This document discusses embedded platform choices and compares microcontroller-based systems to Linux-based systems. It provides examples of popular microcontrollers like AVR, PIC, and STM32. It also discusses developing firmware in C/C++ and the importance of understanding a microcontroller's memory layout and peripheral interfaces. The document then covers building embedded Linux systems using tools like Yocto and choosing hardware like system on modules. It provides an example application that uses a microcontroller connected via serial to control an LCD/keypad and a Linux host for networking, storage, and programming in Python.

1. Embedded platform
choices
Tavish Naruka
BaseApp Systems

2. Microcontroller or linux platform?
Or both?
When prototyping
OR

3. System which uses only microcontrollers to
function
System which has a linux cpu (may have
microcontrollers too)

4. Working with microcontrollers
● First step is component selection
○ Does the controller have all the interfaces required
○ Voltage range, cost, memory sizes(flash and ram)
○ Familiarity with the architecture, supported software,
documentation, even community
○ debugger/programmer, toolchain
○ Is some development kit available? If not, then can
you make a pcb for prototyping yourself? Maybe
someone has published a design using this online

5. Some popular choices
● AVR is probably most popular due to
arduino.
● PIC family of microcontrollers is also popular
● ST microelectronics
● has a few not too expensive dev kits
● TI MSP

6. Some dev kits
STM32F4 discovery Olimex PIC32-T79
MSP launchpad
PinguinoPIC32
Parallaxpropeller8core

7. Firmware
● Most code is in C, or assembly. sometimes
C++
● compilers and chip makers often provides
driver libraries and example code for things
like USB, TCP/IP stack, and other
peripherals
● The reference manual is usually a reference
for a family of similar chips, and the
datasheet is the ultimate reference

8. ARM Cortex M3 memory map

9. Peripherals and clocks on an
STM ARM cortex m3 chip

10. ● Input voltages
● Memory layout
● How does it boot
● How do interrupts work in this chip
● System clocks
● Details about any peripherals you want to
use like USB, SPI, serial, DMA, timers etc.
Things to read in the manuals

11. Difference between bare metal and
hosted programs
● Userspace programs running inside an operating
system are hosted
● Code running on a microcontroller, (and linux kernel)
are compiled for a freestanding environment. You (or
mostly the compiler) provides C ‘runtime’
● C startup =
○ Initialize stack, cannot use variables without it
(mostly), or call functions
○ copy values to Initialized variables’ locations in RAM
○ initialize globals to 0
○ Copy read only data like strings to RAM

12. Why memory layout is important
● There is a well defined way how a chip boots, might
start executing from a particular location
● Peripherals are memory mapped
● Interrupt handlers, are locations reserved for function
pointers in some
● If you want a bootloader, then the application code must
not have any portion located in flash occupied by
bootloader
● This is done using linker scripts
● The C startup code used locations exported by the
linker script to know where from/to copy or paste code

13. Why microcontrollers
● Small codebase, everything can be controlled
● Instant on
● Real-time behavior
● Low power
● Less supporting hardware
● cheap
● Low level things are easier to tweak than on linux

14. Why not microcontrollers
● limited choice of languages, toolchain
● debugging can be difficult
● File Systems, networking, graphics etc.
These things may be done on some
microcontrollers, but with a lot more effort
than if using linux
● application portability to a different system

15. Embedded linux platform
● Embedded, because its not general purpose,
unlike a desktop
● Platform, because it can carry payload of
your application, and become whatever
specific purpose system you design it to be

16. Hardware
● Linux supports many architectures x86,
x86_64, ARM, MIPS, powerpc, AVR32 etc.
● Not designed for small microcontrollers
● RAM requirement depends on application,
but on a minimum is around 8MB

17. Where to get
● Evaluation board from chip manufacturer
AM335x starter kit from TI

18. cont..
● System on module
AM3352 SOM from olimex Carambola SOM placed on board

19. cont..
● Or designs/products by open hardware community
● Could be from chip makers(like beaglebone from TI),
companies releasing usable boards(like A13 olinuxino
from olimex)
● custom design
Allwinner A13 olinuxino iMX233 Locux BaseApp BeagleBone Black

20. cont..
● Consumer devices
A Router running linux
BelkinWemoswitch
Smartphones
Smart tv
And lots of industrial Systems
Kindle

21. ● Some ways to create a linux system(root filesystem) are
○ [Yocto project, openembedded, angstrom]
○ [Buildroot, Openwrt]
○ debian debootstrap
○ Can use full featured distros too, like debian(without
desktop environment)
● Sometimes need to optimize for space/speed
○ provide common tools using busybox
○ simpler init, only necessary programs run on boot
○ only required drivers in kernel
○ compressed fs like squashfs
Making a linux system for an
embedded platform

22. Linux system boot process
● (not x86)
● first instructions execute a bootloader from
ROM which loads another bootloader, or
sometimes linux kernel itself
● The bootloader loads the kernel, passes it
kernel command line(can be hard coded in
kernel image too)
● after loading, it jumps to the kernel’s entry
point
● kernel command line option ‘rootfs’ is
mounted

23. cont..
● After mounting rootfs, kernel looks for init program,
which becomes the first process, and all other
processes spawn from it.
● kernel loads modules as required
● Usually init would be a standard program, like
○ systemV init
○ systemd(debian uses this)
○ upstart(ubuntu)
○ busybox provided sysV init(common in embedded
systems to conserve space)
● Init parses its init scripts to launch programs as required

24. ● Suppose you need character LCD, keypad,
CD ROM, SD card, network, USB hard disk
● Choose linux platform which is most suitable
○ Needs USB, ethernet, fast enough
● Decide to use serial port to connect to a
microcontroller, which connects to LCD and
keypad
● If only one USB port, put usb hub
● Linux has drivers for serial(for this particular
cpu), usb-storage(both hard disk and SD
card), and usb CD ROM
Example application

25. cont..
● Program microcontroller so that it accepts
commands for LCD on serial RX, sends
keypad presses on serial TX
● Choose python to write application
● Can mount/unmount hard disk/SD card(usb
mass storage) and cd-rom with
mount/umount
● can use eject to open tray
● can write to serial port with say pyserial
● can access network easily