August 1998, Issue 97

BitScope
A Mixed-Signal Capture Engine

THE BIG PICTURE

The basic idea behind BitScope is that of a specialized piece of data-capture hardware that doesn’t include any user interface other than an RS-232 plug. Most engineers have more computers, mice, and keyboards than they know what to do with. If I was going to build a cheap ’scope, I certainly didn’t want any more of that stuff.

What I needed was an electronic drone that could capture and disgorge data on command. No more, no less.

The commands had to be simple ASCII characters that are intuitive and easy to learn. The PC-based user interface can then synthesize functionality of arbitrary complexity by sending scripts of command characters and receiving the replies.

The answer: a virtual instrument where specialized hardware does the electronic test job and a PC lets the engineer drive it. One big advantage of this setup is that changing the way the virtual instrument works doesn’t usually involve reprogramming chips (hard) but may be done by downloading a new program from the ’Net (easy).

As described in the sidebar "Virtual Machine Architecture," the microcontroller firmware is designed as a virtual machine (VM). The BitScope design is novel because it has an unusual arrangement of the VM program code. The instructions are not located in memory on the microcontroller but reside in the user interface and are executed atomically direct from the serial port. (Table 1).