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Binary input and Output: reading a Switch and turning On an Led if the Switch iS cLOSed
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In any real-world situation, you ll be reading inputs either as discrete single-line inputs or as resistances, frequencies, pulse widths, or similar signals. In this chapter, we will consider reading one external input as being either high or low. Because we need to know whether we have actually read the input successfully, we will turn an LED on if and only if the input is read as being low. We will define one pin as an input for the input signal and one pin as an output to turn on the LED. All the work in this program is being done in one cog, although in a parallel-processing environment we could assign the input to one cog and the output to another, as we will do in many of the following programs. First, let s take a look at the program and then we will discuss what each part of the program is doing. For our purposes, the signal will go between a logical 0 and logical 1 at the Propeller input pin. For the Propeller with its CMOS circuitry, anything over half the 3.3 DC volts power (or 1.65 volts) is a seen as a high. The input impedance of the lines in the input state is so high that they tend to float and will drift between 0 and 1 if they are not connected to anything (when defined as inputs). You can observe this if you run a finger close to a free input line with a wire sticking in the air like an antenna. The stray capacitance (usually a positive charge) of your body provides enough current to switch the line to a high condition as you approach it. For us, this means that all input lines have to be tied high or low, with a high resistance when we use them as inputs (or defined as outputs and then made high or low). If you are going to pull the line low in your circuit to read it, tie it high with a 1 meg resistor, and if you are going to pull the line high to read it, tie it to ground with a 1 meg resistor. Keeping all this in mind, Figure 15-1 shows the circuit for reading a line that is connected to a switch that grounds it.
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figure 15-1 Switch controls LED on line P27 by pulling line P23 low.
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note The power to drive the LED comes from the power to the 7404, not the Propeller. The 7404 inverts each signal it processes.
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In Figure 15-1, we are using a buffer in the 7404 to drive the LED. We are using a 1 meg resistor to pull line P23 up to 5 volts. The current to the LED is limited by the 470 ohm resistor. The switch grounds line P23. This is pin 28 on the chip. The Spin language uses the internal numbers, never the external chip pin numbers. The code that runs the circuitry in Figure 15-1 is listed in Program 15-1. After you have studied it, we can discuss it.
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program 15-1 Reading a Switch and Turning an LED on while the Switch Is Down
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{{12 Sep 09 Harprit Sandhu ButtonLED.spin Propeller Tool 1.2.6 15 Program 1 This program turns an LED ON if an input that has been pulled up is grounded It demonstrates the use of subroutines in an absolutely minimal way. First define the constants we will use. }} CON _CLKMODE=XTAL1+ PLL2X _XINFREQ = 5_000_000
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'The system clock spec (continued)
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diScuSSiOn
program 15-1 (continued)
Reading a Switch and Turning an LED on while the Switch Is Down
input_pin output_pin inv_high inv_low
=23 =27 =0 =1
'select 'select 'define 'define
the the the the
pin to be used for input pin to be used for the LED inverted High state inverted Low state
'High is defined as 0 and low is defined as a 1 because we are using an 'inverting buffer on the output PUB Go dira [output_pin]~~ 'sets pin to an output line with ~~ notation outa [output_pin]~ 'makes the pin a low output dira [input_pin]~ 'sets pin to an input line with the ~ notation repeat 'repeat forever because there is no number after repeat if ina[input_pin]==0 'check pin for high or low turnOn_LED 'subroutine call else turnOff_LED 'subroutine call PRI turnOn_LED 'subroutine to set the LED line high outa[output_pin] :=inv_high 'line that actually sets the LED high PRI turnOff_LED outa[output_pin] :=inv_low 'subroutine to set the LED line low 'line that actually sets the LED low
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