Make the doc slightly more complete and add notes on how to add a new target in OpenWrt, some serial console and JTAG tips and tricks
SVN-Revision: 13880v19.07.3_mercusys_ac12_duma
Florian Fainelli
16 years ago
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Debugging hardware can be tricky especially when doing kernel and drivers
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development. It might become handy for you to add serial console to your
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device as well as using JTAG to debug your code.
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\subsection{Adding a serial port}
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Most routers come with an UART integrated into the System-on-chip
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and its pins are routed on the Printed Circuit Board to allow
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debugging, firmware replacement or serial device connection (like
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modems).
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Finding an UART on a router is fairly easy since it only needs at
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least 4 signals (without modem signaling) to work : VCC, GND, TX and
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RX. Since your router is very likely to have its I/O pins working at
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3.3V (TTL level), you will need a level shifter such as a Maxim MAX232
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to change the level from 3.3V to your computer level which is usually
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at 12V.
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To find out the serial console pins on the PCB, you will be looking
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for a populated or unpopulated 4-pin header, which can be far from
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the SoC (signals are relatively slow) and usually with tracks on
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the top or bottom layer of the PCB, and connected to the TX and RX.
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Once found, you can easily check where is GND, which is connected to
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the same ground layer than the power connector. VCC should be fixed
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at 3.3V and connected to the supply layer, TX is also at 3.3V level
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but using a multimeter as an ohm-meter and showing an infinite
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value between TX and VCC pins will tell you about them being different
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signals (or not). RX and GND are by default at 0V, so using the same
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technique you can determine the remaining pins like this.
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If you do not have a multimeter a simple trick that usually works is
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using a speaker or a LED to determine the 3.3V signals. Additionnaly
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most PCB designer will draw a square pad to indicate ping number 1.
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Once found, just interface your level shifter with the device and the
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serial port on the PC on the other side. Most common baudrates for the
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off-the-shelf devices are 9600, 38400 and 115200 with 8-bits data, no
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parity, 1-bit stop.
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\subsection{JTAG}
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JTAG stands for Joint Test Action Group, which is an IEEE workgroup
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defining an electrical interface for integrated circuit testing and
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programming.
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There is usually a JTAG automate integrated into your System-on-Chip
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or CPU which allows an external software, controlling the JTAG adapter
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to make it perform commands like reads and writes at arbitray locations.
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Additionnaly it can be useful to recover your devices if you erased the
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bootloader resident on the flash.
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Different CPUs have different automates behavior and reset sequence,
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most likely you will find ARM and MIPS CPUs, both having their standard
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to allow controlling the CPU behavior using JTAG.
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Finding JTAG connector on a PCB can be a little easier than finding the
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UART since most vendors leave those headers unpopulated after production.
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JTAG connectors are usually 12, 14, or 20-pins headers with one side of
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the connector having some signals at 3.3V and the other side being
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connected to GND.
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