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Reverse engineering surrounding circuitry of PIC device

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DaveDexter

DaveDexter

Electrical
Apr 22, 2002
1
Hi

I'm attempting to reverse engineer a PIC device and I have the operation of the PIC down very well - it sends an ID, a specific time after power up.

The device is connected to a limited current source and two parralel 0603 capacitors are used to provide a voltage for the PIC. I have the voltage waveform of the charging parralel capacitors and I want to calculate their values. Does anyone know how I can do this accurately?

Surrounding the PIC is a SOT-23 device which I believe to be a Digital Transistor connected to the charged capacitors, the output and the pin that drives the logic on the PIC. Why doesn't the PIC pin drive the output directly and what benefit does the device have? Is it likely to be something different to a Digital Transistor (eg a MOSFET or diode of some kind) and how can I find out.

I have the voltage waveforms and 1 pin is high during operation (base?) whilst the output (emitter?) follows the pulse seen at the input (collector?). Why is this implemented?

The markings on the SOT-23 device are N4 46 (N4 is written perpendicular to the device) but I have done a search for what it may be and I haven't been too successful. Does anyone know a place which may help me narrow the device down.

Many thanks in advance for your help - it's much appreciated.

Dex
 
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Sounds like a transistor to me. Typical PIC devices can only source 20-25mA max. I am not familiar with the markings you stated because we don't use SMT that much yet. To calculate the capacitor values will be quite tricky and not very accurate. Are the caps polarity marked? If they are then the typical tolerance (through hole electrolytics) are typically 20%. So if you came up with a value it would be quite a bit different from unit to unit. I suppose to come up with a value you would need to look at the ripple on the DC supply as well as the equivalent resistance. This would seem to be a waste of time, if you know the input frequency from the current source then you should be able to determine the appropriate cap values and verify experimentally. What are you trying to do?
 
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Open Collector circuit? It this output signal is run to multiple locations, you need to have an open collector configuration. Let's assume that all connected outputs are at the same value, no problem. However, if one of those outputs changes state, then that output will have to source / sink the outputs of all the remaining devices (VCC connected to GROUND).

Hope this helps! Keep us posted!!!
 
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