Chap 4 lesson02emsysnewinterruptbasedi_os

DEVICE DRIVERS AND INTERRUPTS
SERVICE MECHANISM
Lesson-2: Interrupt and Interrupt Service
Routine Concept

Chapter-4 L02: "Embedded Systems - " , Raj Kamal,
2008 1
Publs.: McGraw-Hill Education

Interrupt Concept
• Interrupt means event, which invites
attention of the processor on occurrence of
some action at hardware or software
interrupt instruction event.

2008 2

Action on Interrupt

In response to the interrupt, the routine or
program, which is running presently
interrupts and an interrupt service routine
(ISR) executes.

2008 3

Interrupt Service Routine

ISR is also called device driver in case of
the devices and called exception or signal
or trap handler in case of software
interrupts

2008 4

Interrupt approach for the port or device
functions
Processor executes the program, called
interrupt service routine or signal handler
or trap handler or exception handler or
device driver, related to input or output
from the port or device or related to a
device function on an interrupt and does
not wait and look for the input ready or
output completion or device-status ready
or set
2008 5

Hardware interrupt
Examples─ When a device or port is ready,
a device or port generates an interrupt, or
when it completes the assigned action or
when a timer overflows or when a time at
the timer equals a preset time in a compare
register or on setting a status flag (for
example, on timer overflow or compare or
capture of time) or on click of mice in a
computer
Hardware interrupt generates call to an ISR

2008 6

Software Interrupt
Examples
When software run-time exception condition (for
examples, division by 0 or overflow or illegal
opcode detected) the processor-hardware
generates an interrupt, called trap, which calls an
ISR
When software run-time exception condition
defined in a program occurs, then a software
instruction (SWI) is executed─ called software
interrupt or exception or signal, which calls an
ISR .

2008 7

Software Interrupt
When a device function is to be invoked, for
example, open (initialize/configure) or read
or write or close , then a software
instruction (SWI) is executed─ called
software interrupt to execute the required
device driver function for open or read or
write or close operations.

2008 8

Interrupt
Software can execute the software instruction
(SWI) or Interrupt n (INT n) to signal execution of
ISR (interrupt service routine). The n is as per the
handler address.
Signal interrupt [The signal differs from the
function in the sense that execution of signal
handler (ISR) can be masked and till mask is reset,
the handler will not execute on interrupt. Function
on the other hand always executes on the call after
a call-instruction.]
2008 9

Interrupt Driven Actions

On Interrupt Interrupt Service
Running Routine (ISR)
Program Return

No continuous checks of device status

2008 10

How does call to ISR differ from a function
(routine) call?

2008 11

Routine (function) and ISR call features
On a function call, the instructions are
executed from a new address
Execute like in as function in the ‘C’ or in a
method in Java.
ISR also the instructions are executed from
a new address like in as function in the ‘C’
or in a method in Java

2008 12

Routine (function) call and ISR call features
A function call is after executing present
instruction in any program and is a planned (user
programmed) diversion from the present
instruction in sequence of instructions to another
sequence of instructions; this sequence of
instructions executes till the return from that.
An ISR call is after executing present instruction
in any program and is interrupt related diversion
from the current sequence of instructions to
another sequence of instructions; this sequence of
instructions executes till the return from that or till
another interrupt of higher priority
2008 13

Nesting of function calls and ISRs
Nesting the function-calls ─ When a
function 1 calls another function 2 and
that call another function 3, on return
from 3, the return is to function 2 and
return from 2 is to function. Functions
are said to be nested
The ISR calls in case of multiple interrupt
occurrences can be nested or can be as per
priority of the interrupts
2008 14

Using the Interrupt(s) and ISR(s) for
each device function (for example, read,
write)

- Use of ISRs (Interrupt Service
Routine) by SWIs is the main
mechanism used for the device
accesses and actions

2008 15

Interrupt driven IO and device accesses
feature
1. There is no continuous monitoring of the
status bits or polling for status by the
application.
2. Between two interrupt calls the program
task(s) continue. There are many device-
functions, and each of which executes on
a device interrupt.

2008 16

Example─ A 64-kbps UART Input
When a UART transmits in format of 11-bit
per character format, the network transmits
at most 64 kbps ÷ 11 = 5818 characters per
second, which means every 171.9 µs a
character is expected.
Before 171.9 µs, the receiver port is not
checked. Only on interrupt from the port the
character is read
There is no in-between time-gap for polling

2008 17

Ports A and B with interrupt generation and
interrupt service (handling) mechanism

Port A be at in a PC, and port B be its
modem input which puts the characters on
the telephone line.
Let ISR_ PortA_Character be a routine that
receives an input character from Port A on
interrupt and Out_B routine re-transmits an
output character to Port B.

2008 18

ISR_ PortA _Character
Step f function (vi): Read Port A character.
Reset the status bit so that modem is ready
for next character input (generally resetting
of status bit is automatic without need of
specific instruction for it). Put it in memory
buffer. Memory buffer is a set of memory
addresses where the bytes (characters) are
queued for processing later.
Return from the interrupt service routine.
2008 19

ISR_ PortA _Character

Current program

On interrupt call ISR_ PortA _Character
Step f
Save PC, status word and registers (current
program context) on stack
Execute function vi codes (read character,
reset port status bit, character save in
memory buffer)
ReturnChapter-4 L02: "Embedded Systems - " , Raj Kamal,
(retrieve the saved context)
2008 20

Out_B routine
• Step g: Call function (vii) to decrypt the
message characters at the memory buffer
and return for next instruction step h.
• Step h: Call function (viii) to encode the
message character and return for next
instruction step k
• Step k: Call function (ix) to transmit the
encoded character to Port B
• Return from the function.
2008 21

Out_B Routine
Out_B Routine
Step g

Step h

Step k

Each step has three parts, save context, execute
codes, and retrieve the context for return

2008 22

Application of Interrupt based IOs and device
functions

In case of multiple processes and device
functions with no device status polling or
wait

2008 23

An automatic chocolate vending machine
(ACVM)
Without interrupt mechanism, one way is the
‘programmed I/O- transfer in which that the
device waits for the coin continuously, activates
on sensing the coin and runs the service routine
In the event driven way is that the device should
also awakens and activates on each interrupt after
sensing each coin-inserting event.

2008 24

Interrupt-service routine (ISR) or device
driver function
The system awakens and activates on an
interrupt through a hardware or software
signal. The system on port interrupt collects
the coin by running a service routine. This
routine is called interrupt handler routine for
the coin-port read.

2008 25

Use of ISR in the ACVM example
1 2 ISR_CoinRead
Coin
Coin event event Call Read port P0-
Event: Coin input P3 bits
insertion, processor Reset port
sorting and PA0- at a Data bus
status
coin amount PA3 device Save amount at
(1 or 2 or 5 a memory
or 10) sent Return buffer
to an input 4 Signal a task
port PA) 3
Signal AVCM
System ISRs and
Tasks
2008 26

Digital camera system
Has an image input device.
The system awakens and activates on interrupt
from a switch.
When the system wakes up and activates, the
device should grab an image frame data.
The interrupt is through a hardware signal from
the device switch. On the interrupt, an ISR (can
also be considered as camera’s imaging device-
driver function) for the read starts execution, it
passes a message (signal) to a function or program
thread or task

2008 27

Image sense and device frame-buffer
The thread senses the image and then the
function reads the device frame-buffer
(called frame grabbing),
then the function passes a signal to another
function or program thread or task to
process and compress the image data and
save the image frame data compressed file
in a flash memory

2008 28

Use of ISR in the digital camera example
Interrupt
Shoot start 1 Keyed 2 ISR_FrameRead
Start ADC scan
event events Call Signal task read
Event: Key input frame status and
pressed to processor data of all pixels
take a PA0- at camera Data bus of image frame at
picture and PA3 6
CCD co-processor
Signal task for
pressed key saving pixels data
code sent at Return
at a frame memory
port buffer
Signal a CCD co-
4
processor task for
3 subtracting offsets
To CCD pixels ADC scan
4 5
Signals camera System Tasks
2008 29

Interrupt through a hardware signal from a
print-switch at device

Camera system again awakens and activates
on.
System on interrupt then runs another ISR.
The ISR Routine is the device-driver write-
function for the outputs to printer through
the USB bus connected to the printer.

2008 30

Mobile phone system
Has a system reset key, which when pressed resets
the system to an initial state.
When the reset key is pressed the system
awakens and activates a reset interrupt through a
hardware signal from the reset key.
On the interrupt, an ISR (can also be considered as
reset-key device-driver function) suspends all
activity of the system, sends signal to display
function or program thread or task for displaying
the initial reset state menu and graphics on the
LCD screen, and also activates LCD display-off
timer device for 15 s timeout (for example).
2008 31

After the timeout
The system again awakens and activates on
interrupt through internal hardware signal
from the timer device and runs another ISR
to send a control bit to the LCD device.
The ISR Routine is the device-driver LCD
off-function for the LCD device. The
devices switches off by reset of a control bit
in it.

2008 32

Use of ISR in the mobile phone reset key interrupt example
Interrupt
Reset event 1 Mobile 2 ISR_ResetKey
Read key state at
Keypad Call Port bits P0-P7
Event: events Reset key state
Reset Key input status bit
pressed to P0-P7 processor Data bus Sent message for
at mobile tasks
reset the 5 Signal timer start
mobile for enabling display
Return
off after 15 s (preset
time)
4

4 3
Signal timer Tasks Sent message
for Tasks
2008 33

Summary

2008 34

We learnt
Interrupt means event, which invites attention of
the processor for some action on hardware or
software interrupt instruction (SWI) event.
When software run-time exception condition is
detected, either processor hardware or an SWI
generates an interrupt─ called software interrupt
or trap or exception.
An embedded system executes many
actions or functions or tasks on the
interrupts from hardware or software.

2008 35

We learnt
An ISR call due to interrupt executes an
event. Event can occur at any moment and
event occurrences are asynchronous.
Event can be a device or port event or
software computational exceptional
condition detected by hardware or
detected by a program, which throws the
exception. An event can be signaled by
soft interrupt instruction in routine.
An interrupt service mechanism exists in a
system to call the ISRs from multiple
sources.
2008 36

We learnt

Interrupts and interrupt-service routines
(device-drivers) play the major role in using
the embedded system hardware and devices.
Interrupt examples of UART IO, ACVM,
Camera and mobile phone

2008 37

End of Lesson 2 of Chapter 4

2008 38

Chap 4 lesson02emsysnewinterruptbasedi_os

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