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. |
62 | |