1. Akos Ledeczi EECE 6354, Fall 2017 Vanderbilt University
Synchronization
Akos Ledeczi
EECE 6354, Fall 2017
Vanderbilt University

2. Semaphores They work for synchronization too ISR to Task or
Task to Task

3. Unilateral Rendezvous
Task calling Pend() does not need to know about the details: the Pend() call will return when the event has occurred
Maximizes use of CPU; no busy waiting
Event (i.e., Post()) will cause the waiting task to run immediately (assuming it has the highest priority)

4. Credit Tracking
Semaphore accumulates the number of times Post() has been called /without a corresponding Pend()/

5. Broadcasting Multiple tasks waiting on a semaphore OS_OPT_POST_ALL
All waiting tasks become ready to run
Care must be taken because some of the tasks may not have called Pend() yet

6. Task Semaphores In µC/OS-III each task has a built-in semaphore
More efficient
OSTaskSemPend(); OSTaskSemPost(); etc.
Post()-er (ISR or Task) needs to know which task to notify

7. Bilateral Rendezvous Only between tasks, as an ISR cannot Pend()
void Task1(void *p_arg) { … OSTaskSemPost(&Task2_TCB, OS_OPT_POST_1, &err); OSTaskSemPend(0,OS_OPT_PEND_BLOCKING,&ts,&err); } void Task2(void *p_arg) OSTaskSemPost(&Task1_TCB,
Only between tasks, as an ISR cannot Pend()

8. Event Flags When tasks need to synchronize on multiple events
When any of the events is enough: disjunctive synchronization (OR)
When all events are needed: conjunctive synchronization (AND)
OSFlag???();
OSFlagPendGetFlagsRdy(): which flags caused task to become ready

9. EventFlags Example #define TEMP_LOW (OS_FLAGS)0x01
#define BATT_LOW (OS_FLASG)0x02
#define SW_PRESSED (OS_FLAGS)0x04
OS_FLAG_GRP MyEventFlagGrp;
void main() { …
OSFlagCreate(&MyEventFlagGrp, “My flags”,0,&err); }
void Task1(void *p_arg)
for( ; ; )
OSFlagPend(&MyEventFlagGrp,
TEMP_LOW + BATT_LOW,
OS_OPT_PEND_FLAG_SET_ANY,
ts,&err);
void Task2(void *p_arg) { …
for ( ; ; )
OSFlagPost(&MyEventFlagGrp,
BATT_LOW,
OS_OPT_POST_FLAG_SET,
&err); }

10. Status and Transient Events
Typically:
Status info monitored by one task, handled by another task using a non-blocking call
A transient event is generated by an ISR or task and handled and consumed by a task using a blocking call

11. Event Flag Internals
struct os_flag_grp { OS_OBJ_TYPE Type CPU_CHAR *NamePtr OS_PEND_LIST PendList; OS_FLAGS Flags CPU_TS TS }; OSFlagPost(&flagGrp, (OS_FLAGS)0x0C, OS_OPT_FLAG_SET, &err);
Pend list is not in priority order as all tasks need to be looked at anyways
Look at code on the left:
If OS_OPT_FLAG_SET and flags contain 0x03, result will be 0x0F
If OS_OPT_FLAG_CLR and flags contain 0x0F, result will be 0x03

12. Synchronizing Multiple Tasks