Introduction to Firmware
- 3. 3 // Background An Embedded System is: ● A computer processor with a dedicated purpose ● Part of a larger electronic device (“embedded”) ● Not intended to be a general-purpose computer Any device that contains a processor, but isn’t a PC or smartphone!
- 4. 4 // Background Firmware is Embedded Software ● Runs on embedded systems ● Directly controls electronic (or mechanical) hardware ● Historically, this software was set in stone at manufacturing time, essentially becoming part of the hardware → “firmware” ○ Today, many devices have updatable firmware
- 6. 6 // What’s different? ● Runs on a general-purpose device ● User-centric, graphical ● Operating system mediates hardware access ● State of the art hardware ● A variety of higher-level programming languages ● An integral part of the device ● Primitive or no user interface ● Direct hardware access ● Primitive hardware capabilities ● Programmed in low-level languages such as C or assembly
- 7. What’s different? System constraints define firmware programming
- 8. ● RAM ● Program Storage ● Speed ● Math ● Libraries ● Operating System ● Debugging Capabilities 8 // What’s different? Constraints
- 10. ● Digital ○ General Purpose Input/Output (GPIO) ○ Serial communications (SPI, I2 C, etc) ○ USB ● Analog ○ Analog-to-digital converters (ADC) ● Specialized Peripherals ○ Radio on-chip (Bluetooth, ZigBee, Thread, RF4CE, etc) 10 // Hardware Interfacing to the outside world
- 11. 11 // Hardware Memory Mapped I/O Address Decoder Memory Cell Enable Read/Write Address Bus Data Bus Normal Memory Latch & Amplifier Address Decoder Enable Read/Write Address Bus Data Bus LED GPIO Address Decoder Peripheral Enable Read/Write Address Bus Data Bus Generic Peripheral
- 12. 12 // Hardware Harvard Architecture Von Neumann Architecture
- 13. ● The most primitive form of multitasking ● The interrupt vector stores callback functions ● Hardware triggers the CPU to invoke these functions in response to certain events ○ Timer elapsing ○ Data arriving on serial interface ○ Many more! 13 // Hardware Interrupts
- 15. ● Usually C, sometimes assembly ○ Must allow addressing specific memory locations ● Low-level programming style -- abstraction is expensive ○ global variables ○ fixed-size arrays ○ avoid passing large objects on the stack ● Very often you’re counting every byte ○ fixed-width types are preferred ○ bitwise operations are common 15 // Tools and Techniques Programming Language
- 16. ● ISRs must be fast! ● Only has access to global state ● Great care must be taken when main thread accesses shared global state ○ Turn off interrupts (“go atomic”) ○ Use CPU instructions that are guaranteed to be atomic ● You can poll the interrupt flags when interrupts are turned off 16 // Hardware Interrupt Service Routines
- 17. 17 // Hardware Concurrency Fun 35 FF FF FF clock FF currentTime 35 FF FF FF FF FF 36 00 00 00 Interrupt fires! 00 FF FF 36 00 00 00 36 00 FF FF currentTime = clock;
- 18. 18 // Tools and Techniques Compiler/IDE ● Expensive, awkward, buggy ● Cross-compiled from your computer ● Emulators are not usually useful because of the tight coupling to external hardware
- 19. ● No obvious sign! It’s just a brick ● Logging is not always available ● Stack traces are not always available ● External hardware is required to debug ● Limited number of breakpoints! ● LEDs are pretty great 19 // Tools and Techniques When things go wrong...
- 20. 20 // Tools and Techniques Programming/Debug Hardware ● Expensive, awkward, buggy ● Some device interfaces are proprietary, but there are also standards such as JTAG or SerialWire
- 21. 21 // Tools and Techniques Test Equipment ● Multimeters ● Oscilloscopes ● Logic Analyzers
- 22. Key Takeaway Embedded systems are “small”