Chapter 1
Introduction to Digital Systems

Digital systems are designed to store, process, and communicate information in digital form. They are found in a wide range of applications, including process control, communication systems, digital instruments, and consumer products. The digital computer, more commonly called the computer, is an example of a typical digital system.

A computer manipulates information in digital, or more precisely, binary form. A binary number has only two discrete values - zero or one. Each of these discrete values is represented by the OFF and ON status of an electronic switch called a transistor. All computers, therefore, only understand binary numbers. Any decimal number (base 10, with ten digits from 0 to 9) can be represented by a binary number (base 2, with digits 0 and 1).

The basic blocks of a computer are the central processing unit (CPU), the memory, and the input/output (I/O). The CPU of the computer is basically the same as the brain of a human. Computer memory is conceptually similar to human memory. A question asked to a human is analogous to entering a program into the computer using an input device such as the keyboard, and answering the question by the human is similar in concept to outputting the result required by the program to a computer output device such as the printer. The main difference is that human beings can think independently, whereas computers can only answer questions that they are programmed for. Computer hardware refers to components of a computer such as memory, CPU, transistors, nuts and bolts. Programs can perform a specific task such as addition if the computer has an electronic circuit capable of adding two numbers. Programmers cannot change these electronic circuits but can perform tasks on them using instructions.

Computer software, on the other hand, consists of a collection of programs. Programs contain instructions and data for performing a specific task. These programs, written using any programming language such as C, must be translated into binary prior to execution by the computer. This is because the computer only understands binary numbers. Therefore, a translator for converting such a program into binary is necessary. Hence, a translator program called the compiler is used for translating programs written in a programming language such as C into binary. These programs in binary form are then stored in the computer memory for execution because computers only understand 1's and 0's. Furthermore, computers can only add. This means that all operations such as subtraction, multiplication, and division are performed by addition.

Due to advances in semiconductor technology, it is possible to fabricate the CPU in a single chip. The result is the microprocessor. Both metal oxide semiconductor (MOS) and bipolar technologies were used in the fabrication process. The CPU can be placed on a single chip when MOS technology is used. However, several chips are required with the bipolar technology. HCMOS (high speed complementary MOS) or BICMOS (combination of bipolar and HCMOS) technology (to be discussed later in this chapter) is normally used these days to fabricate the microprocessor in a single chip. Along with the microprocessor chip, appropriate memory and I/O chips can be used to design a microcomputer. The pins on each one of these chips can be connected to the proper lines on the system bus, which consists of address, data, and control lines. In the past, some manufacturers have designed a complete microcomputer on a single chip with limited capabilities. Single-chip microcomputers were used in a wide range of industrial and home applications.

"Microcontrollers" evolved from single-chip microcomputers. The micro-controllers are typically used for dedicated applications such as automotive systems, home appliances, and home entertainment systems. Typical microcontrollers, therefore, include a microcomputer, timers, and A/D (analog to digital) and D/A (digital to analog) converters - all in a single chip. Examples of typical microcontrollers are Intel 8751 (8-bit) / 8096 (16-bit) and Motorola/Freescale Hc011 (8-bit) / Hc016 (16-bit), and Microchip Technology PIc018F(8-bit)/PIC32(32-bit).

In this chapter, we first define some basic terms associated with the computers. We then describe briefly the evolution of the computers and the microcontrollers. Finally, a typical practical application, and technological forecasts are included.

1.1 Explanation of Terms

Before we go on, it is necessary to understand some basic terms (arranged in alphabetical order).

• Address is a pattern of 0's and 1's that represents a specific location in memory or a particular I/O device. An 8-bit microcontroller with 16 address bits can produce 216 unique 16-bit patterns from 0000000000000000 to 1111111111111111, representing 65,536 different address combinations (addresses 0 to 65,535).
• Addressing mode is the manner in which the microcontroller determines the operand (data) and destination addresses during execution of an instruction.
• Arithmetic-logic unit (ALU) is a digital circuit that performs arithmetic and logic operations on two n-bit digital words. Typical operations performed by an ALU are addition, subtraction, ANDing, ORing, and comparison of two n-bit digital words. The size of the ALU defines the size of the microcontroller. For example, an 8-bit microcontroller contains an 8-bit ALU.
• Big endian convention is used to store a 16-bit number such as 16-bit data in two bytes of memory locations as follows: the low memory address stores the high byte while the high memory address stores the low byte. The Motorola/Freescale Hc011 8-bit microcontroller follows the big endian format.
• Bit is an abbreviation for the term binary digit. A binary digit can have only two values, which are represented by the symbols 0 and 1, whereas a decimal digit can have 10 values, represented by the symbols 0 through 9. The bit values are easily implemented in electronic and magnetic media by two-state devices whose states portray either of the binary digits 0 and 1. Examples of such two-state devices are a transistor that is conducting or not conducting, a capacitor that is charged or discharged, and a magnetic material that is magnetized north to south or south to north.
• Bit size refers to the number of bits that can be processed simultaneously by the basic arithmetic circuits of a microcontroller. A number of bits taken as a group in this manner is called a word. For example, an 8-bit microcontroller can process an 8-bit word. An 8-bit word is referred to as a byte, and a 4-bit word is known as a nibble.
• Bus consists of a number of conductors (wires) grouped to provide a means of communication among different elements in a microcontroller system. The conductors in a bus can be grouped in terms of their functions. A microcontroller normally has an address bus, a data bus, and a control bus. Address bits are sent to memory or to an external device on the address bus. Instructions from memory, and data to/from memory or external devices, normally travel on the data bus. Control signals such as read/write for the other buses and among system elements are transmitted on the control bus. Buses are sometimes bidirectional; that is, information can be transmitted in either direction on the bus, but normally in only one direction at a time.
• Clock is analogous to human heart beats. The microcontroller requires synchronization among its components, and this is provided by a clock or timing circuits.
• The chip is an integrated circuit (IC) package containing digital circuits.
• CPU (central processing unit) contains several registers (memory elements), an ALU, and a control unit. Note that the control unit translates instructions and performs the desired task. The number of peripheral devices depends on the particular application involved and may even vary within an application.
• EEPROM or E2PROM (electrically erasable programmable ROM) is nonvolatile. EEPROMs can be programmed without removing the chip from the socket. EEPROMs are called read most memories (RMMs) because they have much slower write times than read times. Therefore, these memories are usually suited for applications when mostly reading rather than writing is performed. An example of EEPROM is the 2864 (8K x 8).
• EPROM (erasable programmable ROM) is nonvolatile. EPROMs can be programmed and erased. The EPROM chip must be removed from the socket for programming. This memory is erased by exposing the chip to ultraviolet light via a lid or window on the chip. Typical erase times vary between 10 and 30 minutes. The EPROM is programmed by inserting the chip into a socket of the EPROM programmer, and providing proper addresses and voltage pulses at the appropriate pins of the chip. An example of EPROM is the 2764 (8K × 8).
• Flash memory is designed using a combination of EPROM and EEPROM technologies. Flash memory is nonvolatile and is invented by Toshiba in mid 1980s. Flash memory can be programmed electrically while embedded on the board. One can change multiple bytes at a time. An example of flash memory is the Intel 28F020 (256K × 8). Flash memory is typically used in cell phones and digital cameras.
• An FPGA (field programmable gate array) chip contains an array of digital logic blocks along with input and output blocks which can be connected together via...