Ramsey Electronics COMPUTEMP CT255 Manuel d'instructions télécharger pdf (Page 5)

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CIRCUIT DESCRIPTION

In the old days, we used to make things as simple as possible. The original

blinky used two transistors, two capacitors and four resistors. This simple circuit

is now seen everywhere in blinky kits so we decided to use a new and novel

approach with this one. Our new blinky kit requires over 20,000 transistors,

three capacitors, five resistors, and a few other parts. The difference is we can

make the LED do anything we want, well almost anything.

The part which does most of the work is an 8-pin PIC micro-controller chip.

Frankly these little things are amazing. They are perfectly suited to perform

small tasks like this one. They are fast, easy to program, and require almost no

external components to make them function. The PIC controller is programmed

to pulse width modulate each of the 3 colors in various duty cycles to achieve

the variety of colors we are looking for. What to we mean by pulse width modu-

late?

Pulse width modulation means that we are changing the percentage of on time

vs. off time in a known amount to achieve a specific brightness. In our case we

are pulsing width modulating with a resolution of 256 steps for each color. This

means we have a resolution of 256*256*256 = 16,777,216 colors! We may not

actually achieve this number due to limitations on switching speed, but we’ll still

get more than your eye can discern.

Now, let’s say we want the red LED at 50% of full brightness. The red element

needs to turn on and off in equal amounts fast enough that our eye cannot see

it blink. How is this done in code? If you are familiar with PIC, here is a snippet:

toploop: ;Make the RED led turn on at 50% brightness.

clrf pace ;For the full 256 counts

movlw 0x7F ;50%, leave red on for this many counts of 256

movf red ;Into the red register

bsf GPIO,RED ;Turn on the Red LED

pwmloop:

decfsz red,F ;Decrement the red register

goto reds ;If red not at zero, leave the Red LED on

bcf GPIO,RED ;If red at zero, turn the Red LED off

reds: decfsz pace,F ;Decrement the pace

goto pwmloop ;If not at zero, keep looping until 256 counts done.

goto toploop ;Continue on, or loop to toploop

Ok, so you may not have done much assembly programming before. This is a

real neat example to help you learn what assembly code can do. The best way

to read this code is to look at the text after the semicolons (here is where the

descriptive information resides). The text on the far left are labels that identify

points in the program flow where you may whish the program to start from or

jump to (remember Basic’s goto 10? 10 was a label for a line).

Ramsey Electronics COMPUTEMP CT255 Manuel d'instructions Page 5