Hardware Reference
In-Depth Information
LED segment is (4.9 2 1.8 2 0.55) 4 560 5 4.55 mA. This value is well within the current drive
capability of each 74HC244 pin. The total current that the 74HC244 needs to drive for the
segment pattern is about 32 mA which, is within the total drive capability of the 74HC244. A
single 74ABT244 pin needs to sink this amount of current, which is also within the capability
of the 74ABT244. There is no current drive incompatibility problem.
▲
If an I/O pin is driving a peripheral pin that does not contain latches or flip-flops, then timing
is not an issue. A latch or flip-flop usually has a control signal or clock signal to control the
latching of an input signal. As illustrated in Figure 7.25, the D input to the D flip-flop must be
valid for
t
su
ns before the rising edge of the CLK signal and remain valid for at least
t
hd
ns after
the rising edge of the CLK signal in order for its value to be correctly copied to the output signal
Q. The timing parameters
t
su
and
t
hd
are referred to as the
setup
and
hold
time requirements of
the D flip-flop. The main timing consideration is that the setup and hold time requirements for
all latches and flip-flops in a digital system must be satisfied in order for the system to work cor-
rectly. A signal may pass through several intermediate chips before it is used by the final latch or
flip-flop. The time delays of all intermediate devices must be added when considering the tim-
ing analysis. Timing requirement analysis can be very complicated and is best illustrated using
examples. Two examples of timing compatibility analysis are given in Section 14.8.
D
t
su
t
hd
Q
D
CLK
CLK
Q
(a)
(b)
Figure 7.25
■
D flip-flop and its latching timing requirement
Although seven-segment displays are easy to use, they are bulky and quite limited in the
set of characters that they can display. When more than a few letters and digits are to be dis-
played, seven-segment displays become inadequate. Liquid crystal displays (LCDs) come in
handy when the application requires the display of many characters.
A liquid crystal display has the following advantages:
•
High contrast
•
Low power consumption
•
Small footprint
•
Ability to display both characters and graphics
The basic construction of an LCD is shown in Figure 7.26. The most common type of LCD al-
lows light to pass through when activated. A segment is activated when a low-frequency bipolar
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