workflow:

- connect zprobe (note: it currently inverts because it didn't have any
  other chips around. this may change later.)

- capture the USB signals at an interesting moment with a sample rate of
  50 MSa/s

- zoom into the frame(s) of interest

- download the data with
  ./get.py

- decode with
  ./dec.py

  For manual decoding, set the coders to D+ and D- (we need D- for SE0
  and SE1 detection), then click on a rising clock edge left of the
  packet and move the cursor to the right.

- if there are problems with the clock, the analog signal and digital
  signals derived from it can be examined after running dec.py with
  ./plot

  (Note that the digital zprobe hides any analog anomalies.)