the AND Value
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Output the RS Bit Value
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// Output the Nybble // Output the High Order Bit
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// Strobe the Clock // Shift up Nybble for Next Byte
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// Toggle the E Clock Bit
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Analog I/O
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In the following sections I want to introduce you to some of the practical aspects of working with analog data with the PIC microcontroller. This includes position sensing using potentiometers. At rst glance, you may think that an ADC-equipped microcontroller is required for this operation, but there are a number of ways of doing this with strictly digital inputs. Along with discussing how it can be done with ADCless microcontrollers, I ll also show how the IBM PC carries out the function (it doesn t use an ADC either). For analog output, I will focus on the theory and operation behind PWM analog control signals. This method of control is very popular and is a relatively simple way to provide analog control of a device. It also can be used for communication of analog values between devices without needing any type of communication protocol. The PIC microcontroller has some built-in hardware that makes the implementation of PWM input and output quite easy to do. While I discuss audio I/O, I want to make it clear that audio input and output capabilities cannot be provided in the PIC microcontroller without signi cant front-end signal processing and ltering. Output from the PIC microcontroller is limited to simple beeps without special hardware.
ANALOG I/O
PIC Micro
ADC Input Pin
Figure 17.18 A potentiometer wired as a voltage divider can be read using the PIC microcontroller s ADC input.
READING POTENTIOMETERS
One of the more useful human input devices is the dial. Rather than relying on some kind of digital data such as a button or character string, the dial allows users a freer range of inputs, as well as positional feedback information, in a mechanical device. For most people, reading a potentiometer value requires setting the potentiometer as a voltage divider and reading the voltage between the two extremes at the wiper, as shown in Fig. 17.18. However, there is a very elegant way of reading a potentiometer s position using the digital input of a PIC microcontroller, as I will show in this section. Note that I consider the measurement to be of the potentiometer s position, not its resistance. This is an important semantic point; as far as using a potentiometer as an input device is concerned, I do not care what the actual resistance is of its position, just what the position is. The method of reading a potentiometer s position using a digital I/O pin that I am going to show you depends very much on the parts used and will vary significantly between implementations. The method of reading a potentiometer uses the characteristics of a charged capacitor discharging through a resistor. If the charge is constant in the capacitor, then the time to discharge varies according to the exponential curve shown in Fig. 17.19.
Vcc Vcc RC Waveform C
Vrc = Vcc * (1 e t/RC )
Figure 17.19 A capacitor s charge-decay response can be modi ed by a potentiometer, and the time taken can be measured by a PIC microcontroller.
PIC MCU INPUT AND OUTPUT DEVICE INTERFACING
The charge in a capacitor is proportional to its voltage. If a constant voltage (i.e., from a PIC microcontroller I/O pin) can be applied to a capacitor, then its charge will be constant. This means that in the voltage-discharge curve shown in Fig. 17.19, if the initial voltage is known, along with the capacitance and resistance, then the voltage at any point in time can be predicted. The equation in the gure, i.e.,
V(t) = VStart(1 - e ** - t/RC)
can be reworked to nd R if V, Vstart, t, and C are known:
R = -t/C * ln((VStart - V)/ VStart)
Rather than calculate the value, though, you can make the approximation of 2 ms for a resistance of 10 k and a capacitance of 0.1 F with a PIC microcontroller that has a high-to-low threshold of 1.5 V. To measure the resistance in a PIC microcontroller, I use the circuit shown in Fig. 17.20. In this circuit, the PIC microcontroller s I/O pin outputs a high that charges the capacitor (with some leakage through the potentiometer resistor). After the capacitor is charged, the pin is changed to input, and the charge in the capacitor draws through the resistor with a voltage determined by the V(t) formula. When the pin rst changes state, the voltage across the resistor will be greater than the threshold for some period of time. When the voltage across the potentiometer falls below the voltage threshold, the input pin value returned to the software will be 0. If the time required for voltage across the pin to go from a 1 to a 0 is recorded, it will be proportional to the resistance between the potentiometer s wiper and the capacitor. The pseudocode for carrying out the potentiometer read is
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