1. Real-Time Concepts for Embedded Systems
Author: Qing Li with Caroline Yao
ISBN:
CMPBooks

2. Chapter 11 Timer and Timer Services

3. Outline 11.1 Introduction 11.2 Real-Time Clocks and System Clocks
11.3 Programmable Interval Timers
11.4 Timer Interrupt Service Routines
11.5 A Model for Implementing the Soft-Timer Handling Facility
11.6 Timing Wheels
11.7 Soft Timers and Timer Related Operations

4. 11.1 Introduction
System tasks and user tasks often schedule and perform activities after some time has elapsed. For example:
Scheduler
Software-based memory refresh mechanism
Communication protocols schedule activities for data retransmission and protocol recovery
Scheduling future activities is accomplished through timers using timer services

5. Introduction (Cont.) Timer
Scheduling of an event according to a predefined time value in the future, similar to setting an alarm clock
Most embedded systems use two different forms of timers to drive time-sensitive activities:
Hard timers and soft timers

6. Two Different Forms of Timers
Hard timers
Derived from physical timer chips that directly interrupt the processor when they expire
Operations with demanding requirements for precision or latency
Soft timers
Software events that are scheduled through a software facility
Allows for efficiently scheduling of non-high-precision software events

7. Soft Timers
A practical design for the soft-timer handling facility should have the following properties:
Efficient timer maintenance, i.e., counting down a timer
Efficient timer installation, i.e., starting a timer
Efficient timer removal, i.e., stopping a timer
Reasons for using soft timers
Applications requiring timeouts with course granularity
TCP module, RTP (Real-Time Transport) Protocol module, ARP module
To reduce system-interrupt overhead

8. Clocks Used in an Embedded System
Real-time clock (RTC)
Track time, date, month, and year
System clock
Track either real-time or elapsed time following system power up
Programmable interval timer (PIT)
Drives the system clock, i.e. the system clock increments in value per timer interrupt

9. 11.2 Real-Time Clocks and System Clocks
Track time, date, month, and year
Integrated with battery-powered DRAM
As shown in the next slide
Thus, RTC is independent of the CPU and the programmable interval timer
Make the maintenance of real time between system power cycles possible

10. A Real-Time Clock

11. 11.2 Real-Time Clocks and System Clocks
A software clock
Track either real-time or elapsed time following system power up
The initial value of the system clock is retrieved from the real-time clock at power up
The programmable interval timer then drives the system clock
The system clock increments in value per timer interrupt

12. System Clock Initialization

13. 11.3 Programmable Interval Timers
Programmable interval timer (PIT), also known as the timer chip
The functionality of the PIT is commonly incorporated into the embedded processor
Thus, often called an On-chip timer
However, dedicated stand-alone timer chips may also available
To reduce processor overhead

14. Programmable Interval Timers (Cont.)
Timer chips
Feature an input clock source with a fixed frequency, as well as a set of programmable timer control registers
Timer interrupt rate
Number of timer interrupts generated per second
Timer countdown value
Determines when the next timer interrupt occurs
Loaded in timer control registers and decremented by one every input clock cycle

15. Timer-Chip Initialization
Resetting the timer chip into a known hardware state.
Programming timer interrupt frequency into the appropriate timer control register.
Programming other timer control registers with correct values.
Dependent on the timer chip
Programming the timer chip with the proper mode of operation.
e.g., periodic timer interrupt is used or one-shot timer
Installing the timer interrupt service routine.
Enabling the timer interrupt.

16. Programmable Interval Timers (Cont.)
Timer interrupt rate
Number of timer interrupt occurrences per second
Set by the timer interrupt-rate register (TINTR)
Each interrupt is called a tick, which represents a unit of time.
e.g., if the timer rate is 100 ticks, each tick represents an elapsed time of 10 milliseconds.

17. 11.4 Timer Interrupt Service Routines
Updating the system clock: both the absolute time and elapsed time are updated
Absolute time: time kept in date, hours, minutes, and seconds
Elapsed time: usually kept in ticks and indicates how long the system has been running since power up
Calling a registered kernel function to notify the passage of a preprogrammed period
Acknowledging the interrupt, reinitializing the necessary timer control register(s), and returning from interrupt

18. Steps in Servicing the Timer Interrupt

19. 11.5 A Model for Implementing the Soft-Timer Handling Facility
Functions performed by the soft-timer facility, called the timer facility, include:
Allowing applications to start a timer
Allowing applications to stop or cancel a previously installed timer
Internally maintaining the application timers

20. 11.5 A Model for Implementing the Soft-Timer Handling Facility (Cont.)
The soft-timer facility is comprised of two components
One lives within the timer tick ISR
The other lives in the context of a task
Why?
If all of the soft-timer processing is done with the ISR
The timer tick event might be lost since the execution of ISR takes too much time

21. Soft-Timer Handling Facility
Thus, the timer tick handler must be short and must be conducting the least amount of work possible.
Processing of expired soft timer is delayed into a dedicated processing task
Because applications using soft timers can tolerate a bounded timer inaccuracy.

22. Soft-Timer Handling Facility
A workable model for implementing a soft-timer handling facility:
Processing of expired soft timers is delayed into a dedicated processing task (called work task in the text book)
In conjunction with the system timer ISR

23. Example An application requires three soft timers
Timeout values: 200 ms, 300 ms, 500ms
The least common denominator is 100 ms
Hardware timer tick: 10ms
100ms: countdown value of 10
Thus, the ISR decrements the countdown value by one during each invocation
If reach to zero, the ISR wake up the worker task
Reinitialize the countdown value back to 10

24. Example (Cont.)
Worker task must maintain an application-level, timer-countdown table based on 100ms granularity
Three countdown values: 2, 3, and 5
An application-installed, timer-expiration function is associated with each other

25. A model for Soft-Timer Handling Facility

26. Servicing the timer interrupt in the task context

27. A model for Soft-Timer Handling Facility (Cont.)
A single ISR-level timer drives three application timers at the task-level
Decrease in the number of ISR timers installed
Improves overall system performance
Application-installed timers are called soft timers

28. 11.5.1 Possible Processing Delays
An ISR must perform the smallest amount of work possible.
Typical implementations perform real work
Either inside a worker task that is a dedicated daemon task
Or within the application that originally installed the timer

29. Possible Processing Delays (Cont.)
First level of delay
The event-driven, task-scheduling delay
Second level of delay
The priority-based, task-scheduling delay
Third level of delay
Introduced when an application installs many soft timers
Introduced later

30. Level 1 Delays- Timer Event Notification Delay

31. Level 2 Delays-Priority-Based, Task-Scheduling Delays

32. 11.5.2 Implementation Considerations
A soft-timer facility should be efficient in
Timer insertion, timer deletion and cancellation, and timer update
The timer list may be implemented as a double- linked list
Fig. 11.8

33. Fig. 11.8 Maintaining Soft Timers

34. 11.5.2 Implementation Considerations (Cont.)
If the timer list is not sorted
Maintaining timer ticks can prove costly
Timer installation can be performed in constant time
Timer cancellation and timer update require O(N) in the worst case

35. Unsorted Soft Timers

36. 11.5.2 Implementation Considerations (Cont.)
Sorting expiration times in ascending order results in efficient timer bookkeeping
Timer installation requires O(log(N))
Timer cancellation is also O(log(N))
Timer update require constant time
Only the first entry update is necessary

37. Sorted Soft Timers

38. 11.6 Timing Wheels Timing wheel A construct with a fixed-size array
Each slot represents a unit of time with respect to the precision of the soft-timer facility
Within each slot, a doubly linked list of timeout event handlers is stored and invoked on timer expiration
Advantage:
Has the advantage of the sorted timer list for updating the timers efficiently
Provides efficient operations for timer installation and cancellation

39. Timing Wheel

40. Timeout Event Handlers

41. Installing a Timeout Event
When installing a new timer event
The current location of the clock dial is used as the reference point to determine the time slot in which the new event handler will be stored
Example
When the developer want to schedule a 200 ms timeout in the feature
The time slot marked +200 is the time slot in which to store an event handler

42. Installing a Timeout Event

43. Issues
First issue: the number of slots in the timing wheel has a limit
Approaches to deal with timing wheel overflow:
Deny installation of timers outside the fixed range
Use event overflow buffer

44. Timing Wheel Overflow Event Buffer

45. Issues Associated with the Timing Wheel Approach (Cont.)
Second issue: the precision of the installed timeouts
For example, a 150 ms timer event is being scheduled while the clock is ticking but before the tick announcement reaches the timing while
Should the timer event be added to the +150ms slot or placed in the +200ms slot?
On average, the error is approximately half the size of the tick

46. Issues Associated with the Timing Wheel Approach (Cont.)
Third issue: relates to the invocation time of the callbacks installed at each time slot
Many handler may be lined in the same time slot
The length of execution of each handler is unknown
No guarantee or predictable measures exist concerning when a callback in a later position of the list can be called
Introduces non-determinism into the system and is undesirable

47. Unbounded Soft-Timer Handler Invocation

48. 11.6.2 Hierarchical Timing Wheels
Using the hierarchical timing wheel approach can solve the timer overflow problem
Multiple timing wheels are organized in a hierarchical order.
Each timing wheel in the hierarchy set has a different granularity

49. A Hierarchical Timing Wheel

50. 11.7 Soft Timers and Timer Related Operations
Can be cataloged into three groups:
Group 1-provides low-level hardware related operations
Developed and provided by the BSP developers
Group 2-provides soft-timer-related services
Used by both the system modules and applications
Group 3-provides access either to the storage of the real-time clock or to the system clock
Used by user-level applications

51. Group 1 Operations sys_timer_enable sys_timer_disable
Enables the system timer chip interrupts
sys_timer_disable
Disables the system timer chip interrupts
sys_timer_connect
Installs the system timer ISR into the system exception vector table

52. Group 1 Operations (Cont.)
sys_timer_setrate
Sets the system clock rate as the number of ticks per second the timer chip generates
Internally, this operation reprograms the PIT to obtain the desired frequency
sys_timer_getticks
Returns the elapsed timer ticks since system power up

53. Group 2 Operations timer_create timer_delete timer_start timer_cancel
Creates a soft timer by allocating a soft-timer structure
timer_delete
Deletes a timer
timer_start
Starts a timer by installing a previously created soft timer into the timer-handling facility
timer_cancel
Cancels a timer by removing the currently running timer from the timer-handling facility

54. Group 3 Operations clock_get_time clock_set_time
Gets the current clock time from the system clock or the real-time clock
clock_set_time
Sets the system clock or real-time clock to a specified time