Профессионально-коммуникативная подготовка студентов

stages there are, the longer it is before the clock pulse has propagated through the system and all the outputs have settled down to their steady-state values. There are various problems associated with this mode of operation. If you are designing a high speed system, it is essential to know exactly when each switching takes place, or else logic sequences can get out of step; this can cause instability which prevents an apparently sound design from working.

A counter that works in a more predictable way is the synchronous counter. Synchronous counters have the counting sequence controlled by a clock pulse, all the outputs of the flip-flops changing state simultaneously (or synchronously). Synchronous counters are quite complicated.

UP-DOWN COUNTERS

Counters are also available to count up (add to the count) or count down (subtract from the count). Such counters feature an UP/DOWN control input which determines the way the counter works, according to the logic state applied to it.

CMOS 1C COUNTERS

Because most of the circuits so far considered are from the TTL family, you might think that CMOS counters are less common. This is not the case, and there are CMOS counterparts (with slight variations) for all the types discussed already. For mediumand low-speed operation – up to 5 MHz – the advantages of CMOS counters are considerable. The low-power requirements, wide power-supply voltage tolerance and excellent noise immunity make CMOS the first choice for low-speed/low-cost applications – and incidentally for easy and reliable demonstrations!

VII. Ask your partner to answer your questions on the texts given above. Change the roles. Use the following models: a) What parts (components) does … consist of? b) What are the advantages (disadvantages, shortcomings) of … ? c) What are the main characteristics of … ? d) How many transistors (resistors, inputs, outputs) are there in … ?

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VIII. Read the dialogue. Using it as a model, make up your own dialogues describing the advantages and shortcomings of various integrated electronic counters: bistable multivibrators, binary counters, latches, synchronous counters, up-down counters, CMOS IS counters.

–They say you are interested in the counters?

–It’s true. What do you want to know?

–I can’t choose a circuit for my counter. What can you say about binary counters?

–It’s a good idea to use them. But they work better in the counters with an output for ordinary tens counting.

–And what about the synchronous counters?

–They are quite complicated and work rather predictably. But they are slow and it takes much time for the counter to operate.

–Maybe, it’s better to use the simpler circuits?

–Why not? For example, you can take the bistable. It requires only six discrete components.

–As I see you know much about the counting circuits! Thank you for your help!

–You are welcome any time.

IX. Decode the abbreviations: CD, MHz, TTL, DCD, CMOS.

UNIT II

COMPUTER MEMORY CIRCUITS

I. Recognize the international words: binary, magnet, arithmetic, microelectronics, museum, address, standard, multivibrator, element, static, information, dynamic, chip, code, decode, video, fix, diode, compromise, ultraviolet, photoelectric, instruction, universal, result.

II. Study and remember the abbreviations.

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IV. Read and translate the text given below. Mind your intonation and pronunciation.

RANDOM-ACCESS MEMORY: STATIC RAM

One of the requirements of a computer system is circuits that will store very large amounts of binary data. The computer memory has to hold many 8-bit, 16-bit, or 32-bit binary numbers, available for instant recall by the computer. Even the smallest personal computer will have a memory system capable of holding upwards of 6 000 000 binary digits (bits), so clearly the circuits used have to be economic.

Early computers used a system called core storage in which each bit was stored in a ferrite ring, which could be magnetized (1) or not magnetized (0). Core storage was bulky and expensive (but rather attractive to look at), each ferrite ring having threaded onto it a system of wires by hand, rather like weaving. Inside the computer’s arithmetic unit, numbers were held in bistables. With the coming of microelectronics it became possible to use bistables for all the computer’s memory, and ferrite cores are now found only in museums – just a couple of decades after they were introduced. The main memory of a computer is called the random access memory (RAM). An array of bistables is organized so that any one bistable can be separately accessed, or addressed. A standard small RAM chip would hold 1024 bits, organized in a two-dimensional array 32·32.

V. Compose as many sentences as you can using the following words. Binary data, store, amount, computer memory, instant recall, available, core storage, ferrite ring, bulky and expensive, main memory, RAM, array of pistables, can be addressed.

VI. Skim the text given below, find the sentences with the

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given below words and translate them. Refresh circuits, access time, extra lines, static RAM, dynamic RAM, leaks away, can pack a lot on to a chip, data input/output lines, a small CMOS RAM, information to be held for long periods, CHIP SELECT lines, IGFET’s construction.

RANDOM-ACCESS MEMORY: DYNAMIC RAM

The type of RAM that uses multivibrator-like bistable elements is called static RAM, and is available in TTL or CMOS form. Both types are widely used; the CMOS scores, as usual, in having very low power consumption, but the TTL has a faster access time, the time it takes to read or write information into the memory. The access times vary, but are in the region of 100-500ns, with the TTL at the fast end of the scale.

A second type of RAM, dynamic RAM, usually called DRAM (but pronounced “dee-ram”) is used for large-capacity memories. Instead of using multivibrator bistable elements, DRAM stores its Is and Os in the capacitance that appears as a by-product of an IGFET’s construction, between the gate and source. Unfortunately, the charge in this capacitor gradually leaks away, and the memory cell must be refreshed by inputting the data over and over again, every millisecond or so. Special refresh circuits are needed for this, and if the memory is used with a computer, care must be taken with the design to ensure the refresh cycle is continuous, and does not clash with the computer using the memory.

Despite these disadvantages, dynamic RAM is used in almost all systems other than small process controllers and the like. The advantages are that it is quite a lot cheaper than static RAM, has a very short access time and, being simpler, can pack a lot on to a chip. Addressing and data input/output lines are the same on static and dynamic RAM chips, and the computer cannot “see” any difference in a properly designed system.

If the memory is to be extended beyond the capacity of the individual chips then the CHIP SELECT lines are used. The ten address lines can be added to, and the extra lines used to select groups of memory circuits. Using a 16-bit addressing system, a computer has a further six address lines that can be decoded, giving a further 26, or 64, possible combinations that can be decoded. With a 16-bit address, the

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maximum number of addresses is 216, or 65 536. A computer with a 16-bit address system could therefore select, in random order and at less than 200 ns notice, any one of 65 536 8-bit or 16-bit binary numbers from its RAM.

It is usually necessary to have at least a small memory system that will not lose its contents when power is removed from the circuit. This is needed to hold a set of permanent instructions for the computer, and sometimes for sub-systems like the disk drive or video system. Clearly, both static and dynamic RAM will lose the contents of the memory when the power is interrupted.

One approach – the one favoured by most manufacturers of small computers – is to build in a small CMOS RAM, powered by a long-life battery (usually a lithium cell) or by a rechargeable cell (usually Ni-Cd) that is trickle-charged when the machine is turned on. This enables information to be held for long periods, but makes it easy to change when necessary.

VII. Answer the following questions using the phrases: as far as I know, if I am not mistaken, to my mind, of course. 1. What type of RAM is called static RAM? 2. In what forms is it available? 3. What is the difference between CMOS and TTL forms? 4. What is dynamic RAM used for? 5. Where does DRAM store its Is and Os? 6. Why must the memory cell be refreshed? 7. What is needed for this? 8. What care must be taken? 9. What are the advantages of dynamic RAM? 10. Are addressing and data input/output lines the same on static and dynamic RAM chips? 11. When are CHIP SELECT lines used? 12. What else can be added? 13. What is necessary to have not to lose memory? 14. When will static and dynamic RAM lose the contents of the memory? 15. What batteries may be built in a small CMOS RAM?

VIII. Read and translate the text. Ask your partner questions on the text.

READ-ONLY MEMORY

A memory in which data is fixed is called a read-only memory or ROM. The simplest – and cheapest – kind of ROM is the fusible-

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link ROM. This is a system that is addressed in exactly the same way as the RAM, but the bistable elements are replaced with diodes, connected across the array.

Each diode is connected by way of a very thin metal link. As manufactured, every memory location is read as a 1. The ROM can be “programmed” by applying a pulse of current to the relevant data lines

– the current bums through the thin link (like blowing a fuse) and permanently open-circuits that particular cell, which, for ever after, will read as a 0. Purpose-built instruments are available for “blowing” ROMs at high speed. Once the required pattern has been impressed on the ROM, it is fitted in the required location in the computer memory addressing system. The read and write currents available from the computer system are insufficient to affect the ROM. Data can be read out of it, but attempts to write data into it have no effect. Where very large numbers of components are required, a manufacturer may specify a special mask to produce the ROM with the program “built in” at the design stage.

A compromise between the immutable ROM and the volatile RAM has been produced, in the form of the erasable programmable read-only memory (EPROM). The EPROM uses MOSFET technology, and during programming – by the repeated applications of pulses of moderate voltage (a few tens of volts) – charges are built up in the insulation below the gate. This layer has an extremely high resistance, and the charge cannot leak away. The presence of the charge causes the MOSFET to conduct, and the charged cell is read as a 1.

EPROMs can retain their data for many years. It is, however, possible to discharge the cells in the EPROM by exposing it to intense ultraviolet light; the light causes a photoelectric current to flow, conducting the charge away. EPROMs of this type have a transparent window in the encapsulation above the chip, to allow the chip to be illuminated while protecting it from mechanical damage.

IX. In the right column find suitable definitions of the terms given in the left column.

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X. Prepare short reports “Advantages and disadvantages of RAM and ROM. A compromise between them”. One of the students is making a report, others asking him questions for more information.

XI. Read and translate the text. Express the main idea of the text in Russian.

A PRACTICAL COMPUTER SYSTEM

In order to reduce to a minimum the number of interconnecting wires required, all computers use a system in which buses carry information from one place to another inside, and sometimes outside, the computer. A bus is nothing more complicated than a group of wires, used together for the transmission of binary numbers. A bus consisting of eight wires (known as an 8-bit bus) will carry an 8-bit binary number – any number from 0 to 255. A 16-bit bus will carry a number between 0 and 65 535, and a 32-bit bus will carry any number from 0 to 4 294 967 295! Personal computers tend to have 16-bit or 32-bit buses for data, and 32-bit or 64-bit buses for addresses.

The first main component of a typical computer is the CPU. This performs all the calculating, organizing and control functions, and is often thought of (inaccurately) as the “brain” of the computer. The CPU is connected to the rest of the system by three groups of wires: the data bus, the address bus and the control bus. Also connected to the buses are the memory and an input and output system.

The data bus is used by the system to transmit binary numbers (data) from point to point. Data may be part of a calculation, or an instruction (more about that below). An 8-bit bus does not limit the computer to dealing with numbers less than 255, of course. Larger numbers are simply sent in two or more pieces, one after the other.

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Numbers up to 65 535 can thus be sent in two pieces, called the lowbyte and the high-byte. A byte is simply eight bits (it is a contraction of “by eight”) and is, in small computers, the amount of information that is dealt with at one time by the CPU.

The address bus is used by the CPU (and other system components) to look at any individual byte of memory (either RAM or ROM); in small computers, a 16-bit address bus is used to provide a total memory capability of 65 535 bytes. This is referred to (confusingly) as “64 kilobytes”, or “64 K”, the capital K being used to denote the “computer kilo” of 1024.

The memory itself consists of a large number of bistable memory elements along with appropriate decoding circuits. The memory is used for storing data (information processed by the computer) and program (instructions telling the CPU what to do). Input may consist of a keyboard, or anything else that can feed information into the computer. Output may be a printer, visual display unit (VDU), or some other device enabling the computer’s output to be passed on to the outside world.

XII. Read and translate the text given below. Prepare to ask as many questions as you can about: a) minimum components of the computer; b) the component which brings the system to life; c) input devices; d) output devices.

COMPONENT PARTS OF THE COMPUTER SYSTEM

In looking at the computer, it is convenient to consider the individual components of the machine – the CPU, memory, keyboard, etc. But in order to make any sense of the computer, it is important to understand the way the whole thing operates as a system.

Remember the text “A practical computer system” describing the way the system works. The same principle applies to almost every digital computer, from the smallest to the largest. The three buses are the channels through which the parts of the computer communicate with each other, and every computer is organised in much the same way. Almost all computers are designed in such a way that extra components can be added, simply by plugging them into a multi-way socket that connects directly to the three buses.

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The minimum components are a CPU and a memory. The lower addresses of the memory are generally ROM, so that immediately power is applied to the CPU it starts executing a program, bringing the system to life. The CPU can be directed by the program to look at an input device. The most common input device is the keyboard. The keyboard may itself incorporate circuits that translate the key-pushes into the relevant binary code. So pushing a letter V would generate a binary number 10100110, to be applied to the data bus at the right instant and read into the CPU. Alternatively, the CPU itself could run a short program, itself part of a larger program, designed to look at the connections to the keys, thus saving on hardware. Such a program is called a subroutine, and saves electronic components at the price of a small loss of operating speed.

Output devices also work by using the buses. The circuits driving a VDU can interrupt the CPU briefly to examine the relevant memory locations for the screen, then interpret them as characters or graphics shapes for display on the screen; the way in which the “screen memory” is addressed is exactly the same method as the CPU uses, but the address and data buses are temporarily under the control of the VDU system.

At this point it is interesting to consider the way memory locations are translated into a picture on the screen, and to examine what is known as the memory map of a typical computer. A memory map is shown in Figure 1. The memory map simply shows the allocation of the addresses that the CPU can access. This is the memory map of a typical low-cost microcomputer that can address 64 K (65 536 bytes).

The first (“lowest”) 24 K is occupied by the ROM. The ROM contains the operating system, the programs that do the housekeeping

– all the operations of looking at the keyboard, loading and saving programs, controlling input and output, etc. The ROM also contains a BASIC interpreter that enables the computer to run programs written in that language – more about that later. Remember that the microprocessor always starts at address 0 when switched on, so the ROM is always at the bottom of memory. The entire memory map above the ROM is filled with RAM – 40 K of it. Pans of the RAM are reserved for special purposes. The 9 K between 24 K and 33 K is the screen RAM and is used to store the information which the computer translates into a picture on the screen of the monitor or television.

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