Root/docs/debugging.tex

1Debugging hardware can be tricky especially when doing kernel and drivers
2development. It might become handy for you to add serial console to your
3device as well as using JTAG to debug your code.
4
5\subsection{Adding a serial port}
6
7Most routers come with an UART integrated into the System-on-chip
8and its pins are routed on the Printed Circuit Board to allow
9debugging, firmware replacement or serial device connection (like
10modems).
11
12Finding an UART on a router is fairly easy since it only needs at
13least 4 signals (without modem signaling) to work : VCC, GND, TX and
14RX. Since your router is very likely to have its I/O pins working at
153.3V (TTL level), you will need a level shifter such as a Maxim MAX232
16to change the level from 3.3V to your computer level which is usually
17at 12V.
18
19To find out the serial console pins on the PCB, you will be looking
20for a populated or unpopulated 4-pin header, which can be far from
21the SoC (signals are relatively slow) and usually with tracks on
22the top or bottom layer of the PCB, and connected to the TX and RX.
23
24Once found, you can easily check where is GND, which is connected to
25the same ground layer than the power connector. VCC should be fixed
26at 3.3V and connected to the supply layer, TX is also at 3.3V level
27but using a multimeter as an ohm-meter and showing an infinite
28value between TX and VCC pins will tell you about them being different
29signals (or not). RX and GND are by default at 0V, so using the same
30technique you can determine the remaining pins like this.
31
32If you do not have a multimeter a simple trick that usually works is
33using a speaker or a LED to determine the 3.3V signals. Additionnaly
34most PCB designer will draw a square pad to indicate ping number 1.
35
36Once found, just interface your level shifter with the device and the
37serial port on the PC on the other side. Most common baudrates for the
38off-the-shelf devices are 9600, 38400 and 115200 with 8-bits data, no
39parity, 1-bit stop.
40
41\subsection{JTAG}
42
43JTAG stands for Joint Test Action Group, which is an IEEE workgroup
44defining an electrical interface for integrated circuit testing and
45programming.
46
47There is usually a JTAG automate integrated into your System-on-Chip
48or CPU which allows an external software, controlling the JTAG adapter
49to make it perform commands like reads and writes at arbitray locations.
50Additionnaly it can be useful to recover your devices if you erased the
51bootloader resident on the flash.
52
53Different CPUs have different automates behavior and reset sequence,
54most likely you will find ARM and MIPS CPUs, both having their standard
55to allow controlling the CPU behavior using JTAG.
56
57Finding JTAG connector on a PCB can be a little easier than finding the
58UART since most vendors leave those headers unpopulated after production.
59JTAG connectors are usually 12, 14, or 20-pins headers with one side of
60the connector having some signals at 3.3V and the other side being
61connected to GND.
62

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