Where to solder glitching/measurement points on a target


#1

I am currently trying to glitch a specific PCB but I am not sure where exactly should I cut the lines and add the connectors for the glitch/measurement part.

  1. Should I solder the wire directly on the VCC IC chip? Without any resistor or decoupling capacitor? similar to the CW303 board.

  2. If I need to add some passive components, what is reasoning behind it?

Cheers!


#2

I think you’re looking at voltage glitching with the ChipWhisperer crowbar circuit. If that’s your goal, you want the glitch output to be connected directly to the chip’s VCC pins - when you apply a glitch, you want to cut power to the chip as quickly as possible. If there are any decoupling capacitors nearby, then your glitch output will have to drain the charge out of them before the chip’s voltage actually drops.

Also, remember that you’re shorting out the power supply when you do this, so you can cause some pretty huge current spikes. The only passive component you might need is a resistor to limit this current. If you have a shunt resistor for power measurements already, then you shouldn’t need anything else.

Good idea to emulate the CW303 board - see if you can use the ChipWhisperer targets as a reference.


#3

Thanks a lot for the reply!

  1. Should I decrease the decoupling capacitance so they are discharged faster?

  2. Having to discharge the decoupling capacitors it only affects the exact timing when the actual glitch happens after it has been triggered. Is this correct?


#4

You’re right - if you have to discharge a big decoupling capacitor, then the effect of the glitch will be delayed a little bit. That isn’t a big deal here because the Glitch Explorer lets you automatically search for the right glitch settings, so you shouldn’t have to worry much. I wouldn’t bother changing any of the decoupling capacitors.

Things do get a bit tricky with lots of capacitance because it’s hard to time short glitches very precisely, but this shouldn’t be a huge problem.


#5

Awesome, thanks for the reply!