Ee3954 spring 16 08 interrup Notes PDF

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Microprocessors and Microcontrollers

Interrupts

EE3954 by Maarten Uijt de Haag, Tim Bambeck, Harsha Chenji Interrupts.1

Interrupts Definitions

•  Polling and interrupts are used when interfacing and communicating with internal and external devices and programs •  Polling: –  Internal or external devices are checked on a regular basis to find out if they need service; if they require a particular function to be executed.

•  Interrupts: –  An event from either an internal or external source where a processor will suspend the execution of its current process (program) and switch to a different instruction sequence. Interrupts.2

Polling If the switch is closed, Then turn the LED on!

VDD R

RB4

Polling the Button

Set up the I/O pins

RB0 Switch Closed ?

N

Y PIC16F877

Turn LED on

Turn LED off

VSS VSS

Interrupts.3

Polling

PORTB TRISB STATUS RP0 RP1

INIT:

POLL: LED_OFF LED_ON:

Source Code + Comments

equ equ equ equ equ

0x06 0x06 0x03 d’5’ d’6’

; in Bank 0 & 2 ; in Bank 1 & 3 ; in all Banks

org nop bcf bsf bsf bcf bcf bcf btfss goto bcf goto bsf goto end

0x000

; program starts at address 0

STATUS, RP1 STATUS, RP0 TRISB, 0 TRISB, 4 STATUS, RP0 PORTB, 4 PORTB, 0 LED_ON PORTB, 4 POLL PORTB, 4 POLL

; access Bank 1… ; … so we can get to TRIS Regs. ; set pin 0 of PORTB to input ; set pin 4 of PORTB to output ; access bank 0 ; initially, turn the LED off ; check if PORTB pin 0 is ‘0’ or ‘1’ ; goto TURN LED on ; Turn LED off, (set RB4=‘0’) ; go back and keep polling ; Turn LED on, (set RB4=‘1’) ; go back and keep polling

Interrupts.4

Interrupt Usage of a System

“Push Button” Interrupt

=

“Timer Interrupts” ”

“Serial Interrupt”

Main Program Loop

Interrupts.5

Interrupts If the switch is closed, Then turn the LED on!

Some Main Program

Interrupt Service Routine

Set up the I/O pins

Set up the I/O pins

VDD R

RB4

Set up Interrupts

RB0

Switch Closed ?

N

Y Process A

Turn LED on

Turn LED off

PIC16F877 Go Back to Process A (RETFIE)

VSS VSS

Interrupts.6

Interrupts

Transfer of Control Instruction N

Interrupt Occurs

Instruction N+1 Return of the program Control from the end of the ISR to the next (N+1) instruction of The main program

End main program

Program control is transferred to the first instruction of the interrupt service routine (ISR)

Instruction ISR-1 Interrupt Service Routine (ISR) Return from Interrupt Interrupts.7

PIC Microcontroller Microcontroller has various on-chip peripheral devices that can interrupt the main program

•  •  •  • 

Analog-to-Digital Converter (ADC), Timers and counters (8-bit, 16-bit), Parallel port, Serial communication, –  RS232 (UART), I2C, SSP

•  Comparator, •  Pulse-Width-Modulation (PWM) •  External signal or interrupt Interrupts.8

Signal: on-change ADC Signal: edge

Parallel port UART

Comparator, PWM

SSP Parallel port Multiplexing of I/O Pins

Pin Diagram from Datasheet

Interrupts.9

Example:

Signal: on-change

On-change interrupt (RB4, RB5, RB6, RB7): If the input on the pin is changed (“0” => “1” or “1” => “0”) an interrupt will occur

Interrupts.10

Interrupt

So what happens when an interrupt occurs

Interrupt Vector 0 0000 0000 0100 (13bit) ( = 0x0004 ) In other words, the current value of the PC is stored on the STACK and 0x0004 is put into PC: next instruction is at 0x0004

Interrupts.11

Interrupt

So what happens when an interrupt occurs

INIT: MAIN:

ISR:

org goto org goto … … … … goto … … retfie

0x000 INIT 0x004 ISR

Remember from the Labs

Interrupt Vector

MAIN

instead of ‘return’

Interrupt Service Routine (ISR) Works just like a subroutine

Interrupts.12

Interrupt

Returning from an ISR

Put the value on top of the STACK back into the Program Counter (PC); in other words, return to whatever the main program was doing.

Interrupts.13

Interrupt Service Routine Example

INIT: MAIN:

SUMISR:

NXT:

org goto org goto … … goto … movlw movwf movlw xorwf incf btfss goto movwf retfie

0x000 INIT 0x004 SUMISR

MAIN 0x20 FSR 0x00 INDF,W FSR,F FSR,6 NXT PORTE

Suppose that we want to write an ISR that XORs the values stored in memory locations 0x20 through 0x3f and write the result to PORTE.

Interrupts.14

Interrupts •  •  •  •  •  •  •  •  • 

INT Pin Interrupt (external interrupt-RB0) Timer overflow interrupt, Comparator change interrupt, PORTB change interrupt (RB4-RB7), Receive (communication) interrupt, Transmit (comm.) interrupt, A/D conversion complete interrupt, Parallel slave port interrupt, Etc. Interrupts.15

Interrupts •  INTCON (address 0x0B, 0x8B, 0x10B, 0x18B) –  Control and Status of interrupts

Ending with an E indicates an enable bit: if the bit is set the corresponding interrupt is enabled

Ending with an F indicates a flag: if the bit is set, the corresponding interrupt has Interrupts.16 occurred

rrupts.17

Interrupts

Control Registers Used for: Configuration for Peripherals

•  PIE1 & PIE2 (address 0x8C and

0x8D respectively

)

–  Contain bits to enable/disable peripheral interrupts for use

•  PIR1 & PIR2 (address 0x0C and

0x0D respectively

)

–  Contain bits to identify which interrupt occurs (flags): –  Corresponding bits are set when the interrupt occurred; Interrupts.18

PIE & PIR

from PIC16F877 datasheet

From page 15 and 16 of PIC16F877 datasheet (DS)

Interrupts.19

in: INTCON

in: PIE1, PIE2 PIR1, PIR2

For PIC16F877 (See section 12 of DS) Interrupts.20

In General for Mid-range devices

pts.21

Let’s Look at an Example VDD

•  Use the PORTB on-change interrupt to turn on the LED.

R

RB1

RB4

PIC16F877

VSS VSS

Interrupts.22

Step 1 Set up the interrupt in main program INTCON Register ( 0x0B in all Banks ):

Interrupts.23

Step 1

Set up the interrupt in main program INTCON TRISB PORTB

INIT:

equ equ equ

0x0B 0x06 0x06

org goto org goto clrf bsf bsf bcf bcf bsf bcf … …

0x000 INIT 0x004 ISR STATUS STATUS,5 TRISB, 4 TRISB, 1 STATUS,5 INTCON,3 INTCON,0

; access ; bank 1 ; pin 4 of PORTB: input ; pin 1 of PORTB: output ; access bank 0 ; enable PORTB on-change interrupt ; YOU MUST CLEAR THE FLAG

(see next slide)

Interrupts.24

Step 2

Write the ISR MAIN:

ISR: LED_OFF:

LED_ON:

… … goto btfss goto bcf bcf retfie bsf bcf retfie

MAIN PORTB, 4 LED_ON PORTB, 1 INTCON,0 PORTB, 1 INTCON,0

; check if pin 4 changed from “0” to “1” ; if not so, turn on LED (“1” to “0”) ; otherwise, turn off LED ; YOU MUST CLEAR THE FLAG ; return to whatever you were doing ; turn on LED ; YOU MUST CLEAR THE FLAG ; return to whatever you were doing

The flag is the only indication for the microcontroller that the interrupt occurred, if you do not clear it in the ISR the interrupt condition will remain!!!

Interrupts.25

Step 3

Enable the Global interrupt in main program INTCON TRISB PORTB

INIT:

MAIN:

equ equ equ

0x0B 0x06 0x06

org goto org goto clrf bsf bsf bcf bcf bsf bcf bsf … …

0x000 INIT 0x004 ISR STATUS STATUS,5 TRISB, 4 TRISB, 0 STATUS,5 INTCON,3 INTCON,0 INTCON,7

; access ; bank 1 ; pin 4 of PORTB: input ; pin 0 of PORTB: output ; access bank 0 ; enable PORTB on-change interrupt ; YOU MUST CLEAR THE FLAG ; enable all interrupts Interrupts.26

Interrupt Latency Time from interrupt event (the interrupt flag bit gets set) to the time that the instruction at address 0x004 starts execution

Synchronous (typical internal) events: latency is 3TCY Asynchronous (typical external) events: latency is 3TCY to 3.75TCY with the instruction cycle time

In general: see the individual sections of the peripheral devices for the exact latency

Interrupts.27

Interrupt Latency

Interrupts.28

Oops What can go wrong with the combination of the following main program and ISR?

MAIN:

ISR:

movf addlw movwf … … goto

TEMP1, W 0x23 TEMP2

movlw movwf retfie

0x20 ISR_TMP,F

suppose that the interrupt occurs here

MAIN

Interrupts.29

Oops … Again What can go wrong with the combination of the following main program and ISR?

MAIN:

ISR:

movf sublw btfss … … goto

TEMP1, W 0x23 STATUS,Z

movlw addwf retfie

0x20 ISR_TMP,F

suppose that the interrupt occurs here

MAIN

Instruction alters the ‘Z’ bit

Interrupts.30

Context Saving During Interrupts

•  Save the CONTEXT: –  at least W and STATUS, –  but also all Registers you plan to use in the ISR that are used in the Main Program.

•  Save CONTEXT in RAM Common Area –  0x71 -0x7F (recall 0x70 used by ICD2) Interrupts.31

Common RAM Area

Advantage of this common area is that you DO NOT have to change banks

Remember this Data Memory area from Lab #1

Interrupts.32

Context Saving Why Not:

ISR:

movwf W_TEMP STATUS,W movf movwf STATUS_TEMP … … … movf STATUS_TEMP,W movwf STATUS movf W_TEMP,W

; DOES NOT affect STATUS ; DOES affect STATUS ; DOES NOT affect STATUS

; DOES affect STATUS ; DOES NOT affect STATUS ; DOES affect STATUS

So, while you are saving ‘W’ and ‘STATUS’ you actually changing the contents of STATUS. You do not want this to happen so you use SWAPF which does not affect any flags in STATUS Example: suppose STATUS = 0x00?

Interrupts.33

Context Saving Why Not:

Contents After Execution Label

Instr.

Argument

ISR:

movwf

W

STATUS

W_TEMP

STATUS_TEMP

W_TEMP

0x12

0x00

0x12

-

movf

STATUS,W

0x00

0x04

0x12

-

movwf

STATUS_TEMP

0x00

0x04

0x12

0x00

… … …

0x04

movf

STATUS_TEMP, W

0x00

0x04

0x12

0x00

movwf

STATUS

0x00

0x00

0x12

0x00

movf

W_TEMP,W

0x12

0x00

0x12

0x00

Before this program segment: W = 0x12 and STATUS = 0x00

Interrupts.34

Context Saving Why Not:

Contents After Execution Label

Instr.

Argument

ISR:

movwf

W

STATUS

W_TEMP

STATUS_TEMP

W_TEMP

0x00

0x12

0x00

-

movf

STATUS,W

0x12

0x12

0x00

-

movwf

STATUS_TEMP

0x12

0x12

0x00

0x12

movf

STATUS_TEMP, W

0x12

0x??

0x00

0x12

movwf

STATUS

0x12

0x12

0x00

0x12

movf

W_TEMP,W

0x00

0x16

0x00

0x12

… … …

Before this program segment: W = 0x00 and STATUS = 0x12

Interrupts.35

So … What instruction moves to/from register without affecting any flags?

swapf Problem is that this instruction swaps the nibbles of the byte

Solution: Just swap them back

Interrupts.36

Context Saving Contents After Execution Label

Instr.

Argument

ISR:

movwf

W

STATUS

W_TEMP

STATUS_TEMP

W_TEMP

0x00

0x12

0x00

-

swapf

STATUS,W

0x21

0x12

0x00

-

movwf

STATUS_TEMP

0x21

0x12

0x00

0x21

swapf

STATUS_TEMP, W

0x12

-

0x00

0x21

movwf

STATUS

0x12

0x12

0x00

0x21

swapf

W_TEMP,F

0x12

0x12

0x00

0x21

swapf

W_TEMP,W

0x00

0x12

0x00

0x21

… …

Before this program segment: W = 0x00 and STATUS = 0x12

Interrupts.37

Context Saving During Interrupts

ISR:

movwf swapf movwf … … … swapf movwf swapf swapf

W_TEMP

; Copy W to a Temporary Register ; regardless of current bank STATUS,W ; Swap STATUS nibbles and place ; into W register STATUS_TEMP ; Save STATUS to a Temporary register ; in Bank0

STATUS_TEMP,W ; Swap original STATUS register value ; into W (restores original bank) STATUS ; Restore STATUS register from ; W register W_TEMP,F ; Swap W_Temp nibbles and return ; value to W_Temp W_TEMP,W ; Swap W_Temp to W to restore original ; W value without affecting STATUS Interrupts.38

Multiple Interrupts What to do when the Microcontroller has to deal with interrupts for multiple sources, e.g. timer and PORTB change interrupt?

Check for the flags

Interrupts.39

Multiple Interrupts Example:

suppose we could get an interrupt for 1) a PORTB pin level change, 2) a TIMER 0 overflow (0xFF -> 0x00)

Interrupts.40

Multiple Interrupts

INIT: MAIN: RB_ISR:

T0_ISR:

org goto org btfsc goto btfsc goto retfie … … … … … … … retfie … … retfie

0x0000 INIT 0x0004 INTCON, RBIF RB_ISR INTCON, T0IF T0_ISR

Interrupts.41

Multiple Interrupts Or

INIT: MAIN:

RB_ISR:

T0_ISR:

org goto org btfsc goto goto … … … … … … … retfie … … retfie

0x0000 INIT 0x0004 INTCON, RBIF RB_ISR T0_ISR

Interrupts.42

Multiple Interrupts Or

INIT: MAIN:

ISR: T0_ISR:

RB_ISR:

org goto org goto … … … … … btfsc goto … … retfie … … retfie

0x0000 INIT 0x0004 ISR

INTCON, RBIF RB_ISR

Interrupts.43

Example in the Timer Section

Interrupts.44...