HARDWARE REQUIREMENTS FOR COMPUTER CONTROL
General Purpose Computers
In general, although almost any digital computer can be used for digital control there are some requirements that should be satisfied before a computer is used for such an application. Today, the majority of small and medium scale DDC-type applications are based on microcontrollers which are used as embedded controllers. Applications where user interaction and supervisory control are required are commonly designed around the standard PC hardware.
As shown in Figure 1.8, a general purpose computer consists of the following basic building elements:
• central processing unit (CPU);
• program memory;
• data memory;
• input–output devices.
The CPU is the part which contains the arithmetic and logic unit (ALU), the control unit (CU) and the general purpose registers (GPR). The ALU consists of the logic circuitry necessary to carry out arithmetic and logic operations, for example to add or subtract numbers, to compare numbers and so on. Some ALU units are equipped to carry out multiplication and division and floating point mathematical operations. The CU supervises the operations within the CPU, fetches instructions from the program memory, decodes these instructions and controls the ALU and other parts of the computer so that the required operations can be implemented. The GPR are a set of fast registers which are generally used to carry out fast operations within the CPU.
The program memory of a general purpose computer is usually an external unit and attached to the computer via the data bus and the address bus. A bus is a collection of conductors which carry electrical signals. The data bus is a bidirectional bus which carries the data to be sent or received between the CPU and the other parts of the computer. The size of this bus is 8 bits in most microprocessors and microcontrollers. Some microcontrollers have data buses that are 16 or even 32 bits wide. Minicomputers and mainframe computers usually have 64 or even higher data widths. The address bus is a unidirectional bus which is used to address the peripheral
devices attached to the computer. For example, when data is to be written to the memory the address of the memory location is sent on the address bus and the actual data byte is sent on the data bus. The program memory is usually a nonvolatile memory, such as electrically programmable read-only memory (EPROM), EEPROM or flash memory. EPROM memory can be programmed using a suitable programmer device. This type of memory has to be erased using an ultraviolet light source before the contents can be changed. EEPROM memory can be programmed and erased by sending electrical signals to the memory. The disadvantage of this memory is that it is usually a slow process to write or read data from an EEPROM memory. Currently, flash memory is one of the most popular types of nonvolatile memory used. Flash memory is fast and can be erased under program control.
The data memory is usually a volatile memory, used to store the user data. RAM type memories are commonly used for this purpose. The size of this memory can vary from several tens of kilobytes to tens of gigabytes.
Minicomputers and larger computers are equipped with auxiliary storage mediums such as hard disks and magnetic tapes. These devices provide bulk storage for programs and data. Magnetic tape is usually used to store the entire contents of a hard disk for backup purposes. Input–output devices are also known as the peripheral devices. Many different types of input devices – scanner, camera, keyboard, microphone and mouse – can be connected to the computer. The output devices can be printers, plotters, speakers, visual display units and so on.
General purpose computers are usually more suited to data processing type applications. For example, a minicomputer can be used in an office to provide word processing. Similarly, a large computer can be used in a bank to store and manipulate the accounts of thousands of customers.
Microcontrollers
A microcontroller is a single-chip computer that is specifically manufactured for embedded computer control applications. These devices are very low-cost and can be used very easily in digital control applications. Most microcontrollers have the built-in circuits necessary for computer control applications. For example, a microcontroller may have A/D converters so that the external signals can be sampled. They also have parallel input–output ports so that digital data can be read or output from the microcontroller. Some devices have built-in D/A converters and the output of the converter can be used to drive the plant through an actuator
(e.g. an amplifier). Microcontrollers may also have built-in timer and interrupt logic. Using the timer or the interrupt facilities, we can program the microcontroller to implement the control algorithm accurately.
Microcontrollers have traditionally been programmed using the assembly language of the target device. As a result, the assembly languages of the microcontrollers manufactured by different firms are totally different and the user has to learn a new language before being able program a new type of device. Nowadays microcontrollers can be programmed using high- level languages such as BASIC, PASCAL or C. High-level languages offer several advantages compared to the assembly language:
• It is easier to develop programs using a high-level language.
• Program maintenance is much easier if the program is developed using a high-level language.
• Testing a program developed in a high-level language is much easier.
• High-level languages are more user-friendly and less prone to making errors.
• It is easier to document a program developed using a high-level language.
In addition to the above advantages, high-level languages have some disadvantages. For exam- ple, the length of the code in memory is usually larger when a high-level language is used, and the programs developed using the assembly language usually run faster than those developed using a high-level language.
In this book, PIC microcontrollers are used as digital controllers. The microcontrollers are programmed using the high-level C language.