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 programming using a hardware description language (HDL) such as Verilog or VHDL. There are two types of components inside an FPGA. These are lookup table (stored in memory), and switch matrices. The concept of FPGA is based on the fact that a combinational circuit can be implemented using memory. In the past, digital logic circuits were built using all hardware (logic gates). It was a time-consuming task to debug the circuits. However, digital circuits implemented using FPGA's are faster to debug since they are programmable. Note that it is much faster to debug software than hardware. Hence, products can be developed using FPGA from conceptual design via prototype to production in a very short time. Therefore, use of FPGA in digital logic is very common these days.

The term gate refers to digital circuits which perform logic operations such as AND, OR, and NOT. In an AND operation, the output of the AND gate is one if all inputs are one; the output is zero if one or more inputs are zero. The OR gate, on the other hand, provides a zero output if all inputs are zero; the output is one if one or more inputs are one. Finally, a NOT gate (also called an inverter) has one input and one output. The NOT gate produces one if the input is zero; the output is zero if the input is one.

Harvard architecture is a type of CPU architecture which uses separate instruction and data memory units along with separate buses for instructions and data. This means that these processors can execute instructions and access data simultaneously. Processors designed with this architecture require four buses for program memory and data memory. These are one data bus for instructions, one address bus for addresses of instructions, one data bus for data, and one address bus for addresses of data. The sizes of the address and data buses for instructions may be different from the address and data buses for data. Several microcontrollers including the PIc018F are designed using the Harvard architecture. This is because it is inexpensive to implement these buses inside the chip since both program and data memories are internal to the chip.

Instruction set of a microcontroller is a list of commands that the microcontroller is designed to execute. Typical instructions are ADD, SUBTRACT, and STORE. Individual instructions are coded as unique bit patterns which are recognized and executed by the microcontroller. If a microcontroller has three bits allocated to the representation of instructions, the microcontroller will recognize a maximum of 2³, or eight, different instructions. The microcontroller will then have a maximum of eight instructions in its instruction set. It is obvious that some instructions will be more suitable than others to a particular application. For example, in a control application, instructions inputting digitized signals to the processor and outputting digital control variables to external circuits are essential. The number of instructions necessary in an application will directly influence the amount of hardware in the chip set and the number and organization of the interconnecting bus lines.

Little 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 low byte while the high memory address stores the high byte. The PIc018F microcontroller follows the little-endian format.

Microcomputer typically consists of a microprocessor (CPU) chip, input and output chips, and memory chips in which programs (instructions and data) are stored.

Microcontroller is implemented on a single chip containing a CPU, memory, and IOP (I/O and peripherals). Note that a typical IOP contains I/O unit of a microcomputer, timers, A/D (analog-to-digital) converter, analog comparators, serial I/O, and other peripheral functions (to be discussed later).

Microprocessor is the CPU of a microcomputer contained in a single chip, and must be interfaced with peripheral support chips in order to function.

Pipelining is a technique that overlaps instruction fetch (instruction read) with execution. This allows a microcontroller's processing operation to be broken down into several steps (dictated by the number of pipeline levels or stages) so that the individual step outputs can be handled by the microcontroller in parallel. Pipelining is often used to fetch the microcontroller's next instruction while executing the current instruction, which speeds up the overall operation of the microcontroller considerably. Microchip technology's PIc018F (8-bit microcontroller) uses a two-stage instruction pipeline in order to speed up instruction execution.

Program contains instructions and data. Two conventions are used to store a 16-bit number such as 16-bit data in two bytes of memory locations. These are called little endian and big endian byte ordering. In little endian convention, the low memory address stores the low byte while the high memory address stores the high byte. For example, the 16-bit hexadecimal number, 2050 will be stored as two bytes in two 16-bit locations (Hex 5000 and Hex 5001) as follows: Address 5000 will contain 50 while address 5001 will store 20. In big endian convention, on the other hand, the low memory address stores the high byte while the high memory address stores the low byte. For example, the same 16-bit hexadecimal number, 2050 will be stored as two bytes in two 16-bit locations (Hex 5000 and Hex 5001) as follows: Address 5000 will contain 20 while address 5001 will store 50. Motorola / Freescale Hc011 (8-bit microcontroller) follows big endian convention. Microchip PIc018F (8-bit microcontroller), on the other hand, follows the little endian format.

Random-access memory (RAM) is a storage medium for groups of bits or words whose contents cannot only be read but can also be altered at specific addresses. A RAM normally provides volatile storage, which means that its contents are lost in case power is turned off. There are two types of RAM: static RAM (SRAM) and dynamic RAM (DRAM). Static RAM stores data in flip-flops. Therefore, this memory does not need to be refreshed. An example of SRAM is 6116 (2K × 8). Dynamic RAM, on the other hand, stores data in capacitors. That is, it can hold data for a few milliseconds. Hence, dynamic RAMs are refreshed typically by using external refresh circuitry. Dynamic RAMs (DRAMs) are used in applications requiring large memory. DRAMs have higher densities than static RAMs (SRAMs). Typical examples of DRAMs are the 4464 (64K × 4), 44256 (256K × 4), and 41000 (1M × 1). DRAMs are inexpensive, occupy less space, and dissipate less power than SRAMs.

Read-only memory (ROM) is a storage medium for the groups of bits called words, and its contents cannot normally be altered once programmed. A typical ROM is fabricated on a chip and can store, for example, 2048 eight-bit words, which can be accessed individually by presenting to it one of 2048 addresses. This ROM is referred to as a 2K by 8-bit ROM. 10110111 is an example of an 8-bit word that might be stored in one location in this memory. A ROM is a nonvolatile storage device, which means that its contents are retained in case power is turned off. Because of this characteristic, ROMs are used to store permanent programs (instructions and data).

Reduced Instruction Set Computer (RISC) contains a simple instruction set. In contrast, a Complex Instruction Set Computer (CISC) contains a large instruction set. The PIc018F is an RISC-based microcontroller while Motorola/Freescale Hc011 is a CISC-based microcontroller.

Register can be considered as volatile storage for a number of bits. These bits may be entered into the register simultaneously (in parallel) or sequentially (serially) from right to left or from left to right, 1-bit at a time. An 8-bit register storing the bits 11110000 is represented as follows:

The speed power product (SPP) is a measure of performance of a logic gate. It is expressed in picojoules (pJ). SPP is obtained by multiplying the speed (ns) by the power dissipation (mW) of a gate.

Transistors are basically electronic switching devices. There are two types of transistors. These are bipolar junction transistors (BJTs) and metal-oxide semiconductor (MOS) transistors. The operation of the BJT depends on the flow of two types of carriers: electrons (n-channel) and holes (p-channel), whereas the MOS transistor is unipolar and its operation depends on the flow of only one type of carrier, either electrons (n-channel) or holes (p-channel).

von Neumann (Princeton) architecture uses a single memory unit and the same bus for accessing both instructions and data. Although CPUs designed using this architecture are slower compared to Harvard architecture since instructions and data cannot be accessed simultaneously because of the single bus, typical microprocessors such as the Pentium use this architecture. This is because memory units such as ROMs, EPROMs, and RAMs are external to the microprocessor. This will require almost half the number of wires on the mother board since address and data pins for only two buses rather than four buses (Harvard architecture) are required. This is the reason Harvard architecture would be very expensive if utilized in designing microprocessors. Note that microcontrollers using Harvard architecture internally will have to use von Neumann architecture externally. Texas Instrument's MSP 430 uses the von Neumann architecture.

1.2 Design Levels

Three design levels can be defined for digital systems: systems level, logic level, and device level.

Systems level is the type of design in which CPU, memory, and I/O chips are interfaced to build a computer.

Logic level is the design technique in which chips containing logic gates such as AND, OR, and NOT are used to design a digital component such as the ALU.

Finally, device level utilizes transistors to design logic gates.

1.3 Combinational and Sequential Circuits

Digital circuits at the logic level can be classified into two types. These are combinational and sequential.

Combinational circuits contain no memory, whereas sequential circuits require memory to remember the present state in order to go to the next state. A binary adder capable of providing the sum upon application of the numbers to be added is an example of a combinational circuit. For example, consider a 4-bit adder. The inputs to this adder will be two 4-bit numbers; the output will be the 4-bit sum. In this case, the adder will generate the 4-bit sum output upon application of the two 4-bit inputs.

Sequential circuits, on the other hand, require memory. The counter is an example of a sequential circuit. For instance, suppose that the counter is required to count in the sequence 0, 1, 2 and then repeat the sequence. In this case, the counter must have memory to remember the present count in order to go to the next. The counter must remember that it is at count 0 in order to go to the next count, 1. In order to count to 2, the counter must remember that it is counting 1 at the present state. In order to repeat the sequence, the counter must count back to 0 based on the present count, 2, and the process continues. A chip containing sequential circuit such as the counter will have a clock input pin.

In general, all computers contain both combinational and sequential circuits. However, most computers are regarded as clocked sequential systems. In these computers, almost all activities pertaining to instruction execution are synchronized with clocks.

1.4 Digital Integrated Circuits

The transistor can be considered as an electronic switch. The ON and OFF states of a transistor are used to represent binary digits. Transistors, therefore, play an important role in the design of digital systems. This section describes the basic characteristics of digital devices and logic families. These include diodes, transistors, and a summary of digital logic families. These topics are covered from a very basic point of view. This will allow the readers with some background in digital devices to see how they are utilized in designing digital systems.

1.4.1 Diodes

A diode is an electronic switch. It is a two-terminal device. Figure 1.1 shows the symbolic representation.

Figure 1.1 Symbolic representations of a diode.

The positive terminal (made with the p-type semiconductor material) is called the anode; the negative terminal (made with the n-type semiconductor material) is called a cathode. When a voltage, V = 0.6 volt is applied across the anode and the cathode, the switch closes and a current I flows from anode to the cathode.

1.4.2 Transistors

A bipolar junction transistor (BJT) or commonly called the transistor is also an electronic switch like the diode. Both electrons (n-channel) and holes (p-channel) are used for carrier flow; hence, the name bipolar is used. The BJT is used in transistor logic circuits that have several advantages over diode logic circuits. First of all, the transistor acts as a logic device called an inverter. Note that an inverter provides a LOW output for a HIGH input and a HIGH output for a LOW input. Secondly, the transistor is a current amplifier (buffer). Transistors can, therefore, be used to amplify these currents to control external devices such as a light emitting diode (LED) requiring high currents. Finally, transistor logic gates operate faster than diode gates.

There are two types of transistors, namely npn and pnp. The classification depends on the fabrication process. npn transistors are widely used in digital circuits.

Figure 1.2 shows the symbolic representation of an npn transistor. The transistor is a three-terminal device. These are base, emitter, and collector. The transistor is a current-controlled switch. This means that an adequate current at the base will close the switch allowing a current to flow from the collector to the emitter.

Figure 1.2 (a and b) Symbolic representations of an npn transistor.

This current direction is identified on the npn transistor symbol in Figure 1.2(a) by a downward arrow on the emitter. Note that a base resistance is normally required to generate the base current.

The transistor has three modes of operation: cutoff, saturation, and active. In digital circuits, a transistor is used as a switch, which is either ON (closed) or OFF (open). When no base current flows, the emitter-collector switch is open and the transistor operates in the cutoff (OFF) mode. On the other hand, when a base current flows such that the voltage across the base and the emitter is at least 0.6 V, the switch closes. If the base current is further increased, there will be a situation in which VCE (voltage across the collector and the emitter) attains a constant value of approximately 0.2 V. This is called the saturation (ON) mode of the transistor. The active mode is between the cutoff and saturation modes. In this mode, the base current (IB) is amplified so that the collector current, IC = IB, where is called the gain, and is in the range of 10 to 100 for typical transistors. Note that when the transistor reaches saturation, increasing IB does not drop VCE below VCE (Sat.) of 0.2 V. On the other hand, VCE varies from 0.8 V to 5 V in the active mode. Therefore, the cutoff (OFF) and saturation (ON) modes of the transistor are used in designing digital circuits. The active mode of the transistor in which the transistor acts as a current amplifier (also called buffer) is used in digital output circuits.

Operation of the Transistor as an Inverter

Figure 1.3 shows how to use the transistor as an inverter. When VIN = 0, the transistor is in cutoff (OFF), and the collector-emitter switch is open. This means that no current flows from +VCC to ground. VOUT is equal to +VCC. Thus, VOUT is high.

Figure 1.3 An inverter.

On the other hand, when VIN is HIGH, the emitter-collector switch is closed. A current flows from +VCC to ground. The transistor operates in saturation, and VOUT = VCE (Sat) = 0.2 V ≈ 0. Thus, VOUT is basically connected to ground.

Therefore, for VIN = LOW, VOUT = HIGH, and for VIN = HIGH, VOUT = LOW. Hence, the npn transistor in Figure 1.3 acts as an inverter.

Note that VCC is typically +5 V DC. The input voltage levels are normally in the range of 0 to 0.8 volts for LOW and 2 volts to 5 volts for HIGH. The output voltage levels, on the other hand, are normally 0.2 volts for LOW and 3.6 volts for HIGH.

Light Emitting Diodes (LEDs) and Seven Segment Displays

LEDs are extensively used as outputs in digital systems as status indicators. An LED is typically driven by low voltage and low current. This makes the LED a very attractive device for use with digital systems. Table 1.1 provides the current and voltage requirements of red, yellow, and green LEDs.

Table 1.1 Current and voltage requirements of LEDs

Basically, an LED will be ON, generating light, when its cathode is sufficiently negative with respect to its anode. A digital system such as a microcomputer can, therefore, light an LED either by grounding the cathode (if the anode is tied to +5 V) or by applying +5 V to the anode (if the cathode is grounded) through an appropriate resistor value. A typical hardware interface between a microcomputer and an LED is depicted in Figure 1.4. A microcomputer normally outputs 400 μ A at a minimum voltage, VM = 2.4 volts for a HIGH. The red LED requires 10 mA at 1.7 volts. A buffer (current amplifier) such as a transistor is required to turn the LED ON. Since the transistor is an inverter, a HIGH input to the transistor will turn the LED ON. We now design the interface; that is, the values of R1, R2, and the gain β for the transistor will be determined. Note that the outputs of typical microcontrollers such as the PIc018F are buffered.

Figure 1.4 Microcomputer - LED interface.

A HIGH at the microcomputer output will turn the transistor ON into active mode. This will allow a path of current to flow from the +5 V source through R2 and the LED to the ground. The appropriate value of R2 needs to be calculated to satisfy the voltage and current requirements of the LED. Also, suppose that VBE = 0.6 V when the transistor is in active mode. This means that R1 needs to be calculated with the specified values of VM = 2.4 V and I = 400 μA. The values of R1, R2, and β are calculated as follows:

Assuming VCE ≅ 0,

Therefore, the interface design is complete, and a transistor with a minimum β of 25, R1 = 4.5 KΩ, and R2 = 330Ω are required.

An inverting buffer chip such as the 74LS04 can be used in place of a transistor in Figure 1.4. A typical interface of an LED to a microcomputer via an inverter is shown in Figure 1.5. Note that the transistor base resistance is inside the inverter. Therefore, R1 is not required to be connected to the output of the microcomputer. The symbol is used to represent an inverter. Inverters will be discussed in more detail later. In Figure 1.5, when the microcomputer outputs a HIGH, the transistor switch inside the inverter closes. A current flows from the +5 V source, through the 330-ohm resistor and the LED, into the ground inside the inverter. The LED is thus turned ON.

Figure 1.5 Microcomputer - LED interface via an inverter.

Note that if 5V is used to turn the LED ON and 0V to turn it OFF, the LED should be connected as shown in Figure 1.6.

Figure 1.6 An LED connection to be turned ON by 5V and turned OFF by 0V.

However, if 0 is used to to turn the LED ON and 5V to turn it OFF, the LED should be connected as shown in Figure 1.7.

Figure 1.7 An LED connection to be turned ON by 5V and turned OFF by 5V.

Note that an LED must not be connected according to the circuit shown in Figure 1.8. This is because the circuit will not provide 1.7V accross the LED and a current of 10 ma through it.

Figure 1.8 An invalid LED connection.

A seven-segment display can be used to display, for example, decimal numbers from 0 to 9. The name seven segment is based on the fact that there are seven LEDs — one in each segment of the display. Figure 1.9 shows a typical seven-segment display.

Figure 1.9 A seven-segment display.

In Figure 1.9, each segment contains an LED. All decimal numbers from 0 to 9 can be displayed by turning the appropriate segment ON or OFF. For example, a zero can be displayed by turning the LED in segment g OFF and turning the other six LEDs in segments a through f ON. There are two types of seven-segment displays. These are common cathode and common anode. Figure 1.10 shows these display configurations.

Figure 1.10 Seven-segment display configurations.

In a common cathode arrangement, the microcomputer can send a HIGH to light a segment and a LOW to turn it off. In a common anode configuration, on the other hand, the microcomputer sends a LOW to light a segment and a HIGH to turn it off. In both configurations, R = 330 ohms can be used.

Transistor Transistor Logic (TTL) and Its Variations

The transistor transistor logic (TTL) family of chips evolved from diodes and transistors. This family used to be called DTL (diode transistor logic). The diodes were then replaced by transistors, and thus the name TTL evolved. The power supply voltage (VCC) for TTL is +5 V. The two logic levels are approximately 0 and 3.5 V.

There are several variations of the TTL family. These are based on the saturation mode (saturated logic) and active mode (nonsaturated logic) operations of the transistor. In the saturation mode, the transistor takes some time to come out of the saturation to switch to the cutoff mode. On the other hand, some TTL families define the logic levels in the active mode operation of the transistor and are called nonsaturated logic. Since the transistors do not go into saturation, these families do not have any saturation delay time for the switching operation. Therefore, the nonsaturated logic family is faster than saturated logic.

The saturated TTL family includes standard TTL (TTL), high-speed TTL (H-TTL), and low-power TTL (L-TTL). The nonsaturated TTL family includes Schottky TTL (S-TTL), low-power Schottky TTL (LS-TTL), advanced Schottky TTL (AS-TTL), and advanced low-power Schottky TTL (ALS-TTL). The development of LS-TTL made TTL, H-TTL, and L-TTL obsolete. Another technology, called emitter-coupled logic (ECL), utilizes nonsaturated logic. The ECL family provides the highest speed. ECL is used in digital systems requiring ultrahigh speed, such as supercomputers.

The important parameters of the digital logic families are fan-out, power dissipation, propagation delay, and noise margin.

Fan-out is defined as the maximum number of inputs that can be connected to the output of a gate. It is expressed as a number. The output of a gate is normally connected to the inputs of other similar gates. Typical fan-out for TTL is 10. On the other hand, fan-outs for S-TTL, LS-TTL, and ECL, are 10, 20, and 25, respectively.

Power dissipation is the power (milliwatts) required to operate the gate. This power must be supplied by the power supply and is consumed by the gate. Typical power consumed by TTL is 10 mW. On the other hand, S-TTL, LS-TTL, and ECL absorb 22 mW, 2 mW, and 25 mW respectively.

Propagation delay is the time required for a signal to travel from input to output when the binary output changes its value. Typical propagation delay for TTL is 10 nanoseconds (ns). On the other hand, S-TTL, LS-TTL, and ECL have propagation delays of 3 ns, 10 ns, and 2 ns, respectively.

Noise margin is defined as the maximum voltage due to noise that can be added to the input of a digital circuit without causing any undesirable change in the circuit output. Typical noise margin for TTL is 0.4 V. Noise margins for S-TTL, LS-TTL, and ECL are 0.4 V, 0.4 V, and 0.2 V, respectively.

TTL Outputs

There are three types of output configurations for TTL. These are open-collector output, totem-pole output, and tristate (three-state) output.

The open-collector output means that the TTL output is a transistor with nothing connected to the collector. The collector voltage provides the output of the gate. For the open-collector output to work properly, a resistor (called the pullup resistor), with a value of typically 1 Kohm, should be connected between the open collector output and a +5 V power supply.

If the outputs of several open-collector gates are tied together with an external resistor (typically 1 Kohm) to a +5 V source, a logical AND function is performed at the connecting point. This is called wired-AND logic.

Figure 1.11 shows two open-collector outputs (A and B) are connected together to a common output point C via a 1 K resistor and a +5 V source.

Figure 1.11 Two open-collector outputs A and B tied together.

The common-output point C is HIGH only when both transistors are in cutoff (OFF) mode, providing A = HIGH and B = HIGH. If one or both of the two transistors is turned ON, making one (or both open-collector outputs) LOW, this will drive the common output C to LOW. Note that a LOW (Ground for example) signal when connected to a HIGH (+5V for example) signal generates a LOW. Thus, C is obtained by performing a logical AND operation of the open collector outputs A and B.

Let us briefly review the totem-pole output circuit shown in Figure 1.12. The circuit operates as follows:

Figure 1.12 TTL Totem-pole output.

When transistor Q1 is ON, transistor Q2 is OFF. When Q1 is OFF, Q2 is ON. This is how the totem-pole output is designed. The complete TTL gate connected to the bases of transistors Q1 and Q2 is not shown; only the output circuit is shown.

In the figure, Q1 is turned ON when the logic gate circuit connected to its base sends a HIGH output. The switches in transistor Q1 and diode D close while the switch in Q2 is open. A current flows from the +5 V source through R, Q1, and D to the output. This current is called Isource or output high current, IOH. This is typically represented by a negative sign in front of the current value in the TTL data book, a notation indicating that the chip is losing current. For a low output value of the logic gate, the switches in Q1 and D are open and the switch in Q2 closes. A current flows from the output through Q2 to ground. This current is called Isink or Output Low current, IOL. This is represented by a positive sign in front of the current value in the TTL data book, indicating that current is being added to the chip. Either Isource or Isink can be used to drive a typical output device such as an LED. Isource (IOH) is normally much smaller than Isink (IOL). Isource (IOH) is typically −0.4 mA (or −400 A) at a minimum voltage of 2.7 V at the output. Isource is normally used to drive devices that require high currents. A current amplifier (buffer) such as a transistor or an inverting buffer chip such as 74LS368 needs to be connected at the output if Isource is used to drive a device such as an LED requiring high current (10 mA to 20 mA). Isink is normally 8 mA

The totem-pole outputs must not be tied together. When two totem-pole outputs are connected together with the output of one gate HIGH and the output of the second gate LOW, the excessive amount of current drawn can produce enough heat to damage the transistors in the circuit.

Tristate is a special totem-pole output that allows connecting the outputs together like the open-collector outputs. When a totem-pole output TTL gate has this property, it is called a tristate (three state) output. A tristate has three output states:

A LOW level state when the lower transistor in the totem-pole is ON and the upper transistor is OFF.

A HIGH level when the upper transistor in the totem-pole is ON and the lower transistor is OFF.

A third state when both output transistors in the totem-pole are OFF. This third state provides an open circuit or high-impedance state which allows a direct wire connection of many outputs to a common line called the bus.

A Typical Switch Input Circuit for TTL

Figure 1.13 shows a switch circuit that can be used as a single bit into the input of a TTL gate. When the DIP switch is open, VIN is HIGH. On the other hand, when the switch is closed, VIN is low. VIN can be used as an input bit to a TTL logic gate for performing laboratory experiments.

Figure 1.13 A typical circuit for connecting an input to a TTL gate.

1.4.3 MOS Transistors

Metal-Oxide Semiconductor (MOS) transistors occupy less space in the circuit and consume much less power than bipolar junction transistors. Therefore, MOS transistors are used in highly integrated circuits. The MOS transistor is unipolar. This means that one type of carrier flow, either electrons (n-type) or holes (p-type) are used. The MOS transistor works as a voltage-controlled resistance. In digital circuits, a MOS transistor operates as a switch such that its resistance is either very high (OFF) or very low (ON). The MOS transistor is a three-terminal device: gate, source, and drain. There are two types of MOS transistors, namely, nMOS and pMOS. The power supply (VCC) for pMOS is in the range of 17 V to 24 V, while VCC for nMOS is lower than pMOS and can be from 5 V to 12 V. Figure 1.14 shows the symbolic representation of an nMOS transistor. When VGS = 0, the resistance between drain and source (RDS) is in the order of megaohms (Transistor OFF state). On the other hand, as VGS is increased, RDS decreases to a few tens of ohms (Transistor ON state). Note that in an MOS transistor, there is no connection between the gate and the other two terminals (source and drain). The nMOS gate voltage (VGS) increases or decreases the current flow from drain to source by changing RDS. Popular 8-bit microprocessors such as the Intel 8085 and the Motorola 6809 were designed using nMOS.

Figure 1.14 nMOS transistor symbol.

Figure 1.15 depicts the symbol for a pMOS transistor. The operation of the pMOS transistor is very similar to the nMOS transistor except that VGS is typically zero or negative. The resistance from drain to source (RDS) becomes very high (OFF) for VGS = 0. On the other hand, RDS decreases to a very low value (ON) if VGS is decreased. pMOS was used in fabricating the first 4-bit microprocessors (Intel 4004/4040) and 8-bit microprocessor (Intel 8008). Basically, in a MOS transistor (nMOS or pMOS), VGS creates an electric field that increases or decreases the current flow between source and drain. From the symbols of the MOS transistors, it can be seen that there is no connection between the gate and the other two terminals (source and drain). This symbolic representation is used in order to indicate that no current flows from the gate to the source, irrespective of the gate voltage.

Figure 1.15 pMOS transistor symbol.

Operation of the nMOS transistor as an inverter

Figure 1.16 shows an nMOS inverter. When VIN = LOW, the resistance between the drain and the source (RDS) is very high, and no current flows from VCC to the ground. VOUT is therefore high. On the otherhand, when VIN = high, RDS is very low, a current flows from VCC to the source, and VOUT is LOW. Therefore, the circuit acts as an inverter.

Figure 1.16 A typical nMOS inverter.

Complementary MOS (CMOS)

CMOS dissipates low power and offers high circuit density compared to TTL. CMOS is fabricated by combining nMOS and pMOS transistors together. The nMOS transistor transfers logic 0 well and logic 1 inefficiently. The pMOS transistor, on the other hand, outputs logic 1 efficiently and logic 0 poorly. Therefore, connecting one pMOS and one nMOS transistor in parallel provides a single switch called transmission gate that offers efficient output drive capability for CMOS logic gates. The transmission gate is controlled by an input logic level.

Figure 1.17 shows a typical CMOS inverter. The CMOS inverter is very similar to the TTL totem-pole output circuit. That is, when Q1 is ON (low resistance), Q2 is OFF (high resistance), and vice versa. When Vinput = LOW, Q1 is ON and Q2 is OFF. This makes Voutput HIGH. On the other hand, when Vinput = HIGH, Q1 is OFF (high resistance) and Q2 is ON (low resistance). This provides a low Voutput. Thus, the circuit works as an inverter.

Figure 1.17 A CMOS inverter.

Digital circuits using CMOS consume less power than MOS and bipolar transistor circuits. In addition, CMOS provides high circuit density. That