August 1998, Issue 97

BitScope
A Mixed-Signal Capture Engine

SERIAL CONNECTION

While a serial interface may seem a bottleneck for a capture engine that can potentially store 64 KB of data, this is not a problem. Thanks to the Internet and 56k modems, most PCs now have fast, buffered UARTs.

The transmission speed of the BitScope serial link can be scaled to 115 kbps using a fast microcontroller. At this rate, you can transfer enough samples to draw a 640 ´ 480 screen—at most 640 bytes—in about 55 ms, or 18 screens per second.

For lower frequency data or simple sine waves, it’s necessary to only send a handful of samples to the host and have the user interface do some curve fitting. Small bursts of contiguous sample data may be used to enhance a waveform display to show high-frequency noise.

Logic analyzers don’t need to rapidly update their display at all. After a trigger event, the data may stay in the sample RAM and be downloaded only when the host needs it. At 115 kbps, the total contents of a 16-KB buffer can download in less than 2 s. The user interface may then draw logic state or timing diagrams and manipulate them as necessary.

USER INTERFACE

Don’t think shrink-wrapped monolithic Windows software for this design. Think more about the Linux model where the engineering community builds its own tools and can customize them as needs arise.

Because BitScope uses simple ASCII commands, in a pinch, you can use a terminal program and spreadsheet to display waveforms. For complex applications, you need more advanced software based on C, Delphi, or Visual Basic.

A BitScope user interface can run under many possible environments, including Windows, MAC, Unix, WinCE, Palm Pilot, Psion, DOS, or Amiga. Basically, it can work on any machine with a serial port.

No single person could write all that software. Instead, I made the BitScope design open and documented so you could create what you need.

On INK’s Web site, you’ll find some user-interface software with source listings to start the ball rolling. Via the Internet, you can also find existing programs that already simulate oscilloscopes, logic circuits, and data displays.