ECE 532 Embedded Systems Winter 2005

1
ECE 532 Embedded SystemsWinter 2005

• Session 9

2
Semaphores

• Invented the Edgser Dijkstra in the 1960s
• Use to
• control access to a shared resource (mutual
  exclusion)
• signal the occurrence of an event
• allow two tasks to synchronize their activities
• highest priority task waiting for semaphore gets
  it

OSSemPost(SharedDataSem)
OSSemPend(SharedDataSem)
3
Semaphores in uC/OS-II

• Two types
• binary
• counting (0 65,535)
• Creating a semaphore
• declare its name (global)
• OS_EVENT SampleSem
• create it prior to use (main or in task but not
  inside loop)
• SampleSem OSSemCreate(INT16U value)
• if a counting semaphore, init value to zero (it
  will track the number of times it is called)
• if value 1, binary semaphore
• Using a semaphore
• task that is waiting for signal calls
  OSSemPend(OS_EVENT event, INT16U timeout, INT8U
  err)
• OSSemPend(SampleSem, 0, err)
• task that is sending the signal calls
  OSSemPost(OS_EVENT event)
• OSSemPost(SampleSem)
• Semaphores must be enabled before use
• In file OS_CFG.H set OS_SEM_EN to 1

4
Accessing Peripherals with Semaphores

• Use binary semaphore

5
Buffer Management Using Semaphores

• Counting Semaphore
• Use when resource can be used by more than one
  task at same time
• Example10 buffers
• Buffer manager satisfies first 10 buffer requests
  directly
• When all semaphores used, task requesting buffer
  is suspended until semaphore available

6
Deadlocks and Semaphores

• Situation where two tasks are waiting for
  resource held by the other
• E.g. Task 1 has exclusive use of resource R1 and
  Task 2 has exclusive use of resource R2.
• T1 requires exclusive use of R2 and T2 requires
  exclusive use of R1
• Neither task can complete
• Avoid by
• Acquire all resources before beginning
• Acquire resources in same order
• Release resource in reverse order
• If kernel allows a timeout to be specified with
  semaphore (uC/OS-II does), this allows deadlock
  to be broken
• Deadlocks generally occur in large, multi-tasking
  systems

7
Task Synchronization

• Semaphores
• Event Flags

8
Synchronization

• Synchronize tasks with a semaphore
• Will work with ISR or another task

9
Event Flags

• Used to synchronize task with multiple event
  occurrence
• Synchronize when any of the events have occurred
• disjunctive synchronization
• effectively the logical OR of event occurrence

10
Event Flags (Cont)

• Synchronize when all of the events have occurred
• conjunctive synchronization
• effectively the logical AND of event occurrence

11
Multiple Task Signaling

• Common events may signal multiple tasks

12
Services

• OS_FLAG_EN must be set to 1 in OS_CFG.H
• Create a flag group before using it
• OSFlagCreate()
• OS_FLAG_GRP EngineStatusFlags
• OSFlagCreate(OS_FLAGS flags, INT8U err)
• flags contains the initial values to store in the
  flag group
• example
• OS_FLAG_GRP EngineStatus
• void main (void)
• INT8U err
• .
• .
• OS_INIT() //Initialize OS
• .
• . //create flag group containing engine status
• EngineStatusFlags OSFlagCreate(0x00,err)
• .
• .
• OSStart() //start multitasking

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Services (Cont)

• Set an Event Flag use OSFlagPost()
• OSFlagPost(pgrp, flags, opt, err)
• pgrp is a pointer to event flag group
• flags sets corresponding bit in the flag group
• opt determines whether set to one or zero
• OS_FLAG_SET or OS_FLAG_CLR
• err is pointer to error code
• OS_NO_ERR call successful
• OS_FLAG_INVALID_PGRP passed a null pointer
• OS_ERR_EVENT_TYPE not pointing to event flag
  group
• OS_FLAG_INVALID_OPT invalid option
• Example
• define ENGINE_OIL_PRESSURE_OK 0x01
• define ENGINE_OIL_TEMP_OK 0x02
• define ENGINE_START 0x04
• .
• .
• err OSFlagPost(EngineStatusFlags, EngineStart,
  OS_FLAG_SET, err)

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Services (Cont)

• To wait for a condition, use OSFlagPend()
• OSFlagPend(pgrp, flags, wait_type, timeout,err)
• pgrp is pointer to flag group
• flags is bit pattern indicating which bit(s) to
  check1 implies check
• wait_type specifies all or any bits
• OS_FLAG_WAIT_CLR_ALL check all bits in flags to
  be zero
• OS_FLAG_WAIT_CLR_ANY check any of bits in flags
  to be zero
• OS_FLAG_WAIT_SET_ALL check all bits in flags to
  be one
• OS_FLAG_WAIT_SET_ANY check any of bits in flags
  to be one
• consume flag by adding OS_FLAG_CONSUME (checks
  flag and clears it)
• timeout is number of ticks to wait
• err is pointer to error code
• OS_NO_ERR no error
• OS_ERR_PEND_ISR tried to pend from an ISR which
  is nono
• OS_FLAG_INVALID_PGRP passed null pointer
• OS_ERR_EVENT_TYPE not pointing to event flag
  group
• OS_TIMEOUT flags not available within specified
  amount of time
• OS_FLAG_WAIT_TYPE improper wait_type argument
• example

15
Additional Intertask Communication Methods

• Message Mailboxes
• task can place a pointer in a mailbox for another
  task to access
• kernel provides services
• Enable Set OS_Mbox_En to 1 in OS_CFG.h
• Create OSMboxCreate() create before use
• Pend OSMboxPend() task waiting for message
• Post OSMboxPost() task sending message
• Query OSMboxQuery() get information about
  message mailbox

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Additional Intertask Communication Methods

• Message Queues
• array of mailboxes
• messages may be accessed FIFO or LIFO