Interrupt and Time Management in µC/OS-III

Akos LedecziEECE 6354, Fall 2015Vanderbilt University

Interrupt Basics• Context• Interrupt Service Routine (ISR)• Enable/Disable• Nesting• Interrupt disable time• Interrupt response: time between interrupt and start of

user ISR execution• Interrupt recovery: time required to return to interrupted

code (or higher priority task)• Task latency: time to return to task level code

Interrupt Handling• Interrupt controller handles many issues including

providing the address of the required ISR in many cases• Two models:

– All interrupts vector to a single interrupt handler– Each interrupt vector directly to its own ISR

µC/OS-III ISR Pseudo CodeMyISR:

Disable all interrupts;Save CPU registers;OSIntNestingCtr++;if (OSIntNestingCtr == 1)

OSTCBCurPtr->StkPtr = current task’s CPU stackpointer register value

Clear interrupting device;Re-enable interrupts (optional);Call user ISR;OSIntExit();Restore CPU registers;Return from interrupt;

• Written in assembly• Context: some CPUs do this automatically• Some CPUs have separate stack (and SP) for interrupts (µC/OS-III port can implement this in software

saving space on all task stacks)• User code typically needs to call one of the Post() functions• Must call OSIntExit()

ISR Prologue

ISR Epilogue

Short ISRMyShortISR:

Disable all interrupts;Save necessary CPU registers;Clear interrupting device;DO NOT re-enable interrupts;Call user ISR;Restore saved CPU registers;Return from interrupt;

• When no Post() call is needed• No OSIntExit() call is needed• Should be the exception as the OS does not know about this at

all…

One Common Master ISR

• The while loop above can be part of a user ISR written in C• Interrupt controller may provide an actual address of the required ISR or just an index that

can be used to get the address from a table in memory• ISR needs to look like this (one for each kind of interrupt):

void MyISRHandler(void)

• Note that there is no nested interrupts even though it does handle multiple interrupts, so ISR Prologue and Epilogue need to be called only once

ISR Prologue;while (there are still interrupts to process) {

Get vector address from interrupt controller;Call interrupt handler;

}ISR Epilogue;

Direct PostInterrupt Latency =

maximum interrupt disable time;

Interrupt Response = interrupt latency +vectoring to ISR +ISR prologue;

Interrupt Recovery =handling of device +Post() + ISR epilogue;

Task Latency = interrupt response +interrupt recovery +scheduler lock time

Deferred PostInterrupt Latency =

maximum interrupt disable time;

Interrupt Response = interrupt latency +vectoring to ISR +ISR prologue;

Interrupt Recovery =handling of device +Post() + ISR epilogue;

Task Latency = interrupt response +interrupt recovery +re-issue Post() +context switch to task +scheduler lock time

Direct vs. Deferred

Tick Handling

• Hook is called first to give the application more precise timing• Your low power app may not need tick processing at all: it is OK, but no

time delays and timeouts can be used.

void OS_CPU_SysTickHandler (void) /* ISR */{

CPU_CRITICAL_ENTER();OSIntNestingCtr++; /* Tell uC/OS-III that we are starting an ISR */CPU_CRITICAL_EXIT();

OSTimeTick(); /* Call uC/OS-III's OSTimeTick() */ OSIntExit(); /* Tell uC/OS-III that we are leaving the ISR */}

void OSTimeTick(void){

OSTimeTickHook();#if OS_CFG_ISR_POST_DEFERRED_EN > 0u

Get timestamp;Post “time tick” to the interrupt queue;

#elseSignal the Tick Task;Run round robin scheduling if enabled;Signal the Timer Task;

#end}

Pend Lists• The first few fields are the same: OS_PEND_OBJ• Type: four characters for easy debugging• Head and Tail pointer point to OS_PEND_DATA and not TCB

Pend Lists cont’d.

• Prev and Next: doubly linked list

• TCBPtr: corresponding TCB• PenObjPtr: object task is

pending on• Rest: only needed when

pending on multiple objects

Pend List example

Time Management: Relative Delay

• OS_OPT_TIME_DELAY• Timing is not precise: – The only thing guaranteed is that the task will be

delayed at least: ((ticks – 1) * tick-period)

Absolute Delay

• Regardless of CPU load, the task remains “synchronized”: for example, it will be called 100 times during 1000 ticks. In relative mode under heavy CPU load, it may miss ticks, so it might get called less than 100 times. A time-of-day clock may be “late” if relative mode is used.

Void MyTask(void *p_arg){

OS_ERR err;OS_TICK current;::current = OSTimeGet();while (1) {current = current + 10;OSTimeDly(current, OS_OPT_TIME_MATCH, &err);if (err == OS_ERR_NONE) {/* code resumes here after waiting for 10 ticks or resumed by another task */}else {/* some error in Dly function call */}}

}

Time Management

• OSTimeDlyHMSM() only works in relative mode– OS_OPT_TIME_HMSM_STRICT : only valid ranges– OS_OPT_TIME_HMSM_NON_STRICT: hr: 0-999, sec: 0-

9999• OSTimeDlyResume()• OSTimeSet()• OSTimeGet()

Timers• Counters that perform an action via a callback function when

0 is reached• OS_CFG_TMR_EN• Timer rate is typically (much) lower than tick rate• Callback is run from the context of the Timer Task: must not

make blocking calls

void OSTmrCreate (OS_TMR *p_tmr, CPU_CHAR *p_name, OS_TICK dly, /* initial delay */ OS_TICK period, /* repeat period */ OS_OPT opt, OS_TMR_CALLBACK_PTR p_callback, void *p_callback_arg, OS_ERR *p_err)

One-shot Timers

• Can be retriggered• Can be used to implement

watchdog timers

Periodic Timers

Timer States

Timing

• Tick ISR signals Timer Task at the timer rate• Timer Task eventually runs and calls callback

functions of all timers that have expired

Timer wheel• The exact same concept as the tick wheel: only a fraction

of the timers need to be looked at at any one time