检查引脚上的电压并打开LED

Question

I'm starting my way into PICs microcontrollers, and I want to start with simple things. I already tried reading a book, but it was more a C book than a microcontrollers book. I learned about ADCs, timers, memory banks, etc., but I didn't get some practical examples.

What are the basics steps to check for some voltage on a pin or turn on an LED ? I mean, I'm not requesting for all the code but a guide on how to do these basic tasks. Also, I know that the code depends on the microcontroller, so I have a 16F628A.

Answer 1

Controlling an LED like this is a great way to introduce yourself to microcontrollers and electronics. You'll learn how to read data sheets, how to perform low-level system configuration, and how to build some electronic circuits. But as you've probably seen already, even the simplest tasks require a bit of bit-twiddling and trial-and-error at first. The good news is that, once you understand the basics, you can apply the same techniques to entire classes of microcontollers, and you'll develop your own library of functions that will make it increasingly easier to build new projects.

---

The 16F628A has memory-mapped I/O , which means its internal registers control the behavior of its special-function pins.

According to the data sheet (PDF), the 28A has two analog comparators . Each comparator is connected to three pins on the 28A. Two pins are used for input: they're connected to the voltages you want to compare. The third pin is used for output: the comparator indicates which input voltage is higher by setting the voltage on the output pin high or low.

The 28A also has a voltage reference that's connected to another pin. This can generate a known voltage, and you can connect it to one of the comparator inputs. Connect your test voltage to the other comparator input, and use the output pin to drive the LED. Then the LED will turn on or off when your test voltage is higher than the reference. (The 28A is actually configurable: you can choose which condition will light the LED.)

The data sheet includes some assembly code that shows you how to configure the comparator by setting the appropriate bits in its control register.

You'll also need to find out whether the 28A can drive an LED directly. Some devices contain the appropriate current-limiting circuitry internally; others require you to provide it yourself.

You may have some luck with a web search for "16F628A application notes" (for manufacturer's suggestions) or just "16F628A application" (for apps created by users). This looks interesting, and the author offers to email his software to you. Maybe he'll offer some tips as well.

Good luck!

Answer 2

Actual code will depend slightly on the compiler you're using, and greatly on whether you want to use some wrapper library (I don't recommend it, as often the code to call the wrapper is longer than just setting the right registers).

In general, what you are wanting to do is called GPIO (general purpose input and output), and you need to do the following:

1. Make sure the GPIO peripheral clock is enabled. For PICs, there isn't a separate clock for GPIOs, so nothing to do here.
2. Configure the pins for their GPIO function (most pins have multiple purposes, and can be connected to SPI controllers, analog-to-digital-converters, etc.)
3. Write the initial value for output pins.
4. Configure the pin direction. (input or output)
5. Configure pull-up and pull-down registers, or lack thereof.
6. Configure interrupt-on-change. (if you want to use it)
7. Install the interrupt handler. (if you are using it) PICs place interrupt handlers at specific locations in memory, so nothing needed here.
8. Read and write the GPIO pins as your application requires.