Embedded platform choices

Embedded platform
choices
Tavish Naruka
BaseApp Systems

Microcontroller or linux platform?
Or both?
When prototyping
OR

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

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

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

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

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

Peripherals and clocks on an
STM ARM cortex m3 chip

● 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

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

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

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

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

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

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

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

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

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

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

● 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

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

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

● 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

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

Embedded platform choices

More Related Content

What's hot (20)

Similar to Embedded platform choices (20)

More from Tavish Naruka (6)

Recently uploaded (20)

Embedded platform choices