Embedded Software Design

1. Embedded Software Design Peter R. Wihl (former Guest Lecturer)

2. Overview • Data flow practices (Throughput) • Real time systems • Software design overview • Communication protocols • An example software design Feel free to ask questions at any time

3. Execution Flow • A diagram that describes the major steps in the sequential software flow. • Illustrates each logical step and decision point in the overall design of the software. • Provides a starting point for the design of the software. • Is to embedded software as logical architecture is to hardware. • Is mapped later to Routines, Functions or Objects

4. Before Designing Execution Flow • Identify which logical architectural modules are mapped to software. • Identify what time period all logical software function must be completed in. • Identify any additional overhead functionality due to modules mapped to software and/or characteristics of the physical design. ie. Polling/Interrupts

5. Logical Design Architecture Example

6. Physical Architecture Mapping

7. Logical Design Architecture Example

8. Role of Microcontroller • Read 16-bit word from ADC • Calculate Error Correction Code (2-bit) • Inject possible error • Detect if there is an error • If there is an error correct it • Write new 16-bit word to DAC

9. Execution Flow Diagram

10. Data Throughput in SW • What is synchronous data flow? • Data arrives at regular intervals • What is asynchronous data flow? • Data does not arrive at regular intervals • What is isochronous data flow? • Data must be delivered within certain time constraints

11. Data Flow Practices • Polling • Interrupt triggered (blocking) • Interrupt triggered (non-blocking) • Event driven • Often referred to as interrupt driven

12. Sample Problem • Need to receive 16 bytes into a buffer and then process the buffer • Bytes arrive asynchronously

13. Polling Overview

14. Polling main do forever count = 0 while count < 16 while byte not ready nop get byte buffer[count] = byte incr count process buffer

15. Interrupt (Blocking) Overview

16. interrupt rx_byte disable interrupts count = 0 while count < 16 get byte buffer[count] = byte incr count process buffer enable interrupts return main enable interrupts do forever nop Interrupt Triggered(Blocking)

17. Interrupt (Non-Blocking) Overview

18. interrupt rx_byte if count < 16 get byte buffer[count] = byte incr count else if count = 16 process buffer count = 0 return main count = 0 enable interrupts do forever nop Interrupt Triggered(Non-blocking)

19. Event Driven Overview

20. interrupt rx_byte if count < 16 get byte buffer[count] = byte incr count return main count = 0 enable interrupts do forever if count = 16 process buffer count = 0 Event Driven

21. Real Time • Hard real time • Absolute deterministic response to an event • Soft real time • Average response to an event

23. Embedded Software Practices Peter R. Wihl ECE 164 Spring 2004

24. Overview • Data flow practices • Real time systems • Communication protocols • Software design overview • An example software design Feel free to ask questions at any time

25. Data Flow Types • What is synchronous data flow? • What is asynchronous data flow? • What is isochronous data flow?

26. Data Flow Types • What is synchronous data flow? • Data arrives at regular intervals • What is asynchronous data flow? • Data does not arrive at regular intervals • What is isochronous data flow? • Data must be delivered within certain time constraints

27. Data Flow Practices • Polling • Interrupt triggered (blocking) • Interrupt triggered (non-blocking) • Event driven • Often referred to as interrupt driven

28. Sample Problem • Need to receive 16 bytes into a buffer and then process the buffer • Bytes arrive asynchronously

29. Polling main do forever count = 0 while count < 16 while byte not ready nop get byte buffer[count] = byte incr count process buffer

30. interrupt rx_byte disable interrupts count = 0 while count < 16 get byte buffer[count] = byte incr count process buffer enable interrupts return main enable interrupts do forever nop Interrupt Triggered(Blocking)

31. interrupt rx_byte if count < 16 get byte buffer[count] = byte incr count else if count = 16 process buffer count = 0 return main count = 0 enable interrupts do forever nop Interrupt Triggered(Non-blocking)

32. interrupt rx_byte if count < 16 get byte buffer[count] = byte incr count return main count = 0 enable interrupts do forever if count = 16 process buffer count = 0 Event Driven

33. Real Time • Hard real time • Absolute deterministic response to an event • Soft real time • Average response to an event

34. Trick Questions • Which is better, hard or soft real time? • Which design methods are hard real time? • Which design methods are soft real time?

35. Communication Protocols • What is a communication protocol? • An established set of conventions by which multiple systems exchange data • The speech analogy • The sounds you can make are the communication medium • The language you use is the protocol

36. Sample Protocol

37. interrupt rx_byte get byte checksum = byte  checksum switch (state) SYNC if byte = sync checksum = byte state = SIZE SIZE size = byte if size > 0 count = 0 state = PAYLOAD else state = CHECKSUM PAYLOAD buffer[count] = byte incr count if count = size state = CHECKSUM CHECKSUM if checksum = 0 state = ACCEPT else state = SYNC ACCEPT drop byte return Protocol Interrupt

38. main state = SYNC enable interrupts do forever if state = ACCEPT process buffer state = SYNC This is a simple event loop that provides mutual exclusion for the buffer Protocol Main

39. Time For A Break

40. Software Design Overview • Requirements Analysis • Architecture • Design • Implementation • Module Testing • Design Validation Testing

41. Example Problem • I want to build a Heads Up Display for my car. • I would like to see both my engine RPM and fuel economy on my windshield. • My car has a serial diagnostic interface that provides this data.

42. Requirements • System shall have a Heads Up Display (HUD) • System shall interface with vehicle’s onboard diagnostic system • HUD shall display current RPM • HUD shall display current fuel economy (MPG)

43. Hardware Constraints • Vehicle’s onboard diagnostic system • Needs a wake-up signal sent every 1 second • Operates at 10,500 bps • Heads Up Display • 256x128 pixels • Full display must be calculated and mirrored • Operates at 115,200 bps

44. Processor Requirements • Processor shall have 2 UARTs • Vehicle’s onboard diagnostic system • Heads Up Display • Processor shall have a timer • Vehicle’s onboard diagnostic system wake-up • Processor shall have more than 8192 bytes of memory • Processed display image (4096 bytes) • Mirrored display image (4096 bytes)

45. Hardware Design

46. Software Architecture

47. Software Design • Modules • Vehicle Diagnostic Interface (VDI) • RPM Data Formatting (RDF) • MPG Data Formatting (MPG) • Display Processing (DP) • Display Control Interface (DCI)

48. Software Design • Modules • Vehicle Diagnostic Interface (VDI) • RPM Data Formatting (RDF) • MPG Data Formatting (MPG) • Display Processing (DP) • Display Control Interface (DCI) • Main/Initialization

49. Vehicle Diagnostic Interface

50. interrupt rx_byte … rx_state = ACCEPT … return extract_data if data type = RPM extract RPM data rpm_format(data) else if data type = MPG extract MPG data mpg_format(data) return main … do forever … if rx_state = ACCEPT extract_data rx_state = SYNC … Vehicle Diagnostic Interface