MICROCONTROLLERS AND EMBEDDED PROCESSORS

This chapter begins with a discussion of the role and importance of microcontrollers in everyday life. In Section 1.1 we also discuss criteria to consider in choosing a microcontroller, as well as the use of microcontrollers in the embedded market. Section 1.2 covers various members of the 8051 family such as the 8052 and 8031, and their features. In addition, we discuss various versions of the 8051 such as the 8751, AT89C51, and DS5000.
SECTION 1.1: MICROCONTROLLERS AND EMBEDDED PROCESSORS
In this section we discuss the need for microcontrollers and contrast them with general-purpose microprocessors such as the Pentium and other x86 microprocessors. We also look at the role of microcontrollers in the embedded market. In addition, we provide some criteria on how to choose a microcontroller.
Microcontroller versus general-purpose microprocessor
What is the difference between a microprocessor and microcontroller? By microprocessor is meant the general-purpose microprocessors such as Intel’s x86 family (8086, 80286, 80386, 80486, and the Pentium) or Motorola’s 680×0 family (68000, 68010, 68020, 68030, 68040, etc.). These microprocessors contain no RAM, no ROM, and no I/O ports on the chip itself. For this reason, they are commonly referred to as general-purpose microprocessors.

Figure 1-1. Microprocessor System Contrasted With Microcontroller System
A system designer using a general-purpose microprocessor such as the Pentium or the 68040 must add RAM, ROM, I/O ports, and timers externally to make them functional. Although the addition of external RAM, ROM, and I/O ports makes these systems bulkier and much more expensive, they have the advantage of versatility such that the designer can decide on the amount of RAM, ROM, and I/O ports needed to fit the task at hand. This is not the case with microcontrollers. A microcontroller has a CPU (a microprocessor) in addition to a fixed amount of RAM, ROM, I/O ports, and a timer all on a single chip. In other words, the processor, RAM, ROM, I/O ports, and timer are all embedded together on one chip; therefore, the designer cannot add any external memory, I/O, or timer to it. The fixed amount of on-chip ROM, RAM, and number of I/O ports in microcontrollers makes them ideal for many applications in which cost and space are critical.



Table 1-1: Some Embedded Products Using Microcontrollers
In many applications, for example a TV remote control, there is no need for the computing power of a 486 or even an 8086 microprocessor. In many applications, the space it takes, the power it consumes, and the price per unit are much more critical considerations than the computing power. These applications most often require some I/O operations to read signals and turn on and off certain bits. For this reason some call these processors IBP, “itty-bitty processors” (see “Good Things in Small Packages Are Generating Big Product Opportunities” by Rick Grehan, BYTE magazine, September 1994; www.byte.com, for an excellent discussion of microcontrollers).
It is interesting to note that some microcontroller manufacturers have gone as far as integrating an ADC (analog-to-digi-tal converter) and other peripherals into the microcontroller.
Microcontrollers for embedded systems
In the literature discussing microprocessors, we often see the term embedded system. Microprocessors and microcontrollers are widely used in embedded system products. An embedded product uses a microprocessor (or microcontroller) to do one task and one task only. A printer is an example of embedded system since the processor inside it performs only one task ; namely, getting the data and printing it. Contrast this with a Pentium-based PC (or any x86 IBM-compatible PC). A PC can be used for any number of applications such as word processor, print server, bank teller terminal, video game player, network server, or internet terminal. Software for a variety of applications can be loaded and run. Of course the reason a PC can perform myriad tasks is that it has RAM memory and an operating system that loads the application software into RAM and lets the CPU run it. In an embedded system, there is only one application software that is typically burned into ROM. An x86 PC contains or is connected to various embedded products such as the keyboard, printer, modem, disk controller, sound card, CD-ROM driver, mouse, and so on. Each one of these peripherals has a microcontroller inside it that performs only one task. For example, inside every mouse there is a microcontroller that performs the task of finding the mouse position and sending it to the PC. Table 1-1 lists some embedded products.
X86 PC embedded applications
Although microcontrollers are the preferred choice for many embedded systems, there are times that a microcontroller is inadequate for the task. For this reason, in recent years many manufacturers of general-purpose microprocessors such as Intel, Freescale Semiconductor Inc. (formerly Motorola), AMD (Advanced Micro Devices, Inc.). and Cyrix (now a division of National Semiconductor, Inc.) have targeted their microprocessor for the high end of the embedded market. While Intel and AMD push their x86 processors for both the embedded and desktop PC markets, Freescale is determined to keep the 68000 family alive by targeting it mainly for the high end of embedded systems now that Apple no longer uses the 680×0 in their Macintosh. In the early 1990s Apple computer began using Power PC microprocessors (604, 603, 620, etc.) in place of the 680×0 for the Macintosh. The Power PC microprocessor is a joint venture between IBM and Freescale, and is targeted for the high end of the embedded market as well as the PC market. It must be noted that when a company targets a general-purpose microprocessor for the embedded market it optimizes the processor used for embedded systems. For this reason these processors are often called high-end embedded processors. Very often the terms embedded processor and microcontroller are used interchangeably.
One of the most critical needs of an embedded system is to decrease power consumption and space. This can be achieved by integrating more functions into the CPU chip. All the embedded processors based on the x86 and 680×0 have low power consumption in addition to some forms of I/O, COM port, and ROM all on a single chip. In high-performance embedded processors, the trend is to integrate more and more functions on the CPU chip and let the designer decide which features he/she wants to use. This trend is invading PC system design as well. Normally, in designing the PC motherboard we need a CPU plus a chip-set containing I/O, a cache controller, a flash ROM containing BIOS, and finally a secondary cache memory. New designs are emerging in industry. For example, Cyrix has announced that it is working on a chip that contains the entire PC, except for DRAM. In other words, we are about to see an entire computer on a chip.
Currently, because of MS-DOS and Windows standardization many embedded systems are using x86 PCs. In many cases using x86 PCs for the high-end embedded applications not only saves money but also shortens development time since there is a vast library of software already written for the DOS and Windows platforms. The fact that Windows is a widely used and well understood platform means that developing a Windows-based embedded product reduces the cost and shortens the development time considerably.

Choosing a microcontroller There are four major 8-bit microcontrollers. They are: Freescale’s 6811, Intel’s 8051, Zilog’s Z8, and PIC 16X from Microchip Technology. Each of these microcontrollers has a unique instruction set and register set; therefore, they are not compatible with each other. Programs written for one will not run on the others. There are also 16-bit and 32-bit microcontrollers made by various chip makers. With all these different microcontrollers, what criteria do designers consider in choosing one? Three criteria in choosing microcontrollers are as follows: (1) meeting the computing needs of the task at hand efficiently and cost effectively, (2) availability of software development tools such as compilers, assemblers, and debuggers, and (3) wide availability and reliable sources of the microcontrbller. Next we elaborate further on each of the above criteria.

Criteria for choosing a microcontroller
1. The first and foremost criterion in choosing a microcontroller is that it must
meet the task at hand efficiently and cost effectively. In analyzing the needs
of a microcontroller-based project, we must first see whether an 8-bit, 16-bit,
or 32-bit microcontroller can best handle the computing needs of the task most
effectively. Among other considerations in this category are:

  1. Speed. What is the highest speed that the microcontroller supports?
    1. Packaging. Does it come in a 40-pin DIP (dual inline package) or a QFP
      (quad flat package), or some other packaging format? This is important in
      terms of space, assembling, and prototyping the end product.
    2. Power consumption. This is especially critical for battery-powered prod
      ucts.
  2. The amount of RAM and ROM on chip.
  3. The number of I/O pins and the timer on the chip.
    1. How easy it is to upgrade to higher-performance or lower power-con
      sumption versions.
    2. Cost per unit. This is important in terms of the final cost of the product in
      which a microcontroller is used. For example, there are microcontrollers
      that cost 50 cents per unit when purchased 100,000 units at a time.



  1. The second criterion in choosing a microcontroller is how easy it is to devel
    op products around it. Key considerations include the availability of an assem
    bler, debugger, a code-efficient C language compiler, emulator, technical sup
    port, and both in-house and outside expertise. In many cases, third-party ven
    dor (that is, a supplier other than the chip manufacturer) support for the chip is
    as good as, if not better than, support from the chip manufacturer.
  2. The third criterion in choosing a microcontroller is its ready availability in
    needed quantities both now and in the future. For some designers this is even
    more important than the first two criteria. Currently, of the leading 8-bit
    microcontrollers, the 8051 family has the largest number of diversified (multi
    ple source) suppliers. By supplier is meant a producer besides the originator
    of the microcontroller. In the case of the 8051, which was originated by Intel,
    several companies also currently produce (or have produced in the past) the
    8051. These companies include: Intel, Atmel, Philips/Signetics, AMD,
    Infineon (formerly Siemens), Matra, and Dallas Semiconductor. See Table 1-2.
Table 1-2: Some of the Companies Producing a Member of the 8051 Family





It should be noted that Freescale, Zilog, and Microchip Technology have all dedicated massive resources to ensure wide arid timely availability of their product since their product is stable, mature, and single sourced. In recent years they also have begun to sell the ASIC library cell of the microcontroller.

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