A Microcontroller Survey

A Microcontroller Survey

Markets for microcontrollers can run into millions of units per applica­tion. At these volumes the microcontroller is a commodity item and must be optimized so that cost is at a minimum. Semiconductor manufacturers have produced a mind-numbing array of designs that would seem to meet almost any need. Some of the chips listed in this section are no longer in regular pro­duction, most are current, and a few are best termed "smokeware": the dreams of an aggressive marketing department.

Four-Bit Microcontrollers

In a commodity chip, expense is represented more by the volume of the package and the number of pins it has than the amount of silicon inside. To minimize pin count and package size, it is necessary that the basic data word-bit count be held to a minimum, while still enabling useful intelligence to be implemented.

Although 4 bits, in this era of 64-bit "maximicros," may seem somewhat ludicrous, we must recall that the original 4004 was a 4-bit device, and all else followed. Indeed, in terms of production numbers, the 4-bit microcontroller is today the most popular micro made. The following table lists representative models from major manufacturers’ data books. Many of these designs have been licensed to other vendors.

These 4-bit microcontrollers are generally intended for use in large vol­umes as true 1 -chip computers; expanding external memory, although possible, would negate the cost advantage desired. Typical applications consist of appli­ances and toys_; worldwide volumes run into the tens of millions.

Eight-Bit Microcontrollers

Eight-bit microcontrollers represent a transition zone between the dedi­cated, high-volume, 4-bit microcontrollers and the high-performance, 16- and 32-bit units that will conclude this topic .

Microprocessors and Microcontrollers

Eight bits has proven to be a very useful word size for small computing tasks. Capable of 256 decimal values, or quarter-percent resolution, the 1-byte word is adequate for many control and monitoring applications. Serial ASCII data is also stored in byte sizes, making 8 bits the natural choice for data communications . Most integrated circuit memories and many logic functions are arranged in an 8-bit configuration that interfaces easily to data busses of 8 bits.

Application volumes for 8-bit microcontrollers may be as high as the 4-bit models, or they may be very low. Application sophistication can also range from simple appliance control to high-speed machine control and data collection. For these reasons, the microcontroller vendors have established extensive "fami-lies" of similar models. All feature a common language, but differ in the amount of internal ROM, RAM, and other cost-sensitive features. Often the memory can be expanded to include off-chip ROM and RAM; in some cases, the micro­controller has no on-board ROM at all, or the ROM is an electrically repro­grammable read only memory (EPROM).

The purpose of this diversity is to offer the designer a menu of similar de­vices that can solve almost any problem. The ROMless or EPROM versions can be used by the designer to prototype the application, and then the designer can order the ROM version in large quantities from the factory. Many times the ROM sight ^version is never used. The designer makes the ROMless or EPROM design sufficiently general so that one configuration may be used many times, or pro­duction volumes never justify the cost of a factory ROM implementation. As a further enticement for the buyer, some families have members with fewer ex­ternal pins to shrink the package and the cost; others have special features such .as analog-to-digital (A/D) and digital-to-analog (D/A) converters on the chip.

The 8-bit arena is crowded with capable and cleverly designed contenders; this is the growth segment of the market and the manufacturers are responding vigorously to the marketplace. The following table lists the generic family name for each chip, but keep in mind that ROMless, EPROM, and reduced pin-count members of the family are also available. Each entry in the table has many variations; the total number of configurations available exceeds a total of 80 types for the 11 model numbers listed.

CAUTION

Not all of the pins can be used for general – purpose I/O and addressing external memory al the same time. The sales literature should be read with some care lo see how many of the pins have more than one function. Inspection of the table shows that the designers made tradeoffs: external memory addressing for extra on – chip functions. Generally, the ability to expand memory off the chip implies that a ROMless family member is available for use in limited production numbers where the expense of factory programming can be avoided. Lack of this feature implies that the chip is meant for high production volumes where the expense of factory-programmed parts can be amortized over a large number of devices. A compromise may be reached by using one time-prgrammable (OTP) parts.

Sixteen-Bit Microcontrollers

Eight-bit microcontrollers can be used in a variety of applications that in­volve limited calculations and relatively simple control strategies. As the re­quirement for faster response and more sophisticated calculations grows, the 8-bit designs begin to hit a limit inherent with byte-wide data words. One so­lution is to increase clock speeds; another is to increase the size of the data word. Sixteen-bit microcontrollers have evolved to solve high-speed control problems of the type that might typically be confronted in the control of ser-vomechanisms, such as robot arms.

The designs become much more focused on these types of real-time prob­lems,- some generality is lost, but the vendors still try to hit as many market­ing targets as they can. The following table lists only three contenders. Intel has recently begun vigorously marketing the MCS-96 family. Other vendors are expected to appear as this market segment grows in importance.

The pulse width modulation (PWM) output is useful for controlling motor speed; it can be done using software in the 8-bit units with the usual loss of time for other tasks.

The 16- (and 32-) bit controllers have also been designed to take advantage of high-level programming languages in the expectation that very little assem­bly language programming will be done when employing these controllers in sophisticated applications.

Thirty-Two Bit Microcontrollers

Crossing the boundary from 16 to 32 bits involves more than merely dou­bling the word size of the computer. Software boundaries that separate dedi­cated programs from supervisory programs are also breached. Thirty-two bit designs target robotics, highly intelligent instrumentation, avionics, image pro­cessing, telecommunications, automobiles, and other environments that fea­ture application programs running under an operating system. The line between microcomputers and microcontrollers becomes very fine here.

All of the functions needed for I/O, data communications, and timing and ^x counting are done by adding other specialized chips.

This manufacturer has dubbed all of its microcontrollers embedded controllers, a term that seems to describe the function of the 32-bit 80960 very well.