ELE538 Quiz/Answers (2004)

Name: _______________ Student #: _____________ Time: 30 minutes

Chun,Clowes,Guerkov

Table of Contents

Reference Material
Instructions
A/D system
Questions


Answer all questions.  All questions have equal weight.
  

Reference Material

This material contains technical details that may be required to answer certain questions.

Instructions

Table 1. Instruction Details (Abridged)

AssemblerModeEncodingCycles
ldaaIMM86 ii2
ldabIMMC6 ii2
mulINH3D10

A/D system

The bits in the Control/Status register (ADCTL, mapped to address 0x1030) are:

Figure 1. AD Control/Status Register


       7   6   5   4   3   2   1   0
       CCF  -   M*  S   0   n   n   n
        

The interpretation of the bits is:

CCF

0: conversion NOT complete; 1: conversion complete.

M*

0: Convert 4 channels; 1: Convert single channel.

S

0: continuous conversion; 1: one-shot conversion.

nnn

Channel number (0-7).

Questions

  1. The following program uses the A/D converter subsystem to read some voltages. The program does work. (i.e. there are no logical or syntactical errors.) 

    ; A simple program using adc module.
; Author: Foo Bar
; Date: October 6, 2004

ADCTL equ $1030     ;address of ADC Control register
ADR1  equ ADCTL+1   ;address of first result register
ADR2  equ ADCTL+2   ;address of second result register
ADR3  equ ADCTL+3   ;address of third result register
ADR4  equ ADCTL+4   ;address of fourth result register

        org $6000
main:
        ldaa #%00010100
        staa ADCTL
        
        jsr foo

        ldaa ADR1
        ldab ADR4
        swi

foo:
         tst ADCTL
         bpl foo
         rts

    1. The subroutine "foo" performs an essential task. The name of the subroutine, however, is poorly chosen since it does not hint at the task it performs.

      What is a better name for the subroutine?


           
           
           

    2. Suppose that all 8 analog channels are connected to DC voltages as follows (assume that "full scale analog voltage" is 5.0 V):


              Channel 1:   2.5 V
              Channel 2:   1.25 V
              Channel 3:   3.75 V
              Channel 4:   5.0 V
              Channel 5:   0.0 V
              Channel 6:   3.75 V
              Channel 7:   5.0 V
              Channel 8:   2.5 V
            

      The program is run from address 0x6000. What values will be in Acc. A and Acc B. when the "swi" instruction is encountered?


           
           
           

    ANSWER

    1. A better name would be something like WaitConvDone.

    2. Channels 5-8 are converted. Channel 5 is 0.0 volts (analog), converted to digital %00000000. Channel 8 is 2.5 volts (analog), converted to digital %10000000. Since AccA reads Channel 5, it is $00; since AccB reads Channel 8, it is $80.

  2. Given the following program: 

            org $6000
main:
        ldx #stuff
        ldab #0

loop:
        ldaa 0,x
        beq done
        addb 0,x
        inx
        bra loop
done:
        swi
        
        org $7000
stuff   fcb 3, 1, 4, -1, 0, 2, 7

    Assume that the CPU begins executing at address 0x6000.


         
         
         

    1. What value (in hex) will be in index register X following the execution of the instruction ldx #stuff?

    2. What will the values in index register X, AccA and AccB be just before the swi instruction is executed?


           
           
           

    ANSWER


    1.       $7000
          


    2.       AccA: 0
            AccB: 7
            IX: $7004
          

  3. Write a code fragment that performs the following:

    1. Divides the signed 8-bit binary number in Acc by 2. (For example, if AccA were 15, it would be 7 after division.)

    2. If Acc A is an odd number, convert it to the next smaller even number. (For example, a 7 would become a 6.)

    3. Invert the bits 2 and 3 of the result. (For example, 6—00000110 in binary— would become 10—00001010 in binary.)





         
         
         

    ANSWER

     asra         ;Part a: arithmetic right shift divides by 2 (signed)
 anda #$FE    ;Part b: no effect on even numbers; decrements odd numbers
 eora #%1100  ;Part c: XORing with 1 inverts bit at same position