docs/debugging.tex - T108 - Gitiles

 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.
	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.