How to Choose an Appropriate CPU
by
Eric Thompson, Application Engineer
VersaLogic Corp.

When shopping for technology a business needs to consider what their needs are, and buy enough technology to handle the application. They do this so they can remain competitive in their industry. There are many different levels of technology available that offer a wide range of capabilities. From the powerful Pentium CPU at 166 MHz to a basic 186 CPU at 16 MHz.

The decision is usually based on the issue of price. Many companies simply take the approach to purchase as much technology as they can afford. This seems like a logical approach to most people, but it may not always be the best or most cost effective approach.

For simple "workhorse" applications in an industrial environment, 486 or Pentium technology is not always needed. Sure it's available, and its nice to know that there is always room for expansion, but it comes with a higher price. Slower processors can usually handle simple repetitive functions just as well as faster CPUs.

Processing speed can be compared roughly to how many events per unit time the CPU can perform. Since an "event" can be anything from a simple read of an I/O port to a complex mathematical computation, published benchmarks of CPU performance really aren't of much use.

A detailed analysis of the application could be done to quantify performance issues, but it is not a simple task. The actual machine code would need to be analyzed with the CPU clock cycles for each code added up. This can be a valuable method for small critical sections of code, but in general it would prove to be too time consuming.

Many industrial companies are using the plug-in capability of the STD 32 bus system that offers a simple solution to the performance question. Develop the system with a powerful unit like a 50 MHz 486 based board, and when done, swap the higher power board with a less powerful, less expensive 386 or 186 based CPU. The plug-in compatibility of the STD 32 bus makes this last step easy to do, and can optimize your system for appropriate performance while minimizing cost.

This plug-in capability also gives the flexibility of being able to upgrade the system later on if the requirements of the system are changed.

Limitations
Each type of application requires different amount of memory, both RAM and ROM. If a large amount of memory is required, then CPU choices are quickly limited to higher end CPUs such as the 386 or 486. The 186 CPU will only address 1 MB of memory, while a 386 or 486 will address 16 MB or more. A few applications also require 32-bit code which would also limit choices by requiring a higher end unit.

Cost Concerns
Generally a system using a 16-bit data bus (186, 386SX,486SXLC) will cost less than one using a 32-bit (386DX, 486DX) or a 64-bit Pentium data bus. The reason is that the latter systems have more memory chips with their associated buffers and signal terminators. These extra components add to the total cost. Another cost consideration that many people overlook is the amount of power required for the system and the cooling requirements. As the speed and performance increase, the requirements for power and cooling also increase. Over the long run these cost can add up, making the decision to buy an appropriate amount of computing power even more important.

Availability
Most chip-sets originally developed for the desktop market are only available for 2 to 3 years. The latest desktop technology will only be "the latest" for that short amount of time. After that, desktop manufacturers will be spending most of their time on "new" technology. That means that upgrading a desktop system will be difficult several years down the road.

Most embedded system manufacturers are still upgrading and expanding their product line even though there are newer technologies available. For most industrial and military embedded system uses, the products are expected to have a 5 to 10 year useful life.

Manufacturers of embedded systems understand that these systems stay around longer so they still offer parts for these units. The 186, 386SX, and the 486 SXLC were developed for the embedded market and are still being manufactured and updated. This practice should continue for many years. Also, the low-tech demand from industrial companies will remain as long as there is a cost savings associated with it.

A perfect example is the 186. This CPU has been around for over ten years and it is still being expanded and updated by CPU manufacturers. They do this because many companies are still using the 186 in their "workhorse" systems that don't require the fastest or the latest technology.

Manufacturers of the 186 CPU offer clock speeds of up to 16 MHz for this type of board. If that is fast enough to handle the application, and it doesn't require over 1 MB of RAM or ROM, then the 186 CPU board is a very cost-effective solution.

In this day of increased competition and tight corporate wallets, it is increasingly important to stay current with technological advancements, but to do it in a way that requires no more capital expenditures than necessary. The STD-32 bus is a convenient, affordable system that allows easy upgrading of an embedded system even in the harsh manufacturing environment that is common for most industrial applications.