Assignment 4: Photocell Bargraph

due Wednesday, Oct 16

Overview

Using provided sample code, write a program that repeatedly displays a converted analog reading as a bargraph of 0 to 15 asterisks.

How To

The photocell is wired in a “voltage divider” circuit as shown. The diagram on the left is the electrical schematic; on the right is a visual wiring guide.

The voltage divider generates an output voltage that is a function of the ratio of the two resistances in the legs of the converter. In this instance, the photocell is in the upper leg. Its resistance decreases with increasing light, causing a higher voltage result in this case.

Sample Code

The file analog2hex.s demonstrates how to initialize the HC11’s analog-to-digital converter and make a conversion. The core functionality in the following code snippet:

        bset  turn on A/Ds

loop:   ldaa #0                 ; select channel 0
        staa ADCTL              ; start conversion
donelp: ldaa ADCTL
        bpl donelp              ; if high bit set, it's done
        ldaa ADR1               ; grab result!

• demonstrate the use of subroutines. Make sure to initialize the stack pointer at the beginning of your code.
  
  
• repeatedly print the bar graph of 0 – 15 asterisks based on the high nybble of the converted value, with the linefeed and carriage return characters between each bar. You should see a sideways scrolling bar graph on your terminal emulator when it is working.