Real-Time Concepts for Embedded Systems Author: Qing Li with Caroline Yao

Transcription

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-highprecision 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 Real-Time Clock 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 System Clock 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 softtimer 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 applicationlevel, 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 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 Priority-Based, Task- Scheduling Delays

32 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 Maintaining Soft Timers

34 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 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 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 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 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 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