Electronic Design

Bit Twiddling

Sometimes so many things pile up in the lab, I don't get around to checking them out until much later. Macraigor's JScan is one of those things. JScan builds on Macraigor's usbDemon, which is typically used for JTAG debugging. It provides IEEE 1149.1-compliant boundary-scan support that lets designers see the state of each point in the scan chain. It also allows designers to twiddle the bits.

Boundary scanning is nothing new. In fact, it's a standard. Nevertheless, few developers choose to contend with boundary scan. It's often relegated to testing systems rather than debugging them due to cost and complexity. This is why boundary scan is invaluable in a higher-end development environment. Here, price isn't an issue. Additionally, the valuable capabilities it provides are worth the trouble to learn to take advantage of the hardware.

JScan's target audience tends to be closer to the overall hands-on design and development. It's about time, too, because the higher packing density and multiple-layer boards make probing a bit more difficult, if not impossible, once the design is set in concrete. This is especially true for systems with FPGAs and 32-bit micros that include this support.

The demo board was neat. Jumpers let designers bypass any of the three FPGAs. Each has a set of simple LEDs or switches that can be exercised. Likewise, the JScan's cables are for individual pins. It's a pain in the neck to plug it in, but it works with any hardware. Once it was hooked up, I could twiddle bits to my heart's content with nary a probe in sight. The software makes the difference, but that's a story for another day.

See Figure 1 and Figure 2.

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