Mark Neil - Microprocessor Course 1 Timers and Interrupts.
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Mark Neil - Microprocessor Course 1 Timers and Interrupts
The Need for Processor Interrupts Mark Neil - Microprocessor Course 2 Up to now if you wanted to do something in a program as soon as a bit was set (key pressed, bit set in a register, voltage exceeded a given threshold,…) you had to keep reading the bit until it changed ! This is called Polling This is clearly not an efficient way of doing things CPU time is wasted watching for something to happen This is why interrupts were introduced as a means of getting the processor’s attention on demand Use the CPU for other things until whatever we were waiting for has happened
Processor Interrupts Mark Neil - Microprocessor Course 3 Interrupts are subroutine calls initiated not by an rcall command but by hardware signals. These hardware signals cause the processor to jump to interrupt service routines. At the end they return control to your program just where it was just before the interrupt occurred.
ATmega128 Timers and Interrupts There are 24 different sources of interrupts There are timers on the ATMega128 that can be used to trigger an interrupt at fixed intervals Interrupts can be triggered when certain hardware tasks are completed Eight external inputs can be used to request an interrupt Mark Neil - Microprocessor Course 4
The ATmega103 Memory Map The first 24 2-word addresses in flash program memory are reserved for interrupts: your program jumps to one of these addresses if an interrupt occurs. A table of jump instructions is used to let the processor know where to execute code in case of an interrupt Mark Neil - Microprocessor Course 5
Using interrupts Mark Neil - Microprocessor Course 8 Global enable via the status register You must first tell the processor that it should use interrupts Masks to work at bit level within devices Control registers to select type of signal
Global Level: Status Register In order to use any interrupts on ATmega103 you must set the ‘ I ’ bit in the status register ( SREG ) using the command sei By now you should know what The V,N,Z,C bits are. Mark Neil - Microprocessor Course 9 SREG
At device level: Mark Neil - Microprocessor Course 10 EIMSK – use to mask which external interrupts are used EICR – used to control how external interrupts are recognised EIFR – flags to show which have been triggered
Timer/Counter0 Control Register Mark Neil - Microprocessor Course 12 CTC0 : Clear Timer/Counter on Compare Match COM00 / COMM01 :Compare Output Mode, Bits 1 and 0 PWM0 : Pulse Width Modulator Enable The timer pre-scale factor : CS02; CS01; CS00 TCCR0
Pre-scaling the Timer via TCCR0 Mark Neil - Microprocessor Course 13 Counter Multiplexer CS00 CS01 CS02 f/128 f/64 f/8 f/32 f/1024 f/256 TOSC1 ASO f f f f Clock Generator Timer0 Clock
Pre-scaling the Timer via TCCR0 Mark Neil - Microprocessor Course 14
Timer/Counter0 TCNT0 Mark Neil - Microprocessor Course 15 8-bit registers which contain the value of the Timer/Counters. Both Timer/Counters are realized as up or up/down (in PWM mode) counters with read and write access. If the Timer/Counter is written to and a clock source is selected, it continues counting in the timer clock cycle after it is preset with the written value
Example Program Mark Neil - Microprocessor Course 16 In the example program the 8 Mhz clock of the ATmega128 is pre-scaled by 256 and the timer zero is loaded with $B2. The counter overflows ($00) every 125 x 256 x ($FF- $B2 + 1) nsec (approx every 2.5 msec) and causes an interrupt. Every 200 interrupts a counter is incremented and the result is displayed on the PORTB LEDs.
Interrupt Jump table Mark Neil - Microprocessor Course 17 jmp Init;2 word instruction to set correct vector jmp xxx ;next interrupt nop ;use this two liner if no routine available reti. jmp TIM0_OVF; Timer 0 Overflow interrupt Vector nop; Vector Addresses are 2 words apart reti. At Program Reset your program jumps to the initialization when Timer0 overflows the interrupt controller jumps to this location
Interrupt initialization Mark Neil - Microprocessor Course 18 ; ; **** Timer0 Setup Code **** ; ldi r16,$06; Timer 0 Setup out TCCR0, r16; Timer - PRESCALE TCK0 BY 256 ; (devide the 8 Mhz clock by 256) ldi r16,$b2; load timer with n=178 out TCNT0,r16; The counter will go off ; in (256 - n)*256*125 nsec ; ; **** Interrupts Setup Code **** ; ldi r16, $01; Timer Interrupt Enables out TIMSK, r16; T0: Overflow ;
Interrupt Service Routine Mark Neil - Microprocessor Course 19 TIM0_OVF: in R4,SREG;save SREG inc r17;increment cycle nop cpi r17,$C8;compare cycle with 200 brne again;if <> jump to again out PORTB, r18;send bits to PORTB inc r18;Increment the portB number clr r17 ;clear cycle and start counting ;200 interrupts again: ldi r16,$B2;reload timer value out TCNT0,r16 out SREG,r4;restore sreg reti It is a good idea to save the information stored on the status register and restore it at the end of the program It is a good idea to save the information stored on the status register and restore it at the end of the program ‘reti’ sets the interrupt bit, in the SREG, back to ‘1’ so the next interrupt can be serviced When an interrupt occurs : D7 of SREG is set to ‘0’ and the program jumps to the interrupt service routine
Main Program Mark Neil - Microprocessor Course 20 main:;wasting time ; nop rjmp main;loop The timer is counting in the background and when it overflows it causes an interrupt forcing the program to execute the interrupt service routine. After the interrupt is handled we call reti and the program comes back in the loop where it was when the interrupt occurred The timer is counting in the background and when it overflows it causes an interrupt forcing the program to execute the interrupt service routine. After the interrupt is handled we call reti and the program comes back in the loop where it was when the interrupt occurred
Interrupt Service Routines Interrupts can occur at any time They are asynchronous to the operation of the rest of your program Your program may be doing something else important The interrupt service routine should leave the program in a state so that in can continue running after the reti General Hints Keep the service routines short – don’t do any heavy computation, don’t do long and involved I/O if you can avoid it Better to simply flag that the interrupt has happened so that elsewhere in your program you can deal with it Make sure to save and restore any registers that you use – including the status register Only if you can guarantee that you are not using a register elsewhere in your program can you avoid saving and restoring Very unpredictable effects can happen if you don’t do this Mark Neil - Microprocessor Course 21