10/9/20011Mani SrivastavaUCLA - EE DepartmentRoom: 7702-B Boelter HallEmail: [email protected]: 310-267-2098WWW: http://www.ee.ucla.edu/~mbsCopyright 2001  Mani SrivastavaEmbedded System S/W Organization & Static SchedulingEE202A (Fall 2001): Lecture #32Copyright 2001  Mani SrivastavaReading List for This Lecture Required [Melkonian00] Melkonian, Michael. Get by without an RTOS.Embedded Systems Programming, September 2000. http://www.embedded.com/2000/0009/0009feat4.htm [Lee87] Lee, E.A.; Messerschmitt, D.G. Static scheduling of synchronous data flow programs for digital signal processing.IEEE Trans. on Computers, vol.C-36, (no.1), Jan. 1987. p.24-35. Others [Bhattacharyya99] Bhattacharyya, S.S.; Murthy, P.K.; Lee, E.A.Synthesis of embedded software from synchronous dataflow specifications. Journal of VLSI Signal Processing Systems for Signal, Image, and Video Technology, vol.21, (no.2), KluwerAcademic Publishers, June 1999. p.151-66. http://ptolemy.eecs.berkeley.edu/publications/papers/99/synthesis/10/9/200123Copyright 2001  Mani SrivastavaCourse Project Proposal Due Friday Oct 12 One page proposal describing the proposed project, and listing reference material talk to me for suggestions Individually or groups of two Project may cover any topic of interest in embedded and real-time systems need not be on a topic covered in lectures ties with your research are acceptable A conference-style report and a project presentation on your results will be due at the end of the quarter Submit as proj1.zip or proj1.tgz4Copyright 2001  Mani SrivastavaReal-time Implementation in Software on a Single Processor Typical implementation approaches Synchronous– single program Asynchronous– foreground/background system– multi-tasking10/9/200135Copyright 2001  Mani SrivastavaExample A process controller with following modules a clock tick comes every 20 ms when a clock module must run a control module must run every 40 ms three modules with soft constraints– operator display update– operator input– management information logs6Copyright 2001  Mani SrivastavaSingle Program Approachwhile (1) {wait for clock;do clock module;if (time for control) do control;else if (time for display update) do display;else if (time for operator input) do operator input;else if (time for mgmnt. request) do mgmnt. output;} Must have t1 + max(t2, t3, t4, t5) ≤20 ms may require splitting tasks… gets complex!10/9/200147Copyright 2001  Mani SrivastavaAnother Exampleint main(void) {Init_All();for (;;) {IO_Scan();IO_ProcessOutputs();KBD_Scan();PRN_Print();LCD_Update();RS232_Receive();RS232_Send();TMR_Process();}// should never ever get here// can put some error handling here, just in casereturn (0); // will keep most compilers happy} 8Copyright 2001  Mani SrivastavaObservations on the Single Program Approach Each function called in the infinite loop represents an independent task  Each of these tasks must return in a reasonable time, no matter what thread of code is being executed  We have no idea at what frequency our main loop runs. In fact, the frequency is not constant and can significantly change with the changes in system status (as we are printing a long document or displaying a large bitmap, for example) Mix of periodic and event-driven tasks Most tasks are event driven e.g. IO_ProcessOutputs is an event-driven task dedicated input event queue associated with them• e.g. IO_ProcessOutputs receives events from IO_Scan, RS232_Receive, and KBD_Scan when an output needs to be turned on Others are periodic No trigger event, but may have different periods, and may need to change their period over time10/9/200159Copyright 2001  Mani SrivastavaObservations on the Single Program Approach (contd.) Need some simple means of inter-task communications e.g. may want to stop scanning the inputs after a particular keypad entry and restart the scanning after another entry require a call from a keypad scanner to stop the I/O scanner task e.g. may also want to slow down the execution of some tasks depending on the circumstances say we detect an avalanche of input state changes, and our RS-232 link can no longer cope with sending all these messages like to slow down the I/O scanner task from the RS-232 sending task May need to perform a variety of small but important duties e.g. dim the LCD exactly one minute after the very last key was pressed, flash a cursor on the LCD at a periodic, fixed and exact frequency.  dedicating a separate task to each of these functions is definitely overkill – so handled through timers and TMR_Process 10Copyright 2001  Mani SrivastavaHandling Periodic Tasks with Multiple and Variable Rates Two variables, execution counters and reload value, associated with tasksvolatile unsigned int LCD_ReloadValue = LCD_TASK_DEFAULT_SPEED;volatile unsigned int LCD_ExecCounter = LCD_TASK_DEFAULT_SPEED; How it works? execution counter counts down from reload value in exact timing system, the execution counter is decremented in a fixed frequency interrupt routine in relative timing system, the task itself decrements the counter when it reaches zero, the task is called, otherwise the entry function to the task exits without further processing using a variable rather than a constant for reload value is that this helps us to dynamically manipulate the execution speed of the task setting execution counter to special value TASK_DISABLED prevents task execution  can be set by any other task Issues fair bit pf processing in the interrupt handler need mutually exclusive access to execution counter by task and interrupt code task must be written so that they return in a reasonable time – may need to split10/9/2001611Copyright 2001  Mani SrivastavaHandling Event-driven Tasks Each event driven task has an input event queue e.g. ring buffer with asynchronous GetEvent and PutEvent Unique input event structure for each tasktypedef unsigned int word;typedef struct {word InPtr; /* Head of buffer */word OutPtr; /* Tail of buffer */word Count; /* Counter */EVENT_TYPE Store[BUFFER_SIZE]; /* buffer space */} INPUT_EVENT_QUEUE_TYPE;12Copyright 2001  Mani SrivastavaEvent-driven Tasks (contd.) Sending an event// somewhere in RS232 moduleOUTPUT_EVENT_TYPE OutputEvent; OutputEvent.NewState = 1; // new state - on