Interrupts, Clock Generation, Resets, and Operation Modes - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 290

Example 6.5
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Write a C program to use the RTI to time-multiplex four seven-segment displays using
the circuit shown in Figure 4.18 and shift the seven-segment display pattern as described in
Example 6.4. Turn on one display at a time and light each display for about 1 ms, then switch
to the next display. Use display 0 to display 3. Use CodeWarrior and a demo board programmed
with serial monitor to implement the circuit.
Solution: To implement the digit sequence shifting program in C language, we make the follow-
ing arrangement:
• Place the segment patterns in one array ( segPat[] ). This array is the overlapping
arrangement of 10 display sequences as shown in Figure 5.2.
• Place digit select values in one array ( digit[] ).
• Use a variable ( seq ) as an index to the segment pattern array that identifies the first
digit of the current sequence.
• Use a variable ( ix ) as an index to the digits within one sequence. The range of this
variable is from 0 to 3.
• Use a variable ( count ) to specify the repetition count of a sequence.
The main function that performs the initialization and the RTI service routine that performs
time multiplexing is
#include “c:\cwHCS12\include\hcs12.h”
#include “c:\cwHCS12\include\SetClk.h”
int seq; // start index to segPat[] of a sequence of digits (0 to 9)
int ix; // index of digits of a sequence (0 to 3)
int count; // repetition count of a sequence
char segPat[13] 5 {0x06, 0x5B, 0x4F, 0x66, 0x6D, 0x7D, 0x07, 0x7F, 0x67, 0x3F, 0x06, 0x5B, 0x4F};
char digit[4] 5 {0xFE, 0xFD, 0xFB, 0xF7};
void main (void) {

seq 5 0; // initialize the start index to segPat[] for the display sequence

ix 5 0; // initialize the index of a new sequence

count 5 400; // initialize the RTI count of a sequence

SetClk8(); // set E-clock to 24 MHz from an 8-MHz crystal oscillator

RTICTL 5 0x40; // RTI interval set to 2**10 OSCCLK cycles

DDRB 5 0xFF; // configure Port B for output

DDRP 5 0xFF; // configure Port P for output

CRGINT| 5 RTIE; // enable RTI

asm(“CLI”); // enable interrupt globally

while(1);

}
// RTI service routine
interrupt void rtiISR(void) {

CRGFLG 5 0x80; // clear RTIF bit

PTB 5 segPat[seq 1 ix]; // send out digit segment pattern

PTP 5 digit[ix]; // turn on the display

ix 11 ; // increment the index to digits of a sequence

if (ix 55 4) // make sure the index to digits of a sequence is from 0 to 3

ix 5 0; // “