.net barcode scanner sdk Our First Program in Software

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Our First Program
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Install a 7404 on the education kit board and wire in one LED, as was shown in Figure 3-3 (refer to 3). Before we get into the intricacies of the Spin language, let s write a short program that is as simple as we can make it. In this program we are going to blink an LED fed from one line of a 7404 buffer. At the same time, we will vary the resistance in the line feeding the 7404 buffer to determine the value of a suitable in line resistance that the circuit will tolerate and still operate reliably. We could calculate this value, but we will instead do it using the trial-and-error method. We know that the 7404 buffer has a high impedance input. This means that it looks like a high resistance device to the Propeller. Even so, we will add more resistance to this line to further figure 4-1 Circuitry between limit the current drawn from the Propeller (see the output line of the Propeller Figure 4-1). and LED
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Software Setup: the propeller tool environment
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figure 4-2
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Photo of Propeller: 7404 and LED
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note Adding this resistance will slow down the operation of the gate ever so slightly, but we may not be able to detect the change with the instruments at our disposal.
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The program we are about to consider blinks an LED approximately once a second a half second on and a half second off for each complete cycle with the hardware extensions shown in Figure 4-2. We do not have to go through a buffer, but doing it this way limits the current load on the Propeller. Program 4-1 appears in the Parallax font here. This font will be used for all programs developed and listed in this book to differentiate the program listings from the general book verbiage.
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program 4-1 Blinking an LED about Once a Second
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Re-listed {{12 Sep 09 Harprit Sandhu BlinkLED.spin Propeller Tool Ver. 1.2.6 4 Program 1 This program turns an LED ON and OFF, with a programmable delay It demonstrates the use of methods in an absolutely minimal way. The clock is 10 MHz (continued)
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our firSt program
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program 4-1
Blinking an LED about Once a Second (continued)
Define the constants we will use. }} CON _CLKMODE=XTAL1 + PLL2X _XINFREQ = 5_000_000 inv_high inv_low waitPeriod output_pin =0 =1 =5_000_000 =27
'The system clock spec 'the crystal frequency 'define the inverted High state 'define the inverted Low state 'about 1/2 sec switch cycle 'line the led is on
'High is defined as 0 and low is defined as a 1 because we are using an 'inverting buffer on the Propeller output PUB Go dira [output_pin]~~ outa [output_pin]~~ repeat turnOff_LED wait turnOn_LED wait
'sets pin to an output line with ~~ 'makes the pin high 'repeat forever, no number after repeat 'method call 'method call 'method call 'method call
PRI turnOn_LED 'method 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 PRI wait waitCnt(waitPeriod + cnt) 'method to set the LED line low 'line that actually sets the LED low 'delay method 'delay is specified by the waitPeriod
noteS on the firSt program
Make note of the following items in Program 4-1, starting from the top and moving down:
If we do not specify a frequency as such for the system, the crystal frequency is
multiplied by two as specified in the first two lines of the program.
We are using a 5 MHz external crystal and a 2 multiplier for a 10 MHz operation. All the constants are defined up front on top of the program. This makes it easy to
make changes to the constants.
The direction of the pin being used has to be specified. This is done with the ~~
notation for output and ~ for input in the dira command.
We are using a wait period of freq/1000 500 (or 5,000,000) cycles (half a second).
Software Setup: the propeller tool environment
Methods are used to do the following:
Turn the LED on. Turn the LED off. Create the wait period.
(If an LCD is attached, which should not be the case at this time, its contents will not be cleared or affected by this program.)
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