Embedded Systems

1. Embedded Systems John Regehr CS 5460, Dec 04

2. Embedded Systems • Account for >99% of new microprocessors • Consumer electronics • Vehicle control systems • Medical equipment • Sensor networks

3. Definitions of “Embedded System” • A special-purpose computer that interacts with the real world through sensing and/or actuation • Almost any computer that isn’t a PC

4. More definitions • Microprocessor: A regular CPU • Microcontroller: A small system on a chip that contains extra logic for interfacing with the real world • Analog to digital and digital to analog converters • Pulse width modulation • Networks: serial, I2C, CAN, USB, etc…

5. Embedded Characteristics • Close interaction with the physical world • Often must operate in real time • Constrained resources • Memory • SRAM, DRAM, flash, EEPROM, … • Power • CPU cycles

6. More Characteristics • Often there is no: • Virtual memory • Memory protection • Hardware supported user-kernel boundary • Secondary storage • Hard to upgrade once deployed • Cost sensitive • Per-unit cost often dominates overall cost of a product

7. Important Difference • Unlike PC software, embedded software is developed in the context of a particular piece of hardware • This is good: App can be tailored very specifically to platform • This is bad: All this tailoring is hard work

8. CPU Options • Create custom hardware • May not need any CPU at all! • 4-bit microcontroller • Few bytes of RAM • No OS • Software all in assembly • These are getting less popular

9. More CPU Options • 8-bit microcontroller • A few bytes to a few hundred KB of RAM • At the small end software is in asm, at the high end C, C++, Java • Might run a home-grown OS, might run a commercial RTOS • Typically costs a few $$

10. More CPU Options • 16- and 32-bit microcontrollers • Few KB to many MB of RAM • Usually runs an RTOS: vxWorks, WinCE, QNX, ucLinux, … • May or may not have caches • Wide range of costs • 32- or 64-bit microprocessor • Basically a PC in a small package • Runs Win XP, Linux, or whatever • Relatively expensive in power and $$

11. Axes of Variation • Power • Must run for years on a tiny battery (hearing aid, pacemaker) • Unlimited power (ventilation control) • Real-time • Great harm is done if deadlines are missed (avionics) • Few time constraints (microwave)

12. More Axes of Variation • Importance • Device is safety critical (nuclear plant) • Failure is largely irrelevant (toy, electric toothbrush) • Upgradability • Impossible to update (spacecraft, pacemaker) • Easily updated (firmware in a PC network card)

13. More Variation • Cost sensitivity • A few % in extra costs will kill profitability (many products) • Cost is largely irrelevant (military applications) • The point: Embedded systems are highly diverse, it’s hard to make generalizations in this domain

14. Programming Languages • Assembler • No space overhead • Good programmers write fast code • Non-portable • Hard to debug • C • Little space and time overhead • Somewhat portable • Good compilers exist

15. More Languages • C++ • Often used as a “better C” • Low space and time overhead if used carefully • Java • More portable • Full Java requires lots of RAM • J2ME popular on cell-phone types of devices • Bad for real-time!

16. RTOS • Low end: Not much more than a threads library • High end: Stripped-down version of Linux or WinXP • Important qualities: • Predictability • Reliability • Efficiency • Do Linux and XP have these?

17. What’s Hard? • Concurrency – threads, bottom-halves, interrupts • Debugging • Often printf and gdb don’t exist • Creating correct software – can’t patch a pacemaker • Getting a product out the door quickly • Meeting resource constraints (power, RAM, etc.)

18. Some Tradeoffs • Advanced tools are expensive ($10,000+ per developer) but can help a lot • Buying an OS helps but usually results in less efficient systems • Complex software architecture provides rich functionality but may be slow and hard to debug

19. Creating Embedded SW • Pick: • Cheapest CPU that seems like it’ll work • Appropriate language • Appropriate OS • Appropriate software architecture • Appropriate simulation tools • Code… test… repeat

20. Cyclic Executive main() { init(); while (1) { a(); b(); c(); d(); }} Advantages? Disadvantages?

21. Cyclic Exec. Variations main() { init(); while (1) { a(); b(); a(); c(); a(); }} main() { init(); while (1) { wait_on_clock(); a(); b(); c(); }}

22. Interrupts + Events interrupt2() { time_critical_stuff(); enqueue_event (non_time_critical); } main() { while (1) { event_t e = get_event(); if (e) { (e)(); } else { sleep_cpu(); }}} Advantages? Disadvantages?

23. Multithreading • Threads are usually sleeping on events • Highest priority thread runs except when: • It’s blocked • An interrupt is running • It wakes up and another thread is executing in the kernel Advantages? Disadvantages?

24. Port Based Objects • PBOs periodically invoked by OS • No blocking synchronization • Preemption may or may not be supported my_pbo() { get_inputs (); // No side effects! compute (); store_results (); } Advantages? Disadvantages?

25. Summary • Very different from programming desktop apps • Software architecture is worth thinking about in advance • Embedded systems are fun