An embedded system is some combination of computer hardware and software, either fixed in capability or programmable, that is specifically designed for a particular function \cite{ganssle2008embedded}. Industrial machines, automobiles, medical equipment, cameras, household appliances, airplanes, vending machines and toys (as well as the more obvious cellular phone and PDA) are among the myriad possible hosts of an embedded system. Embedded systems that are programmable are provided with programming interfaces, and embedded systems programming is a specialized occupation.
\subsection{Processing units}
The term embedded system is quite broad, so there is no surprise that the spectrum of used processing units is also wide. Since the general purpose microprocessors require external components, namely memories and peripherals, they tend to consume extra power and a board space. Since the design limitations of an embedded systems are most of the time low physical size, low power consumption and/or long uptime and ruggedness (more components mean more parts could fail), microprocessors are seldom used. However, most of the commonly used architectures and word lengths are covered. Due to aforementioned reasons, microcontrollers are favored over microprocessors.
\subsection{System-on-chip}
Today's state of chip integration allows production costs of a complex system on chip devices to be relatively low, thus making very SoCs attractive choice for embedded systems. Soc could be described as an integrated circuit (IC) that integrates all components of a computer or other electronic system into a single chip. It may contain digital, analog, mixed-signal, and often radio-frequency functions - all on a single chip substrate \citep{flynn2011computer}. SoCs are very common in the mobile electronics market because of their low power consumption.
A typical SoC consists of:
\begin{itemize}
\item a microcontroller, microprocessor or digital signal processor (DSP) core(s)
\item memory blocks including a selection of ROM, RAM, EEPROM and flash
\item timing sources including oscillators and phase-locked loops
\item peripherals including counter-timers, real-time timers and power-on reset generators
\item external interfaces, including industry standards such as USB, FireWire, Ethernet, USART, SPI
\item analog interfaces including ADCs and DACs
\item voltage regulators and power management circuits
\item a bus connecting these blocks
\end{itemize}
SoCs can be implemented as an application-specific integrated circuit (ASIC) or using a field-programmable gate array (FPGA).
GL.inet Smart Router is a remake of a common TP-Link router TL-WR703N. The board changes include, but are not limited to, increased RAM and Flash memory, custom firmware and what is the most important - 5 usable GPIO pins exposed to the 2cm pin header for utility. Whole thesis is revolving around taking advantage of this fact. Basic information about the router are shown in Table \ref{t:charact}.
\begin{table}[h]
\caption{The basic characteristics of the GL.inet board}\label{t:charact}
The Atheros AR9331 is a highly integrated and cost effective IEEE 802.11n 1x1 2.4 GHz Systemon-a-Chip (SoC) for wireless local area network (WLAN) AP and router platforms. The block diagram of the chip can be seen on Figure \ref{f:ar_block}. The main processor frequency is 400 MHz and it is suited for running Linux distributions for embedded devices, preferably OpenWrt or DD-Wrt.
Features of this SoC are following:
\begin{itemize}
\item Complete IEEE 802.11n 1x1 AP or router in a single chip
\item MIPS 24K processor operating at up to 400 MHz
\item External 16-bit DDR1, DDR2, or SDRAM memory interface
\item SPI NOR Flash memory support
\item No external EEPROM needed
\item 4 LAN ports and 1 WAN port IEEE 802.3 Fast Ethernet switch with auto-crossover, auto polarity, and auto-negotiation in PHYs
\item Fully integrated RF front-end including PA and LNA
\caption{The block diagram of the Atheros AR9331 SoC used as a main processing unit on GL.inet board}\label{f:ar_block}
\end{figure}
\subsection{From TL-WR703N to GL.inet}
TP-Link TL-WR703N router is a popular choice among hacker community because of it's cheap price tag compared to processing power and usage of a full-grown Linux distribution. People have figured out how to upgrade RAM / Flash memories or to make use of not used GPIO / UART ports for their own needs. These solutions however were mostly crude and expensive to replicate. The GL.inet team saw an opportunity to grasp this public knowledge and rolled out their own improved board clone to the marked.
Whole printed circuit board of TL-WR703N was remade by the GL.inet team to expose the unused GPIO ports on the SoC, utilize two Ethernet port instead of one and utilize the USB 2.0 port. Memory chips were replaced by their higher capacity alternatives.
