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PART 1: WHATıS IT ALL ABOUT?
by George Martin

Start ı Narrowing Specs ı Selecting the CPU ı Packaging ı The Software ı Memory ı Your Job ı Sources and PDF

PACKAGING

Packaging is next. We probably need to leave the through-hole world and enter the surface-mount arena for the CPU and memory. The high pin counts of the available devices make this a necessary shift.

But, in the surface-mount arena, there are still several packaging density choices. Components with pads at 0.050ı spacing (SOP, SOJ, PLCC) offer 0.010" line width and 0.010" line spacing, which allows one line between each pad. Components with pads at 0.020" spacing (TSOP) require a 0.008" line width and spacing. Then, you probably donıt want to put lines between pads.

An IC-packaging technique (BGA) puts solder balls in an array on the bottom of the chip. These solder balls melt and bond to pads when the devices are heated to the proper temperature. You have to inspect these bonds with x-ray equipment or at least x-ray glasses.

If you donıt have the sophisticated equipment in-house to assemble the surface-mount devices, you can find contract manufacturers to provide these services.

For this design, letıs select 0.050" packages and use 0.010" design rules.

DEVELOPMENT TOOLS

What are we going to use for development tools? Iıd like to have an emulator. But, that causes two problems: I donıt want to invest those kinds of dollars in capital equipment (neither does your management) and most of the chips I mentioned donıt have an emulator.

How is that possible? As devices have been getting smaller and faster, the special chips that were used as the basis of emulators were no longer practical. These special chips were special faster devices with special emulator-only functions available on the pins. Essentially, the manufacturers built these and other support features into the later generations of CPUs.

And, if we use surface-mount technology for manufacturing, then we wonıt be able to plug the emulator in and out. So, todayıs devices have an emulator interface built in and/or are so complete that you just need to provide power, clock, and memory to load startup code.

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