UBB pattern capture ("logic analyzer")

[Werner Almesberger]

I made a counterpart for the pattern generator: a capture solution
that uses the MMC controller and DMA to sample DATx.

Characteristics:

- four digital channels

- capture rate 41 kSa/s to 56 MSa/s (142 different rates
  selectable), "overclocking" to 84 MSa/s can be attempted

- maximum continuous capture length about 8070 samples

- external trigger input. Trigger on any pattern on DATx and
  TRIG.  Delay to first sample is about 530 ns plus one sample
  time.

- GUI (running on the Ben) for exploration of the captured
  waveforms

The code lives here:
http://projects.qi-hardware.com/index.php/p/ben-blinkenlights/source/tree/master/ubb-la

The usage is similar to ubb-patgen. It is described in
http://projects.qi-hardware.com/index.php/p/ben-blinkenlights/source/tree/master/ubb-la/README

I made an extremely simple passive adapter that consists just
of UBB and two resistors:

http://downloads.qi-hardware.com/people/werner/ubb/la/ubb-la-passive.jpg

It has an input impedance of 1.1 kOhm on DAT0 (11 kOhm during
capture) and 10 kOhm on DAT1-3, so it can only be used on signals
that are not easily disturbed. Adding buffers with better input
impedance is left as an exercise to the reader :-)

Schematics are here:
http://projects.qi-hardware.com/index.php/p/ben-blinkenlights/source/tree/master/ubb-la/hw

ubb-la can use the same cut-out labels as ubb-patgen.

The following picture shows ubb-la displaying a fake SPI pattern
generated by ubb-patgen running on the Ben sitting in the
background:

http://downloads.qi-hardware.com/people/werner/ubb/la/ubb-la-pspi.jpg

- Werner