From Intrepid's Thursday lecture series, our developer Ryan Meador explains the basics of firmware. Includes firmware background, what's different between a mobile device and a firmware device, underlying technology, & tools and techniques.

1. FIRMWARE
2016

2. 01
Background

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

5. 02
What’s different?

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

9. 03
What Lies Beneath

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

14. 04
Tools and Techniques

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”