ADC Frequency in ChipWhisperer-Lite Setup

1

Hello everyone,

I’m working with a ChipWhisperer-Lite and encountered something puzzling during the setup. When I run the Setup_Generic script, it configures the adc_freq to be 160 MHz and sometimes even higher. However, according to the documentation, the maximum recommended value for adc_freq should be 105 MHz.

Here’s an example of the output from the Setup_Generic script:

INFO: Found ChipWhisperer😍
scope.gain.mode                          changed from low                       to high                     
scope.gain.gain                          changed from 0                         to 30                       
scope.gain.db                            changed from 5.5                       to 24.8359375               
scope.adc.basic_mode                     changed from low                       to rising_edge              
scope.adc.samples                        changed from 24400                     to 5000                     
scope.adc.trig_count                     changed from 231058                    to 3194540                  
scope.clock.adc_src                      changed from clkgen_x1                 to clkgen_x4                
scope.clock.adc_freq                     changed from 0                         to 162209632                
scope.clock.adc_rate                     changed from 0.0                       to 162209632.0              
scope.clock.clkgen_div                   changed from 1                         to 26                       
scope.clock.clkgen_freq                  changed from 192000000.0               to 7384615.384615385        
scope.io.tio1                            changed from serial_tx                 to serial_rx                
scope.io.tio2                            changed from serial_rx                 to serial_tx                
scope.io.hs2                             changed from None                      to clkgen                   
scope.io.cdc_settings                    changed from bytearray(b'\x01\x00\x00\x00') to bytearray(b'\x00\x00\x00\x00')

I’m curious to understand what this higher frequency value represents. Is it an expected behavior under certain conditions, or could there be a mistake in the configuration process?

Any insights or explanations would be greatly appreciated!

Thanks in advance for your help.
John

2

Indeed 105 MHz is the maximum clock frequency at which things are guaranteed to work.
For historical reasons, when CW-lite is first connected, clocks are set higher; however, running Setup_Generic (or just calling scope.default_setup()) sets the clocks to “normal” values.

Now sometimes you may still see strange values for some clock measurements, because Setup_Generic makes the clock changes and then reads and caches scope.clock properties before they may be settled. This commit should fix this; you can also manually set scope.clock._cached_adc_freq = None to force the clock frequency to be re-read at any time.

Thank you for bringing this to our attention!

3

Thank you for your answer.

I appreciate the explanation about the clock settings and the caching behavior. I have a follow-up question: when I run scope.capture() and observe this strange adc_freq value, what is the real adc_freq being used at that point? Should I assume that it is equivalent to the clkgen_x4 setting, or is there a more reliable way to determine the actual frequency during capture?

Thanks again for your help!

4

The strange adc_freq value you see is because it was read and cached before the ADC clock settled. Apply the fix above or set scope.clock._cached_adc_freq = None to force a fresh read of the clock frequency to get its actual value.