This is a free module. Its free for use in universities and institutes on condition of prior notification. Please, do not use it for commercial purposes. If you need full set of accompanying labs and software tool feel free to contact me by email (amraldo@hotmail.com) or by mobile (+201223600207).

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Embedded Software
Architectures
Prerequisite(s)
 Programming in any high-level programming language preferably C
 Knowledge of µ-controller programming is a plus
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Notes
 Ask any time.
 Cell phones silent, please!
 Certificates are not for attendance.
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Reference(s)
 The slides are compilation of books below.
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License
 Universities and colleges are allowed to use it on condition that we are
informed ahead and we issued a written approval.
 Please, do not use in any commercial activity without written permission
from:
 Swift Act Servces
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Outline
 Basic Definitions
 Cyclic Executives
 Interrupt-Driven Executives
 Preemptive Executives
 Conclusion
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System
 A system processes inputs and generates outputs.
 For every input, we need to determine:
 Occurrence (periodic or aperiodic)
 Response time (fixed or bounded)
 Turnaround time (fixed or bounded)
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Event
Time
Response
Time
Turnaround time
Result
Real-Time System = Correct Functions
@Correct Time
 They can be:
 Hard: Correct time is unnegotiable
 Soft: Correct time can be tolerated
 Factors determining real- time behavior of a system are:
 Determinism (predictability)
 Responsiveness
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Importance of Real-Time in
Systems
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Multitasking
 Executing separate tasks (seemingly) simultaneously
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Multitasking Types
 Non-preemptive multitasking
 Pre-emptive multitasking
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Task = C Function + Timing C/C’s
 Execution Time (E)
 Deadline (D)
 Period (P)
 Blockage (B)
 For a task to be schedulable:
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Embedded Code
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Multitasking Requirements
Multitasking
Context Switching
Scheduling
Intertask Interaction
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Exercise: Compare Multitasking Requirements
from Multitasking Types PoV
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Preemptive Non-Preemptive
Context Switching
Scheduling
Intertask Interaction
Case Study: Digital Clock
 The digital clock displays time the
format “HH.MM”.
 Only 24 hours format
 HH is hours and MM is minutes.
 The “.” is used to separate hours
from minutes.
 The digital clock is adjustable.
 User can adjust hours or
minutes.
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Exercise 1: Identify Needed Tasks
 What are the needed tasks to implement the digital clock?
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Cyclic Executive - The Super Loop
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µController
CPU IO 1
IO 2
IO N

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Exercise 2: The Super Loop
 Write the digital clock in the form of a super loop.
 Identify the timing c/c’s of your tasks from your code.
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Multi-Rate Cyclic Executive – The
Super Loop
 Not all tasks running @ same rate.
 Some tasks need higher rate.
 Integer multiple of base rate
 Major cycle/Minor cycle(s)
 Major Cycle = 1 or more Minor cycle(s)
 If we have a set of tasks with periods P1, P2, ... Pn then:
 Major cycle = LCM (P1, P2, ... Pn)
 Minor cycle = HCF (P1, P2, ... Pn)
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Switch Driver Design
 Sampling switch state periodically where period > de-bounce period (~20
ms)
 According to the state of the latest 3 samples, the switch state stored can
be changed into one of:
1. Released
2. Pre-Pressed
3. Pressed
4. Pre-Released
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Switch Voltage
Level
Sampling Pulses
7 Segment Display Driver Design
 Changing port data periodically
 Rate of change of all all SSD is > 20 Hz and less than 50 Hz
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Exercise 3: The Super Loop Revisited
 Calculate the periods of your tasks .
 Identify minor cycle and major cycle lengths.
 Draw system timeline.
 Rewrite your super loop to achieve this timeline.
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Cyclic Executive - The Super Loop
Revisited
 Pros
 Simple
 Minimal HW resources
 Highly portable
 Cons
 Inaccurate timing
 High power consumption
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Time-Triggered Cyclic Executive (sEOS)
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Time-Triggered Cyclic Executive (sEOS)
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Features of sEOS
 Can run tasks @ specified rates
 ISR’s are easy to manage in high-level language.
 Framework to develop embedded systems
 Puts the processor “to sleep” between tasks:
 To conserve power
 To reduce task jitter
 Easily portable to other controllers
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sEOS Library
 sEOS_Init()  Initializes sEOS with needed tick rate
 sEOS_GoToSleep()  Puts the processor to sleep when nothing to be
done
 sEOS_ISR()  Called periodically to does what the system should do
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The Super Loop Refactored
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The Super Loop – Re-factored
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Schedulability Check for sEOS
 P = HCF (T1, ..., TM)
 TICK = HCF (P1, ..., PN)
 Sum (E1, ..., EZ) in a tick < TICK over a Major Cycle = LCM (P1, ..., PN)
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Exercise 4: sEOS
 Rewrite your super loop and sEOS_ISR to achieve your timeline.
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Interrupt-Driven Executive -
Foreground/Background
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µController
CPU IO 1
IO 2
IO N
Interrupt-Driven Executive -
Foreground/Background cont’d
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Interrupt-Driven Executive -
Foreground/Background cont’d
 Pros
 No upfront cost
 Minimal training required
 No need to set aside memory resources to accommodate RTOS
 Cons
 Difficult to ensure that each operation will meet its deadline
 High-priority code must be placed in the foreground
 Sharing data is not OK!
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Exercise 5: Interrupt-Driven Executive
 What are the needed ISRs?
 What are the needed tasks?
 What are the needed communication?
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Preemptive Executive - RTOS
 RT = Correct function @ Correct
time
 OS = HW + SW manager
 RTOS = HW + SW manager that
can help us ensure having correct
function @ correct time
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µController
CPU IO 1
IO 2
IO N
RTOS
RTOS = Kernel + …
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Application
RTOS
Hardware
File System
GUI
TCP/IP
USB
Bluetooth
RS-232
Kernel

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Kernel
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Objects
Services
Scheduler
Kernel
Scheduler
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Algorithm
Dispatcher
Scheduler

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RTOS Scheduling
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Interrupt-level
scheduling
Task-level
scheduling
RTOS Benefits
 Developers who use RTOS are freed from implementing a scheduler and
related services
 Typically applications that incorporate RTOS are much easier to expand
 The best RTOS have undergone thorough testing
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Exercise 6: RTOS
 How will you change the interrupt-driven implementation to use RTOS?
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Conclusion
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Responsiveness
Determinism
Time-
Triggered
Cyclic
Executive
Interrupt-
Driven
Executive
RTOS

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Contact Us
 https://www.facebook.com/groups/EmbeddedSystemsTraining/
 https://www.youtube.com/channel/UC5NTpGcnlx3XwGAlFaYJnBg
 www.swift-act.com
 training@swift-act.com
 (+2)0122-3600-207
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