Embedded Systems Design Europe - May 2008 - (Page 16)

cover feature Listing 2 How processTasks() might be implemented. loop through tasks if (cooperative_Tasks[i].ready > 0) { returnVal = (*cooperative_Tasks[i].pTask) (cooperative_Tasks[i].param); cooperative_Tasks[i].ready--; if (cooperative_Tasks[i].reload == 0) cooperative_deleteTask (i); } Listing 3 Example of a very simple Protothreads “thread.” Static PT_THREAD(example(struct pt *pt)) { PT_BEGIN(pt); while(1) { PT_WAIT_UNTIL(pt, counter == 1000); printf(“Threshold reached\n”); counter = 0; } PT_END(pt); } Task execution is performed outside of ISRs, which means all threads run in the same context minimizing needs for concurrency control mechanisms. As all of the scheduler code is written in ANSI C and the only hardware dependency is the tick, one can quickly see how easy it would be to port this to a new target. Although an excellent explanation of Protothreads internals can be found on the Protothreads website at www. sics.se/~adam/pt/, minimal information is needed to get up and running quickly. Protothreads are stackless threads that provide conditional blocking. The conditional blocking is accomplished using what some have called a “dodgy” feature of C: many C compilers, including GNU, allow for a nonconventional switch/case construction; a while loop placed in the middle of the switch. Listing 3 and 4 shows the breakdown example. Notice how the PT_BEGIN simply maps to the beginning of the switch/case: #deﬁne PT_BEGIN(pt) switch(pt->lc) { case 0: switch/case, cleans up, and returns control to the scheduler. #deﬁne PT_END(pt) >lc = 0; return 2; } pt- (Source for Listing 3 and 4: www.sics.se/~adam/pt/expansion.html) Each time the function or “thread” is called, execution will resume exactly where the thread blocked. The location is determined from the line number at compilation time. See the expansion of PT_WAIT_UNTIL: #deﬁne PT_WAIT_UNIT(pt, c) pt->lc = __LINE__;case __LINE__:\ If (!(c)) return 0 Finishing up with PT_END closes the 16 Creation of a thread usListing 4 The simple thread with ing Protothreads is simple but macros expanded. requires a few steps as shown in Static the code example in Listing 5. char example(struct pt *pt) Each thread must be declared { switch(pt->lc) { case 0: using the PT_THREAD macro. Each thread must receive a while(1) { pt struct point as a parampt->lc = 12; case 12: if(!(counter == 1000)) return 0; eter. Each thread also requires printf(“Threshold reached\n”); PT_INIT(&pt) to be invoked to counter = 0; conﬁgure the context the ﬁrst } time. } pt->lc = 0; return 2; A PT_BEGIN(pt) is re} quired at the beginning of the thread and a PT_END(pt) is required at the very end. In the virtual concurrency and the cooperamiddle of the thread the implementer tive scheduler used to allow task execuhas a variety of macros to choose from tion to be scheduled. to cause the thread to yield. Listing 6 shows a complete blendThe complete application proing of an application using Protogramming interface, examples, more threads and the cooperative scheduler. implementation details can all be The task control block in this case (pt) found on the Protothreads website. Several threads can be created and is deﬁned by Protothreads and still executed. A simple main program for needs to be initialized before use, by the example above could be like the one invoking the PT_INIT macro. After shown in Listing 5. that, the tasks are created exactly as The inﬁnite loop in main is continexplained above in the cooperative uously executing all of the threads. Each scheduler. time a thread is reentered from main, The tasks will run just ﬁne using code execution will begin from where Protothreads alone, but by adding the the thread had previously yielded, just cooperative scheduler, higher prioras one would expect. Notice how the ity threads can be created that execute data is stored in global variables. more often than lower priority. Both the cooperative scheduler and Since scheduling is based and ocProtothreads can be useful for a variety curs on a tick, a device can even go into of jobs, with Protothreads providing sleep mode and conserve battery life. MAY 2008 | embedded systems design europe | www.embedded.com/europe http://www.sics.se/~adam/pt/expansion.html http://www.sics.se/~adam/pt/ http://www.sics.se/~adam/pt/ http://www.embedded.com/europe

Table of Contents Feed for the Digital Edition of Embedded Systems Design Europe - May 2008

Embedded Systems Design Europe - May 2008 Contents Microsoft Provides Embedded Roadmap Enea Buys Developers Irish Start-Up Raises Funds for Telecom FPGAs Kontron Promotes COM Express Nano Mentor Nucleus Platform Provides UI for Atmel Small Form Factor Boards Head for the SUMIT Proffibus Advances IO-Link Integration Embedded Developers Cautious on Multicore Auto Cooperation Improves Test Altera Launches DO-254 Partner Network Building an ‘Instant-Up’ Real-Time Operating Systems An Architecture for Reusable Embedded Systems Software Free up Bandwidth in PCI Express Evaluating Software in Medical Devices Circuit Sensitivity in Analog Circuits Choosing Flash Memory New Products Advertising Contacts

Embedded Systems Design Europe - May 2008

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