Servo - August 2010

FIGURE 3. I recently found the electronic equivalent that makes it a snap to build a circuit like this. The secret is to use the jumper cables shown here which are sold at SparkFun in combination with standard .100 pin headers.

to those used for the LCD as shown in Figure 2.

A VEX User Interface

Data entry can be either in decimal, floating point, or hexadecimal format since the keypad has 16 keys. In fact, we can edit the values by assigning the unused keys from the keypad as a backspace, insert or delete key, cursor position, or tab key. Our PIC18 C User Interface (UI) application uses the keypad scan routine to scan for the user’s keystrokes and uses the digits entered to build an ASCII representation of the motor speed entered. We’ll slightly alter the firmware from the June ‘ 10 article to work with the keypad.

This application also uses the WPILIB routines to drive the VEX motor to the selected speed which prevented me from using a modified version of printf to display data since the WPILIB could not be modified. Instead, I had to resort to making specific function calls to position the cursor and format the data for the LCD.

Although the example provided is very rudimentary, the reader can build on this to develop custom user interfaces using our source code as a starting point.

Some Assembly Required

In order to connect the keypad and LCD to the microcontroller, just follow the wiring diagram shown in the schematic in Figure 2. The motor is connected to motor block, pin 1, located on the microcontroller. Please note that two of the LCD signals (RS and E pins) have been relocated from IO7 and IO6 to IO10 and IO11 on the microcontroller (this is different from the schematic in the May issue). I had to change the LCD wiring to accommodate the keypad firmware. The 10K potentiometer controls the contrast to the LCD and should be adjusted for ambient lighting conditions.

TABLE 1. Bill of materials required to build the User Interface using a keypad and LCD. One component not shown in the schematic is a VEX motor. I used one to demonstrate how to change the motor speed using this interface.

ITEM QT Y DESCRIPTION

1

VEX Controller

2

1

4 x 4 Keypad

3

1

16 x 2 LCD Display

4

40

5

1

6

1

7

1

8

1

.100 Pin Headers

10K ohm Trim Potentiometer Wire-wrap Cable Package of Jumper Cables VEX Motor

SOURCE

Innovation First, Inc. www.vexforum.com RadioShack www.radioshack.com SparkFun www.sparkfun.com Digi-Key www.Digi-Key.com Digi-Key www.Digi-Key.com RadioShack www.radioshack.com SparkFun