Microcomputer Circuits and Processes

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INPUT AND OUTPUT Devices that input bits of information into the computer are vital: to get the program loaded into memory in the first place, to input users' commands 'via the keyboard, and to make measurements on the external (real, no~-computer bussed) world. Similarly, output devices are needed to drive television screens, printers, and circuits which control robots making cars. Many input and output devices transfer their information in whole bytes, like the keyboard and television shown in figure 2.25. These need to be hung on the data bus, via the usual tristate buffer. In the case of an ouput the CPU must select the correct device by pulsing the enable high while sending out data, on the bus, which is then picked up by the device. D keyboard select television select data bus Figure 2.25 How input and output devices are hung onto the bus via tristate buffers. This implies the need for more control signals to select the devices. The only connection to be made is a signal to enable the output buffer. One way of doing this is to think of the output device as a location in memory which may be written to by a normal WRITE operation. In that case, the output buffer can be enabled by a decoded memory address. Of course, no real memory must live at this address - that would cause bus contention. The method is shown in figure 2.26. It has the disadvantage of using up memory space, and needs a large address bus 16 4 8 CPU o E C o o E R television screen D Figure 2.26 Selecting an output device by decoding one memory address. The output device looks like a memory cell to the CPU. 22
amount of decoding in big systems. It has the advantage of looking like memory to the CPU. Now the CPU spends most of its time shoving bytes between itself and memory. So it does that well: it has many memory-oriented operations which the user may use in a program. If input-output looks like memory, then these operations can be applied also to input-output. Input-output devices which are enabled in this way are called memory-mapped devices. Most processors are designed with special input and output facilities so that the input-output circuits are quite separate from memory. It adds an extra dimension to the computer - ?ften people in computer laboratories talk about 'memory space' and 'input-output space', referring to that particular region in the high-speed bussed dimensions of electrical pulses, where memory or in-out signals rule, respectively. The special input-output facilities involve another control line. We shall call this line MilO, meaning memory or input-output. Note the bar above 10; this is consistent with the following definition of MilO: MilO = 1 (high) MilO = 0 (low) implies a memory type operation implies an input-output type operation. For example, if the number 01011110 is put on to the data bus by the CPU, then it gets sent to memory if MilO = 1, or else to an output device if MilO = O. How can this new line be used to control one single output device, a television screen as shown in figure 2.27? The television screen is buffered as usual on to the data bus. Now, remember that the buffer must be enabled when there is valid data on the bus coming from the CPU. This is true when the write signal WR goes high. So this is the signal that must be used. Also, 'this is input-output, not memory, so MilO must be low. It is easy to see that the enable pulse must be made from MilO inverted and then ANDed with the WR signal. from address decoder CPU RD t----+----~ WR t----+-----tl....-~ television screen D Figure 2.27 Using the memory/input-output control signal (MjIO) to select output device or memory. 23
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 14 and 15: 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 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:
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Microcomputer Circuits and Processes

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