Can I Get More Samples Per Clock Cycle?

Maybe. But why would you? Before answering we need to talk about synchronous sampling.

Synchronous vs Asynchronous Sampling

Before ChipWhisperer, side-channel practitioners usually needed very expensive oscilloscopes with very high sampling rates to get good/useful power traces. They needed a high sampling rate because they were sampling asynchronously, which means that there is no relationship between the target clock and the sampling clock.

One of the defining feature of ChipWhisperer is that it samples synchronously: it uses a PLL to synchronize its ADC sampling clock to the target clock. This, it turns out, allows side-channel attacks to succeed with a single sample per clock cycle. All of our Jupyter course notebooks use either 1 or 4 samples per clock cycle.

If you’re not convinced, read Colin’s paper which compares synchronous sampling at these low rates with asynchronous sampling at much higher rates: “Synchronous Sampling and Clock Recovery of Internal Oscillators for Side Channel Analysis and Fault Injection”. The results are very interesting! Synchronous sampling is shown to outperform asynchronous sampling at a sampling rate that’s nearly two orders of magnitude higher. This is why we’ve settled on 1x or 4x sampling.

But I Still Want a Higher Sampling Rate!

ChipWhisperer Husky has a more powerful and flexible clock scheme: with Husky you can, if you want, set the number of samples per clock to any integer (up to the maximum supported sampling rate) with the scope.clock.adc_mul parameter.

This is not possible on the ChipWhisperer-Lite, Pro, or Nano. If you insist on a achieving a higher sampling rate using one of these, your only option is to sample asynchronously. Although it’s not how ChipWhisperer is not meant to be used, it is possible to configure ChipWhisperer to sample asynchronously. You’ll need to supply your target’s clock from elsewhere (it can’t come from ChipWhisperer). If you are using a CW308 target, you can use its crystal driver as the clock source. Our larger FPGA target boards like the CW305 have on-board PLLs that can generate an independent target clock. Then, on the ChipWhisperer side, simply set the ADC clock source to not be set from the external (target) clock, e.g. scope.clock.adc_src = "clkgen_x1".

To be clear: we don’t recommend you do this – unless you have to. If you don’t have access to the target’s clock, and you’re unable to drive the target’s clock from ChipWhisperer, then asynchronous sampling is your only option. In this case you should probably set the sampling rate as high as your capture hardware allows. Maximum sampling rates as listed here.