Microcomputer Circuits and Processes

(al
driver
displays
0 1
1 0
1 0
0 1
(bl
9
ov
Figure 4.13
Driving two displays with just one driver chip. Switches in the cathode lines of each
display turn that display on when its own 4-bit number is being input. (a) The number
0110 (6 10 ) is sent to the lefthand display by closing its cathode switch. (b) The number
1001 (9 10 ) is sent to the righthand display by closing its cathode switch.
Note the switches connected to the cathodes. For the dual display to
show the number 69, first the cathode switch on the first digit is closed
and 0110 (6 10 ) put into the driver-figure 4.13(a). This causes the
number 6 to be displayed on the first digit. Then this switch is opened
and the switch on the second digit closed-figure 4.13(b). Sending out
1001 will cause the displayed number 9 to shine on the second digit.
This process is repeated a few thousand times per second. The actual
switches are contained inside a special Ie package. Figure 4.14 shows
the final design.
buffer
segment
driver
display
...---...., .-----,
data bus
3
segment
latch
buffer
digit driver
data bus
4
7
digit latch
Figure 4.14
How to connect two or more displays to our microcomputer. One output port of 4-bits
wide selects the segments which shine, and the second 4-bit port determines which digit
these shine on.
Two output ports are needed, one 4-bit number passing to the digitdriver
circuits, and the other passing into the decoder which determines
which digit is shining. Note how the display driver latch is still used.
This holds the digit stable while a different 4-bit number is being output
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