August 1998, Issue 97

BitScope
A Mixed-Signal Capture Engine

THAT IS LOGICAL, CAPTAIN

PLDs such as the Lattice 1016 can swallow a whole swag of logic functions. In this case, about 18 medium TTL devices with all their wiring disappear into a 44-pin PLCC device.

Radial PLDs like the Lattice are like eight PALs in a circle surrounding a big breadboard. This architecture favors the tight timing requirements of counters and glue logic.

Mostly, this PLD is a 16-bit shift register and counter with a configurable comparator for triggering. The PIC can load a five-byte configuration word that sets the operation of the chip, after which it may be clocked at full speed.

THRU THE LENS MEASURING

The SLR lens-mount system from the photographic world is a great design that has stood the test of time. You start with a camera body with a general-purpose 50-mm lens, and for specialized work, you screw in any of a hundred matching lens types. From fisheye to telescopic, as long as the mounts match, you have a new camera.

I tried to use the same SLR principle in the BitScope design. The device on its own is an extremely useful DSO and logic analyzer, but it is not everything.

The pod connector provides an electronic lens mount for test equipment. Think of the sample RAM in BitScope as a roll of 35-mm film, and the data you store there may come from either built-in connectors or any weird and wonderful "data lens" you care to attach via the data pod. Because the pod architecture and protocol is open and documented, anyone may design a specialized data lens for BitScope.