PHY 406 - Microprocessor Interfacing Techniques

Module 3 - Digital Input and Output

Objective

Preparation

Functions>>Data Acquisition>>Digital I/O>>Read From Digital Port.vi

Functions>>Data Acquisition>>Digital I/O>>Write To Digital Port.vi

Make sure (by asking if necessary) that you understand the circuit diagram for the keypad and display interface keypad.pdf

This module will also tax your knowledge of LabVIEW programming. Two very useful concepts that you should be sure you understand are the use of "sub-VIs" to encapsulate code so that you don't have to program it again or have it cluttering up your screen, and the concept of "constant arrays" which allow you to deal with "if the number I get is A, then the result I want is B" when there is no simple mathematical relationship (or there is, but it's a pain to compute) between A and B

Equipment

There has been one problem with a computer failing to run the interface because of a fuse failure in the interface. If it doesn't seem to be working at all - no lights, no numbers - try running the test program to see if it is the interface or your program

The interface consists of a single digit LED display which has seven segments (hence it is often called a "seven-segment" display) and a decimal point each of which can be individually controlled from single bits of the PB port which is an output port. There is a also a 4x4 keypad which is scanned by four bits of the PB port and produces results in the lower four bits of the PA input port. The upper 4 bits of that port are always 0.

Your mission, Jim, should you decide to accept it, is to make this interface work in a logical manner.

Digital I/O

• The demonstration VI e:\l_view\examples\keyboard.vi gives you the port numbers to use with the interface. Why is the shift register used? The output function of the VI activates the lines PB0-7 with a bit pattern which is supplied from the control Output Pattern. Notice the use of a binary display to make things easier. Make a simple VI to display a number from 0-20 on the LED display when you enter it from the computer keyboard (not the keypad beside the LED display). You will have to get inventive to display numbers >9 with a single digit, but you may invent you own patterns for those.
• Input is more of a problem. You will need to "scan" the keypad by raising each of the lines PB4-7 high in turn and then "sampling" the input on PA0-3. (e.g. you can raise PB5 by sending the bit pattern 00100000). The bit inputs will be 0 unless the corresponding key of the matrix is depressed, in which case you will get a "1" in the corresponding location. Write a VI to display the number of the key pressed on the computer screen. You should ensure that an invalid keypress (such as two keys pressed at once) is detected and some sort of alarm condition activated until things clear up.
• Make your two Vis into sub-VIs (see the tutorial) and use them in a VI to take the number from the keypad and put it in the LED display. This will be a problem because the display lines are used for both purposes. Things are therefore not perfect. Use your ingenuity - and some modified timing - to produce the best visual result in the display.

Advanced

• One of the problems that keyboards suffer is "bounce". It isn't visible on your system because the sampling with the VI (which you will remember is of the order of mS) is too slow. How would you defend against "bounce" where a key repeated connects and disconnects before settling to the new state?

Notes