docs/debugging.tex

   1 Debugging hardware can be tricky especially when doing kernel and drivers
   2 development. It might become handy for you to add serial console to your
   3 device as well as using JTAG to debug your code.
   4
   5 \subsection{Adding a serial port}
   6
   7 Most routers come with an UART integrated into the System-on-chip
   8 and its pins are routed on the Printed Circuit Board to allow
   9 debugging, firmware replacement or serial device connection (like
  10 modems).
  11
  12 Finding an UART on a router is fairly easy since it only needs at
  13 least 4 signals (without modem signaling) to work : VCC, GND, TX and
  14 RX. Since your router is very likely to have its I/O pins working at
  15 3.3V (TTL level), you will need a level shifter such as a Maxim MAX232
  16 to change the level from 3.3V to your computer level which is usually
  17 at 12V.
  18
  19 To find out the serial console pins on the PCB, you will be looking
  20 for a populated or unpopulated 4-pin header, which can be far from
  21 the SoC (signals are relatively slow) and usually with tracks on
  22 the top or bottom layer of the PCB, and connected to the TX and RX.
  23
  24 Once found, you can easily check where is GND, which is connected to
  25 the same ground layer than the power connector. VCC should be fixed
  26 at 3.3V and connected to the supply layer, TX is also at 3.3V level
  27 but using a multimeter as an ohm-meter and showing an infinite
  28 value between TX and VCC pins will tell you about them being different
  29 signals (or not). RX and GND are by default at 0V, so using the same
  30 technique you can determine the remaining pins like this.
  31
  32 If you do not have a multimeter a simple trick that usually works is
  33 using a speaker or a LED to determine the 3.3V signals. Additionnaly
  34 most PCB designer will draw a square pad to indicate ping number 1.
  35
  36 Once found, just interface your level shifter with the device and the
  37 serial port on the PC on the other side. Most common baudrates for the
  38 off-the-shelf devices are 9600, 38400 and 115200 with 8-bits data, no
  39 parity, 1-bit stop.
  40
  41 \subsection{JTAG}
  42
  43 JTAG stands for Joint Test Action Group, which is an IEEE workgroup
  44 defining an electrical interface for integrated circuit testing and
  45 programming.
  46
  47 There is usually a JTAG automate integrated into your System-on-Chip
  48 or CPU which allows an external software, controlling the JTAG adapter
  49 to make it perform commands like reads and writes at arbitray locations.
  50 Additionnaly it can be useful to recover your devices if you erased the
  51 bootloader resident on the flash.
  52
  53 Different CPUs have different automates behavior and reset sequence,
  54 most likely you will find ARM and MIPS CPUs, both having their standard
  55 to allow controlling the CPU behavior using JTAG.
  56
  57 Finding JTAG connector on a PCB can be a little easier than finding the
  58 UART since most vendors leave those headers unpopulated after production.
  59 JTAG connectors are usually 12, 14, or 20-pins headers with one side of
  60 the connector having some signals at 3.3V and the other side being
  61 connected to GND.