Im currently working on trying to reverse engineer a USB puzzle board I was given. (its a ATMega168 based device with a standard FTDI USB to UART chip) and all the information you are given is you may need programming and electronic investigation tools to work it out.

enter image description here

Anyway I found the following signal on the SCA pin from the MCU but cant identify what it is. its on the i2c bus, clock line goes high and stays high.


Anyone know what it is? If it helps I can send a dump of the MCU which will runin AVS Studio 4 simulate and let you interact with the uart whrre the main puzzles are.

also is there any way I can work out what pins are used for what? im sure I need to interface with them in some way to activate things. the board has the following pins exposed for connections on the back via pads on the pcb. on the front is a standard SPI bus connector which all I dont know how to check if their is anything on there I can interface with using a bus pirate. (new to MCUs and the bus pirate but did managed to use it to pull a copy of the code from it)

  1. VCC
  2. GND
  3. PCINT19/OC2B/INT1
  5. PCINT1/OC1A
  6. INT0/PCINT18
  7. VCC
  8. GND
  9. ADC1/PCINT9
  10. ADC0/PCINT8
  11. ADC4/SDA/PCINT12
  12. ADC5/SCL/PCINT13
  13. VCC
  14. GND
  15. SCK/PCINT5
  18. PCINT2/SS/OC1B
  • \$\begingroup\$ Welcome to EE.SE. Your not giving us much to work with here. It may take days before someone comes along that has personal experience with this display. \$\endgroup\$ – Sparky256 Jul 22 '16 at 23:06
  • 1
    \$\begingroup\$ That is some very interesting introductory text... \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 22 '16 at 23:17
  • \$\begingroup\$ Did you get the board at a recruiting event? If so pretty cool. \$\endgroup\$ – Andrew W. Jul 22 '16 at 23:24
  • \$\begingroup\$ Hey Andrew.W yes I did. and that was all the info you get. one small board with USB port and some solder pads on the back. no info other then their is 14 keys to find. \$\endgroup\$ – Mathew Jul 23 '16 at 13:35

That is definitely one of those "keys" you're looking for.
It seems to be a UART signal and the 1st 2 bytes are 'k' (0x6b 01101011) 'y' (0x79 01111001).
To decode the rest from your scope trace, remember that the idle state between bytes is high (1) and that each byte begins with a start bit (0).
The data bits themselves are back-to-front, so the least significant bit is first after the start bit.

Alternatively, you could find the baud rate by measuring the period of 1 bit and then feeding the signal to something with a UART to display it for you.

  • \$\begingroup\$ You sir! are a genuis! I had thought it was as I got the K everytime. but the second letter was never correct. didn't notice it was reversed. The scope I was using was a Bitscope (bitscope.com) BS05 I tried to hook up my buspirate to thesignal in UART mode and it pulled garbage regardless of the speed. same with my ttl UART interface. now I just need to find the last 5 keys!. maybe i need to toggle pins high and low lol. \$\endgroup\$ – Mathew Jul 23 '16 at 13:36

Looks a lot like an UART signal. You can output that at any GPIO pin unsing a so-called software UART - which just uses a timer to toggle the pin at the correct times. Your logic analyser software should have a decoder for that.

The numbers in the screenshot make no sense to me, but it could be a common rate like 1200 or 9600 baud.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.