Microcomputer Circuits and Processes

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.
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