From 27b76ab0671089c47506615a796a261e993896a7 Mon Sep 17 00:00:00 2001
From: James <>
Date: Sun, 17 Mar 2013 12:16:37 +0000
Subject: fish

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+Debugging hardware can be tricky especially when doing kernel and drivers
+development. It might become handy for you to add serial console to your
+device as well as using JTAG to debug your code.
+
+\subsection{Adding a serial port}
+
+Most routers come with an UART integrated into the System-on-chip
+and its pins are routed on the Printed Circuit Board to allow
+debugging, firmware replacement or serial device connection (like
+modems).
+
+Finding an UART on a router is fairly easy since it only needs at
+least 4 signals (without modem signaling) to work : VCC, GND, TX and
+RX. Since your router is very likely to have its I/O pins working at
+3.3V (TTL level), you will need a level shifter such as a Maxim MAX232
+to change the level from 3.3V to your computer level which is usually
+at 12V.
+
+To find out the serial console pins on the PCB, you will be looking
+for a populated or unpopulated 4-pin header, which can be far from
+the SoC (signals are relatively slow) and usually with tracks on
+the top or bottom layer of the PCB, and connected to the TX and RX.
+
+Once found, you can easily check where is GND, which is connected to
+the same ground layer than the power connector. VCC should be fixed
+at 3.3V and connected to the supply layer, TX is also at 3.3V level
+but using a multimeter as an ohm-meter and showing an infinite
+value between TX and VCC pins will tell you about them being different
+signals (or not). RX and GND are by default at 0V, so using the same
+technique you can determine the remaining pins like this.
+
+If you do not have a multimeter a simple trick that usually works is
+using a speaker or a LED to determine the 3.3V signals. Additionnaly
+most PCB designer will draw a square pad to indicate ping number 1.
+
+Once found, just interface your level shifter with the device and the
+serial port on the PC on the other side. Most common baudrates for the
+off-the-shelf devices are 9600, 38400 and 115200 with 8-bits data, no
+parity, 1-bit stop.
+
+\subsection{JTAG}
+
+JTAG stands for Joint Test Action Group, which is an IEEE workgroup
+defining an electrical interface for integrated circuit testing and
+programming.
+
+There is usually a JTAG automate integrated into your System-on-Chip
+or CPU which allows an external software, controlling the JTAG adapter
+to make it perform commands like reads and writes at arbitray locations.
+Additionnaly it can be useful to recover your devices if you erased the
+bootloader resident on the flash.
+
+Different CPUs have different automates behavior and reset sequence,
+most likely you will find ARM and MIPS CPUs, both having their standard
+to allow controlling the CPU behavior using JTAG.
+
+Finding JTAG connector on a PCB can be a little easier than finding the
+UART since most vendors leave those headers unpopulated after production.
+JTAG connectors are usually 12, 14, or 20-pins headers with one side of
+the connector having some signals at 3.3V and the other side being
+connected to GND.
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