Interrupts In AVR Microcontrollers (Chapter 10 Of The Text .

Microprocessors, Lecture 9:Interrupts in AVR Microcontrollers(Chapter 10 of the text book)

Contents Interrupts ATmega32 Using interrupts in C programmingUniversity of Tehran 2

Interrupts in AVRInterrupts: A way o improve performanceUniversity of Tehran 3

How do interrupts work?University of Tehran 4

Interrupt vs. polling Polling:– Continuously check the flags to know when theexpected event occurs, then go for the nextevent– Example: Timer time-out problem of the previous lecture:University of Tehran 5

Interrupt vs polling Polling: wasting time to check the devicescontinuously What if we are to generate two delays at thesame time?– Example: Toggle bit PB.5 every 1s and PB.4 every 0.5s. What if there are some task to be donesimultaneously with the timers?– Example: (1) read the contents of port A, process thedata, and send them to port D continuously, (2) toggle bitPB.5 every 1s, and (3) PB.4 every 0.5s.University of Tehran 6

Interrupt vs polling Interrupts– A mechanism to work with peripheral devices No need for the processor to monitor the status ofthe devices and events Let the events notify the processor when theyoccur– By sending an interrupt signal to processorUniversity of Tehran 7

Interrupt vs polling Interrupts:– Example: Copy the contents of port A to port Dcontinuously and toggle bit PB.5 every 1s and PB.4every 0.5s.– Solution:» Copying the contents of port A to port D as the main program» Get timers 0 and 1 to generate the delays» Define two interrupts for timers 0 and 1 to notify the processorwhen they finish counting» Upon an interrupt, stop the main program, service the timers andcontinue the main programUniversity of Tehran 8

Interrupts Interrupting mechanism in all microprocessors andmicrocontrollers is almost the same:– Define the set of devices and events that can generatean interrupt– Write a function for each interrupt that will be executedwhen the corresponding interrupt is activated» The address of this function must be saved somewhere– Set a priority scheme among interrupts– A mechanism is needed to disable all or some interruptsUniversity of Tehran 9

Interrupts ISR: Interrupt Service Routine– The function that is executed when an interruptis enabled Interrupt Vector Table: a table that keeps theaddress of each ISR in the instruction memoryUniversity of Tehran 10

Interrupt sources in AVR.University of Tehran 11

External interrupts 3 pins of ATmega32University of Tehran 12

Interrupts in AVR Just 2-bytes for eachinterrupt service routine Too small to write theinterrupt service routine Æ Write the routinesomewhere in the memoryand put a code to jump to theaddress of the function in the2-byte assigned to the ISRUniversity of Tehran 13

Interrupts in AVR How is an interrupt serviced?1. Stop fetching the next instruction and save PC2. Go to Interrupt Vector Table to find the address of the ISR ofthe interrupting device3. Execute the function4. Resume normal execution by retrieving PCUniversity of Tehran 14

Enabling Interrupts Interrupts can be enabled or disabled byprogrammer– Bit7 (I) in SREG (status register)– SREG keeps the processor status (remember the firstlecture on AVR)– Disabled on reset (I 0)University of Tehran 15

Enabling Interrupts In addition to Bit7 (I) in SREG each interruptshould be enabled independently The enable bit of each interrupt is in some register– Example: TIMSK register to enable/disable timerinterrupts– TIMSK Timer Interrupt Mask– Mask? غيرفعال کردن ، پوشاندن To enable timer1 overflow interrupt:– Bit7 (I) in SREG Å 1– Bit 0 of TIMSK (TOIE0)Å 1University of Tehran 16

Interrupt priority What if two or more interrupts occurat the same time?– The interrupt with lower ISR addressis prioritized (external int. 0 has thehighest priority) When an interrupt is serviced, the Ibit becomes automatically 0 todisable interrupts– Will be enabled when returning fromthe ISRUniversity of Tehran 17

TIMSK registerUniversity of Tehran 18

Interrupt programming in C Enable interrupts Set the mask register (TIMSK for timers)– Example: TIMSK 0x01; Write the ISR function to specify whatoperation should be done when the interruptoccursÆ Compiler dependent: different in differentcompilers!University of Tehran 19

Interrupt programming in C A way to use assemblyinstructions in C:#asm(“instruction”) SEI: (Set I) an assemblyinstruction that enablesinterrupts (bit 7 ofSREG 1) CLI: Clear I #asm(“sei”); enableinterrupts in C No way to accessSREG.7 in C Mazidi’s book uses a differentcompilerÆ differentinstructions: sei() instead of#asm(“sei”), different ISRnames#asm(“sei”);interrupt [TIM0 OVF] void timer0 ovf isr(void)Æ On entering the timers ISR, the TOV0bit is automatically cleared, no need tobe cleared by softwareUniversity of Tehran 20

Interrupt programming in C In codevision, ISR is generated duringproject setup. Just fill the function body!University of Tehran 21

External Interrupts To allow externalsources interrupt themicrocontroller Can be masked by CIGRregister GICR: General interruptcontrol registerUniversity of Tehran 22

External Interrupts- GICRUniversity of Tehran 23

External interrupts Interrupts can be edge triggered or level triggered Edge trigger: activated when a change in signallevel occurs Level trigger: activated when a signal has aspecific value INT0 and INT1 can be programmed to be edge orlevel triggered– Low-level active by default INT 2 is only edge triggeredUniversity of Tehran 24

External interrupts A register called ISC (interrupt sense control) canset the interrupt type of INT0 and INT1ISC RegisterISC10 and ISC11 set the same setting for INT1University of Tehran 25

External Interrupts- Cprogramming#asm(“sei”);University of Tehran 26

Resume normal execution by retrieving PC . University of Tehran 15 Enabling Interrupts Interrupts can be enabled or disabled by programmer – Bit7 (I) in SREG (status register) – SREG keeps the processor status (remember the first lectur