This document discusses various topics related to embedded Linux systems, including common hardware architectures like ARM and MIPS, embedded operating systems like Embedded Linux and uClinux. It covers storage options for embedded devices like NOR flash and NAND flash. Other topics include bootloaders, device trees, toolchains for cross-compiling code, application programming interfaces (APIs), static versus dynamic compiling, and programming languages used in embedded systems like C++, Python and Go.

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IoT Hardware

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Architectures
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MIPS
– 1981
– HDTV, wireless networks, VoIP
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Power Architecture
– Networking and telecommunications-related embedded
applications
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x86
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ARM
– Sony PlayStation Portable (PSP), Apple iPhone

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Operating Systems

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Embedded Linux
●
Why linux?
– Supports a vast variety of hardware devices
– Supports a huge variety of applications and networking
protocols
– Linux is scalable
– Linux can be deployed without the royalties
– Linux has attracted a huge number of active developers
– An increasing number of hardware and software vendors
– Supports many programming languages

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Busybox
The Swiss Army Knife of Embedded Linux
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Tiny versions of many common UNIX utilities
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Small and efficient single executable file
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Can use as init process
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Useful for cross compile
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Multi call Binary‐call Binary

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uCLinux
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Microcontroller Linux
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No MMU (Memory Management Unit)
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No virtual memories
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Architecture supported:
– ARM Cortex-M
– ARC
– MicroBlaze
– Blackfin
– Nios

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Real Time Linux
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Real-Time kernel patch – by Ingo Molnar
– PREEMPT_NONE
– PREEMPT_VOLUNTARY
– PREEMPT_DESKTOP
– PREEMPT_RT

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Storage Options
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NOR flash (few MB to a GB)
– Expensive
– Mapped into the CPU address space
– Useful for bootloader
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NAND flash (tens of MB to tens of GB)
– Much cheaper than NOR
– Higher capacities
– Needs a lot of hardware and software support

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Storage Options...
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Managed flash
– MMC: Multi Media Card
– SD: Secure Digital
– eMMC
– UFS (Universal Flash Storage)

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Bootloaders
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Initialize the system to a basic level
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Loads the operating system kernel

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Bootloaders...

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Bootloaders...
Name Main architectures supported
Das U-Boot ARC, ARM, Blackfin, Microblaze, MIPS, Nios2, OpenRiec, PowerPC, SH
Barebox ARM, Blackfin, MIPS, Nios2, PowerPC
GRUB 2 X86, X86_64
Little Kernel ARM
RedBoot ARM, MIPS, PowerPC, SH
CFE Broadcom MIPS
YAMON MIPS

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UEFI
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Universal Extensible Firmware Interface
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The processor loads the platform initialization
firmware
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load an EFI boot manager from the EFI System
Partition (ESP)
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UEFI boot manager
– systemd-boot
– Tummiboot

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Initramfs
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Ramfs is a very simple filesystem that exports
Linux's disk caching mechanisms
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A dynamically resizable RAM-based filesystem
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gzipped "cpio" format archive
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Linked into the linux kernel image

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Device Tree
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ARM does not come with any hardware
information
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A flexible way to define the hardware
components
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Loaded by the bootloader and passed to the
kernel

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Device Tree...

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Device Tree Example
serial@4806c000 {
compatible = "ti,dra742-uart", "ti,omap4-uart";
reg = <0x4806c000 0x100>;
interrupts = <0x0 0x44 0x4>;
ti,hwmods = "uart2";
clock-frequency = <0x2dc6c00>;
status = "disabled";
dmas = <0xb3 0x33 0xb3 0x34>;
dma-names = "tx", "rx";
};

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Pinmux
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Different embedded projects deal with different
components
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We cannot attach an I2C device to a pin that
follows the protocol of the SPI bus
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We cannot attach an analog sensor to the
GPIO pin
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It is not possible to have that many CPU pins
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share the functionalities of available pins

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Toolchains
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Set of tools that compiles source code into executables
– Compiler
– Linker
– Run-time libraries
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Standard GNU toolchain components:
– Binutils: A set of binary utilities including the assembler and the linker
– GNU Compiler Collection (GCC): compilers for C, C++, Objective-C, Java, Fortran,
Ada, Go
– C library: A standardized application program interface (API) based on the POSIX
specification

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Toolchains...
●
Native: toolchain runs on the same type of system
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Cross: toolchain runs on a different type of system
than the target
Linaro, ( h t t p s ://w w w . l i n a r o . o r g / )
crosstool-NG

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Application Binary Interface (ABI)
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Guarantees that the libraries exist on all compliant
machines
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Ensures the integrity of the interface

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Application Program Interface (API)
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glibc: it is the most complete implementation of the
POSIX API
●
musl libc: a small and standards-compliant
alternative to GNU libc
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uClibc-ng: developed to work with uClinux

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Application Program Interface (API)

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API & ABI

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Static & Dynamic Compiling

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Yocto
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Tools and methods to help you create custom Linux-
based systems
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The Yocto Project is more than a distribution
●
Main Components:
– glibc
– BusyBox
– matchbox (for the windowing system)

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Other Programming Languages
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C++/Qt
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Python
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Golang
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Node.js
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Lua
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Scratch
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Java ...

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Refrences