Microcomputer Circuits and Processes

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comparator digital-to-analogue data bus binary counter stop/continue o 1 enable J1.f"UL clock start conversion Figure 4.25 Components of a linear ramp converter. A D-to-A converter is driven by the binary counter. The voltage produced, which is a staircase waveform, is compared with the input voltage. When the two are equal, the counter stops, and then holds the binary value of the input voltage. number to a voltage which is compared with Jlin. Assume the counter has reached 0011. The comparator receives voltage 0011 from the D-to-A converter, and compares it with Jlin' 0101. It sees that the D-to-A converter's voltage is too low, and so outputs a logical high. This is fed back to the counter, which commands it to continue counting. It continues from 0011 to 0100,to 0101. At this point the comparator receives Jlin' 0101, and also 0101 from the D-to-A converter. The inputs are equal, and the comparator's output goes logical low (it has been v+=t>- V_ - inputs output II m ~ o> 'S 5 V Do 'S o •••••--- logical high o L..-.__ ••••••L-. _ Input voltages V+ < V_ I V+ > V_ I Figure 4.26 Action of a comparator chip. The graph shows the output is 5 V (logical high) when the voltage V+ is greater than V_. When V+ is less than V_, the output is OV (logical low). 60
iased that way). This low, when fed back to the counter, tells it to stop counting. The counter now holds 0101, the binary value of the input voltage. The conversion is complete, and the 4-bit number can be enabled on to the computer's data bus and read into memory. Figure 4.27 shows this conversion; the step output of the D-to-A converter as the counter increments until the D-to-A voltage equals Jlin' V iṉ ------- I I I 1 I I I D-to-A output voltage I Comp ••• ", output Time I conversion done Figure 4.27 The technique oflinear ramp conversion. The D-to-A output increases in equal steps until it is equal to the input voltage. Then, the comparator signals that the conversion is done. The counter, which had been driving the D-to-A converter, is stopped, and holds the binary value of the input voltage. The comparator's output is also shown dropping low at this point. In a working circuit, a little more logic is needed, to reset the counter before each conversion is started and to signal the CPU when the conversion is done, so the result can be enabled on to the bus. It is quite a straightforward procedure, but rather slow. For a large input voltage, the counter may need to count up to a large number. An 8-bit converter would need a maximum of 255 counts. A typical clock driving the converter may run at 1MHz, needing 1 JlS per count, or up to 255 JlS for a single conversion. Such pedestrian behaviour may be fine if you want to measure the height of water in a river once per hour, but totally inadequate to record the behaviour of a millisecond pulsar. Microprocessor-controlled measuring systems are usually used in the two extremes of either very infrequent (once per hour, day ... ) or very frequent (thousands or millions per second). These are areas where humans would not perform very well. The problem with quick or transient measurements is illustrated in figure 4.28. You know that a certain signal has a large initial peak and then decays down. You start the ramp converter at the instant the signal appears (you cannot anticipate), but by the time the counter has counted a hundred or so pulses, the input voltage has long since 61
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General editor. Revised Nuffield Ad
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Longman Group Limited Longman House
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PROLOGUE This Reader explains how m
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transistor, through the logic circu
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tasks (large computing 'power'), wa
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800000 mark. Such high sales result
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CHAPTER 2 THE ELECTRONICS MICROCOMP
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 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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Microcomputer Circuits and Processes

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