CENG 412 Embedded Systems
Lab 8
Period, Frequency, PulseWidth and Duty Cycle measurement
Name ______________
Part A ___________
Part B _____________
Introduction
In this lab you will use the Input Capture function of the HCS12 Timer system to
measure the time between rising and falling edge events of a TTL waveform. Attention to
register detail is a must in this kind of work.
Equipment
1. HCS12 workstation, Standard bench equipment
Procedure: Part A - Period and Frequency measurement
1.
Look at the AD9S12D- H1 Pinouts diagram on the course web site – click on the HCS12
Board Layout and Connector Pins link. Determine what pin is connected to PT0/IOC0. By
looking at the Demo Board Layout and Connector Pins link you can verify that Switch 8
connected to the same pin.
PT0/IOC0 pin number ____________
Toggle Switch 8 until you are sure there a Logic High level at Switch 8. You can verify this with
a scope or DVM.
2.
Set the TTL/SYNC output of the function generator to 1 kHz at 50% duty cycle and
verify using the scope. Connect the signal to the PORT T (PT0) pin and ground on the Demo
Board Primary I/O connector.
3.
Create a CodeWarrior project. The C and Assembler code are shown at the end of the
lab. Copy/paste the code to the appropriate files.
4.
You will need to add the termio.c file to the project in the Sources folder in order to use
the printf function. In Codewarrior select project, AddFiles and then find and select the file.
This file is found at C:\Program Files\Freescale\CodeWarrior for S12(X) V5.01\lib\hc12c\src.
5.
“Make” the program in CodeWarrior. The Assembly code has an unusual bit of code with
a loop that the program will never execute. What is the purpose of this code?
BrkVal:
6.
file.
BRN BrkVal
Determine the addresses of the labels edge1, edge2, and BrkVal from the Project.map
edge1 address __________
edge2 address __________
BrkVal address _________
period address __________
7.
Open AsmIDE and download the .s19 file to your HCS12 workstation. Place a breakpoint
at the address of the label Brkval. Run the program to this breakpoint. Type the T command
twice (trace) until you have executed the instruction std period.
1
8.
Examine the values in the memory locations edge1, edge2 and period. Record the
values found at these locations. Subtract the edge2 and edge 1 values. Does the result agree
with what you see in Register D and the memory location for period?
edge1 value ___________
edge2 value_____________
edge2 – edge1 value __________
period value ____________
9.
The value for Period in memory should match what is in register D. Record the value in
register D.
Register D ____________
In the following paragraph you will do by hand what the C code is doing.
The value in Register D or memory is the raw (unscaled) measure of period. Convert the value
to decimal. Determine the scaled or actual period of the waveform by multiplying this value by
32 and divide by 24. The result is in µsec. Determine the frequency of the waveform in Hz by
dividing the Timer Clock frequency of 750 kHz by the raw (unscaled) measure of period. What
is the value of the period and frequency?
Period ____________
Frequency ____________
10.
Remove the breakpoint. Run the program for input frequencies of 500 Hz, 1 khz and 2
kHz. Record the results.
Case
1
2
3
Function Generator freq
Measured freq
Procedure: Part B - PulseWidth and Duty Cycle measurement
1.
The code in Part A measured the period of a square wave. In this part of the lab you will
write another assembly language function to measure the pulse width or high time (THIGH)
between a rising and falling edge on the input waveform. This can be done in 1 of 2 ways. Do
one or the other but not both.
Method 1
Write a new function called readPW in the main.asm file. Copy/paste the code for the function
readperiod in main.asm. Call the new function readPW
Method 2
Create a new file using Notepad – give it a name such as pw.asm. Copy/paste the code
from main.asm into this new file. Call the new function readPW Add this file to the sources
folder in CodeWarrior.
11.
For either method you will need to address the following issues:
•
Edit the new function called readPW
2
•
•
•
Be sure that you put the correct XDEF statement, the function name and an rts at the
appropriate places in the code
Define memory locations for edge3, edge4 and pw (pulsewidth).
Insert a new line of code (similar to movb #$xx,TCTL4) before the second instance
of brclr TFLG1,C0F,* to configure the program to now capture a falling edge
12.
In the C code declare new variables rawPW (unsigned int), scaledPW, and DC
(unsigned long int). Use the existing C code to guide you. Make a call to each of the
assembly methods (readPeriod and read PW). Scale the value of pulsewidth in the same
manner as was done for period and then calculate the duty cycle of the signal by this
relationship: duty cycle = (Pulse width*100) /Period
Using printf display Period, Frequency, Pulse width, and Duty cycle with proper units. Use
the existing C code to determine how to display these values.
13.
Set the TTL/SYNC output of the signal generator to 2 kHz at 25% duty cycle. Verify
that this is the correct waveform using the oscilloscope. Connect the signal generator to the
HCS12 workstation to pin 13 and ground.
14.
Repeat for 5 KHz at 75% duty cycle. Record the results. Demonstrate the Part B
program to your instructor and have your lab sheet signed.
Case
Funct Gen freq
1
Funct Gen Duty
Cycle
Measured freq
Measured Duty
Cycle
2
C program
;**********************************************
;* Lab 8 - This lab measures the period of a TTL
;* waveform using the HCS12 Input Capture function
;*
;* A C program calls assembler code which returns a raw value
;* for period to the C program. The C code computes the scaled vales
;* of the period and frequency and prints them out.
;**********************************************
#include <hidef.h>
#include "derivative.h"
#include <stdio.h>
/* common defines and macros */
/* derivative-specific definitions */
int readPeriod(void);
void main(void) {
unsigned int RawPer,Freq;
unsigned long int ScaledPer;
RawPer = readPeriod();
RawPer = readPeriod();
ScaledPer = ((long)RawPer*32)/24;
Freq = 750000/RawPer;
/*
/*
/*
/*
3
call assembler
call assembler
scale period –
convert period
function */
function */
pre-scale of 32*/
to freq */
printf("The period is %lu useconds \n\r", ScaledPer);
printf("The frequency is %u Hz \n\r", Freq);
/* \n=LF
\r=CR
*/
asm("swi");
}
Assembler Code
XDEF readPeriod
TIOS:
TSCR1:
TSCR2:
TCTL4:
TFLG1:
TC0:
IOS0:
C0F:
equ
equ
equ
equ
equ
equ
equ
equ
edge1:
edge2:
period:
$40
$46
$4D
$4B
$4E
$50
$01
$01
ds.b 2
ds.b 2
ds.b 2
;memory to hold the first edge
;memory to hold the second edge
;memory to store the period
readPeriod:
movb #$90,TSCR1
bclr TIOS,IOS0
movb #$05,TSCR2
movb #$01,TCTL4
movb #C0F,TFLG1
brclr TFLG1,C0F,*
ldd
TC0
std
edge1
brclr TFLG1,C0F,*
ldd
TC0
std
edge2
subd edge1
BrkVal:
BRN
BrkVal
std
period
rts
;enable timer counter and enable fast timer flags clear
;enable input-capture 0
;disable TCNT overflow interrupt, set prescaler to 32
;set to capture the rising edge of PT0 signal
;clear the C0F flag
;wait for the arrival of the first rising edge
;save the first edge and clear the C0F flag
;store first edge in memory
;wait for the arrival of the second edge
;save the second edge and clear the C0F flag
;store first edge in memory
;compute the period
; return to caller
XDEF readPW
edge3:
edge4:
pw
:
ds.b 2
ds.b 2
ds.b 2
;memory to hold the first edge
;memory to hold the second edge
;memory to store the period
readPeriod:
movb #$90,TSCR1
bclr TIOS,IOS0
movb #$05,TSCR2
movb #$01,TCTL4
movb #C0F,TFLG1
brclr TFLG1,C0F,*
ldd
TC0
std
edge3
movb #$02,TCTL4
brclr TFLG1,C0F,*
ldd
TC0
std
edge4
subd edge3
std
pw
rts
;enable timer counter and enable fast timer flags clear
;enable input-capture 0
;disable TCNT overflow interrupt, set prescaler to 32
;set to capture the rising edge of PT0 signal
;clear the C0F flag
;wait for the arrival of the first rising edge
;save the first edge and clear the C0F flag
;store first edge in memory
;set to capture the falling edge of PT0 signal
;wait for the arrival of the second edge
;save the second edge and clear the C0F flag
;store first edge in memory
;compute the period
; return to caller
4