Knowing that the power adapter may have a DS2502-like chip, I started looking around for the OneWire protocol. There are actually plenty of documents and even source code for this protocol, and I happen to have some DS18B20 for verification purpose, I quickly came up with some code running on MSP430G2553 uC talking in this protocol. And I verified it’s working great with the DS18B20 device: I can read the unique device ID, and even get the room temperature. After that, I wanted to try it with a power adapter. I choose a broken one I got from PC recycle pile. This one has broken laptop side connector, which is exactly what I need to connect the signal pin. So, 5 minutes wiring up, double check, and I ran my code. Guess what? Nothing!
I can’t even get it pass through the reset sequence, which is detected by device pulling down the signal wire, but I didn’t see that at all! I re-checked the wiring, and my code, verified it again with DS18B20, and found nothing wrong on my side.
After spending more than ten minutes checking everything, I decided to open the power brick to see what’s inside. After hurting one of my fingers I finally got it opened. And here is what I saw:
This is the PCB side. The signal pin connects to a resistor (the black thing on top-left), and then go to the central pin of a TO-92 packaged component. The right pin is not connected, and left pin is grounded. There is also a diode-like device between ground and central pin, which looks like a reverse-voltage protection mechanism.
My multi-meter shows that the resistor is larger than 20Mohm, and zero ohm between any two of the three pins of the chip. All these results make me believe that this chip has been damaged. And it may be a problem of DELL’s design, since this signal wire has no shielding and the long wire can easily trap static electricity and damage the chip.
Here is a close look at the chip itself, and the reverse side of the PCB board
There are some text “01MFKVE BYTE TI” on the chip but I couldn’t find anything useful by searching it on internet.
Anyway, it’s not a waste of time, and I got this:
By soldering the connector from power adapter side and a socket from a broken dell laptop, I exposed the signal wires from both sides and also the ground line. This will allow me to listen on the communication between laptop and a working power adapter, which is the next thing I’m gonna do.
To be continued…