Real Time Interrupts

1. Real Time Interrupts Section 4.15

2. Real-Time Interrupt (RTI) • Most operating systems (OS) require an interrupt every T seconds by the RTI • RTI interrupts are often counted to determine time and date (XP, LINUX) • OS use RTI interrupts for process scheduling • IBM PC is interrupted 18.2 times per second by RTI

3. Real-Time Interrupt • Two Bits of Interest • RTIE – Real-Time Interrupt Enable • “0” – Disables Interrupts • “1” – Enables Interrupts • RTIF – Real-Time Interrupt Flag • Set every T Seconds when RTIE=“1” • Generates Maskable Interrupt when “1” • Write “1” to RTIF to Clear Flag

4. Real Time Interrupt68HC12DP256 • Set Up and Controlled using 3 Registers: • RTICTL • CRGINT • CRGFLG

5. Clock Generator • Lab Processor Boards uses a 4 MHz Crystal Oscillator • Frequency of Bus Clock (ECLK) – 2 MHZ • RTI uses the 4 MHZ Crystal Clock

7. 4 MHZ Prescaler: Divides by 1, 2, 4, 8, 16, 32, or 64 Divides by 1, 2, 3, 4, 5, 6, • • •, 16 SETS RTI FLAG

8. Divide 4 MHZ Clock by: 1024 AND BY 1 or 2 or 4 or 8 or 16 or 32 or 64 (Selected by RTR[6:4]) AND BY 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or 11 or 12 or 13 or 14 or 15 or 16 (Selected by RTR[3:0]) to generate rate of RTI Timeout Output.

9. Real Time Interrupt Enable

10. Real Time Interrupt Flag Power-On Reset Flag Self-check Mode Interrupt Flag

11. Real Time Control Register

13. Frequency of RTI /* rtc1 = RTR[6:4], rtc = RTR[3:3] frti = Frequency of RTI */ int rtc1, rtc2; float frti; rtc1 = (RTR>>4)&0x07; rtc2 = RTR&0x0F; if(rtc1==0) frti=0; else frti = 4000000/(1024*(1<<(rtc1-1))*(rtc2+1))

14. 244.14 122.07 61.04 30.52 15.26 7.63 3.81 16

15. CRGFLG Init prescale factor in RTICTL Enable RTIE in CRGINT Reset RTIF in CRGFLG Enable maskable interrupts ENABLE();

16. Text RTI Example • RTI Interrupt every 8.192 msec. • E Clock is 4 MHZ • RTICTL is defined differently • Divide E Clock by 2 raised to power 15 • RTR[6:4] = 6 • RTR[3:0] = 0 • RTICTL = 0x60 • RTI Interrupt Vector Address = 0xFFF0

17. TEXT RTI Examples • Replace #include <912b32.h> with • #include <ece371.h> • #include <ports_D256.h> • Replace interrupt #pragma code with • void RTI_isr() __attribute__ ((interrupt)); • Replace init vector #pragma code but place inside of main program • Initialize Interrupt Vectors with • SETVECT(0xFFF0,RTI_isr);

18. Text RTI Example • Replace RITCTL=0x84 with • Set Interrupt Frequency, enable RTI, clear RTI • RTICTL = 0x60; • CRGFLG=0x80; • CRGINT = CRGINT | 0x80; • Replace RTIFLG=0x80 with • Clear RTI Flag • CRGFLG = 0x80; • Replace CLI() Enable Interrupts with • ENABLE();

19. /* Real Time Interrupt Example Program Page 206-208 of Text Modified to Execute on Lab Microcontroller This program keeps track of clock time using the Real Time Interrupts. The RTI generates an interrupt 8.192 ms. The RTI_isr keeps track of elapsed time. */ /* include files */ #include <ece371.h> #include <ports_D256.h> /* function prototypes */ void RTI_isr(void) __attributes__ ((interrupt)); /* global variables */ unsigned int ms_ctr, sec_ctr, mins_ctr, hrs_ctr, days_ctr;

20. void main(void){ ms_ctr = 0; // initialize timer variables sec_ctr = 0; mins_ctr = 0; hrs_ctr = 0; days_ctr = 0; /* initialize RTI - - replaces pragma abs_address, etc. */ SETVECT(0xFFF0,RTI_isr); // interrupt vector // Replace RTICTL = 0x84; RTICTL = 0x60; // set interrupt frequency CRGFLG=0x80; // clear RTI interrupt flag CRGINT = CRGINT | 0x80; // enable RTI /* Enable Interrupts – replace CLI() */ ENABLE(); while(1); // wait for interrupts }

21. /* RTI_isr: RTI Interrupt Service Routine */ void RTI_isr(void) { // replaces RTIFLG CRGFLG = CRGFLG | 0x80; /* update milliseconds */ ms_ctr = ms_ctr+1; /* update seconds */ if(ms_ctr == 122) /* counter equates 1000 ms at 122 */ { ma_ctr = 0; // reset millisecond counter sec_ctr = sec_ctr +1; // increment seconds counter

22. /* update minutes */ if(sec_ctr == 60) { sec_ctr = 0; // reset seconds counter mins_ctr = mins_ctr+1; // increments minutes counter } /* update hours */ if(mins_ctr == 60) { mins_ctr = 0; // reset minutes counter hrs_ctr = hrs_ctr + 1 // increments hours counter } /* update days */ if(hrs_ctr == 24) { hrs_ctr = 0; // reset hours counter days_ctr = days_ctr + 1; // increment days counter } }