Microcomputer Circuits and Processes

Now this signal goes high only when the CPU executes an OUTput
instruction to this device, and only then will the data be allowed
through the buffer. This signal is also connected to the display driver
'latch' terminal. When the signal is high, the data is allowed into the
display, but when it goes low, after the OUTput instruction is finished,
the data is latched into the display and the number will be displayed. If
this were not done, then you would never see any numbers, since the
output instruction takes, say, a millisecond to be executed. The latch
holds the data firm until the next output instruction is performed, when
the displayed number must be updated.
Extending this to 4, 8, or 10 digits is easy. You could use a displaydriver
chip plus output buffer for each digit, which would then appear
to the CPU as separate output ports. Outputting a number to port 1
would set up that number in the first digit, outputting to the second
port would set up the second digit, and so on. It would work, but uses a
lot of chips. There is a simpler way. To understand how this works, we
must take a look inside the display (figure 4.12).
anode
connections
common
cathode
Figure 4.12
Details of a seven-segment display unit, showing how each segment, a light-emitting
diode, is selected by the anodes. All cathodes are connected together.
Each segm~nt is a light-emitting diode, and all the cathodes of the
diodes are connected together. In the use described above, the driver
chip would supply current (10-20 rnA) to each anode, and the common
cathode would be connected to 0 V to complete the circuit. Now look at
the circuit in figure 4.13. Two display chips are shown being driven by
one driver circuit, each output of this circuit now being connected to
two anodes, one from each display.
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