Chosing shunt resistor

Embedded Security ChipWhisperer Hardware
1

Trying to apply everything that I learned from the tutorials to a real chip… but my limited knowledge of eletronics is causing me some problems.

The chip on which I’m working on has these characteristics (from the datasheet):

  • Working voltage: 2.7-5.5v
  • Max power consumption: 20mA

I would like to undestand better which kind of shunt resistor I have to use to get the best result in power analysis. I’m using chipwhisperer lite and I will provide to the chip the 3.3v from there.

A- There is a value better to get a better power analysis? An high value of the shunt resistor is better?
B- If I undestood correctly the shunt resistor all will be fine if below to 10ohm, or I misunderstand something? A 0.22ohm resistor will be fine?
C- based on the information present on Notduino target I saw that a normal 51ohm resistor has been used as shunt resistor. So it’s the same thing?
D- there is a simple way to calculate the value of the shunt resistor needed?

Thank you in advance

2

Honestly, if you’re not too sure it’s probably easiest to buy a small range of resistor values and just pick the maximum that keeps the target working. Typically going outside the specs of the chip by a little won’t be much of a problem.

Regarding the resistor you listed, 0.22ohms sounds a bit low if it’s a microcontroller. Even a pretty beefy microcontroller like the MPC5748G we use a 1ohm resistor for, with lower value resistors being reserved for our CW305 FPGA target. Based on that voltage range, I’d guess either that you’re targeting an 8-bit microcontroller, in which case I’d start with something around the XMega/Notduino for resistance, or it’s got an internal regulator, in which case you should try to find a way around the regulator.

Alex