Microcomputer Circuits and Processes

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CHAPTER 2 THE ELECTRONICS MICROCOMPUTER OF A If you spend a few thousand pounds on buying a minicomputer - a wardrobe-sized cabinet where the processing is done by several boards full of medium-scale integrated circuits - or if you spend a few hundred pounds or less on a microcomputer, where the processing is done by"an LSI microprocessor like the 8085, then your computer will always have three types of circuit: a central processing unit (CPU); some memory; input and output devices - keyboard, television screen, printer, etc. This chapter is about how these three types of circuit are connected together. We shall think about a small microcomputer, where the CPU is a microprocessor, just like the one in the laboratory or even in your home. GETTING IT TOGETHER - THE BUS CONCEPT The three circuit types must be connected together; figure 2.1 suggests two ways of doing this. The first suggestion involves wires between each device. This may work, but would be clumsy. In the second suggestion, each device is connected in parallel to a set of wires called a bus. The bus is drawn on circuit diagrams as a wide channel and, in reality, consists of many parallel wires carrying signals. Each device - CPU, memory, keyboard, television screen - is connected to or 'hangs on' the bus. (a) television screen D printer cPU Figure 2.1(a) Interconnecting computer devices: wires connect each device. 8
(b) D II CPU _____ ~----- w;'es althe bus Figure 2.1(b) Interconnecting computer devices: the efficient bus connection. It saves a lot of wires, but there isjust one problem. What happens if two devices put their signals onto the bus simultaneously? This is shown in figure 2.2, where just one wire of the bus is shown, and two logic inverters are shown connected. The output of one is high and the output of the second is low. What happens? The bus wire cannot be both low and high Figure 2.2 There is a problem when two devices put different signal levels on to the bus. This situation must be avoided. Almost certainly, something will get very hot and be damaged; at the very least the system will not work. This is called bus contention, and steps must be taken to ensure that no two devices put their signals on the bus together. In technical jargon, no two devices may 'talk' to the bus simultaneously. This is achieved using a type oflogic gate called the 'Tri-State' gate (which is a trademark of National Semiconductor Corporation). A tristate gate does not have three different logic states, but is like a normal logic gate in se'rieswith a switch. Look at figure 2.3 which shows a tristate inverter gate. The idea is that a control signal is able to connect the gate to its output. This is called 'enabling' the gate. If the enable is low, then there is no output from the gate, neither high nor low. The gate is in a 'high-impedance' state, meaning that anything connected to its output does not know it is there. When the gate is 9
Page 2 and 3: General editor. Revised Nuffield Ad
Page 4 and 5: Longman Group Limited Longman House
Page 6 and 7: PROLOGUE This Reader explains how m
Page 8 and 9: transistor, through the logic circu
Page 10 and 11: tasks (large computing 'power'), wa
Page 12 and 13: 800000 mark. Such high sales result
Page 16 and 17: enabled, as the truth table in figu
Page 18 and 19: D memory cell storing binary 0 stor
Page 20 and 21: address bus r----- AND gates RD .--
Page 22 and 23: If you get the chance, look up 7415
Page 24 and 25: Period 2 Period 3 The data bits are
Page 26 and 27: transfers the full 16 bits of addre
Page 28 and 29: INPUT AND OUTPUT Devices that input
Page 30 and 31: A glance at the timing diagram in f
Page 32 and 33: CHAPTER 3 RUNNING A SIMPLE PROGRAM
Page 34 and 35: adding is represented by 1111. Figu
Page 36 and 37: The number moved into register A im
Page 38 and 39: loaded into register A. So the CPU
Page 40 and 41: The FETCH cycles FETCH cycles, whic
Page 42 and 43: The EXECUTE cycles During EXECUTE c
Page 44 and 45: Second EXECUTE cycle MOV M--+Acc, c
Page 46 and 47: Third EXECUTE cycle ADI, clock peri
Page 48 and 49: That is the end of the program, exc
Page 50 and 51: CHAPTER 4 MEMORIES, INTERFACES, AND
Page 52 and 53: measurements made from a microproce
Page 54 and 55: (al (bl Figure 4.6 (a) Same as in f
Page 56 and 57: A typical bubble unit contains 300
Page 58 and 59: (al driver displays 0 1 1 0 1 0 0 1
Page 60 and 61: Where exactly is this common line c
Page 62 and 63: this R is in series with the next h
Page 64 and 65:
That is the principle of the conver
Page 66 and 67:
comparator digital-to-analogue data
Page 68 and 69:
II CI l! ~L--~---- desired measurem
Page 70 and 71:
except that it is sent out bit afte
Page 72 and 73:
These equally spaced voltages are f
Page 74 and 75:
Figure 4.35 shows how the letter 'D
Page 76 and 77:
A CONTROL AND DATA AQUISITION COMPU
Page 78 and 79:
It does this by outputting a signal
Page 80 and 81:
The second application, shown in fi
Page 82 and 83:
INDEX This Index will direct you to
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