Hardware Reference
In-Depth Information
#7
#6
#0
560
a
b
c
d
e
g
a
a
a
f
b
f
b
f
b
g
g
g
e
c
e
c
e
c
d
d
d
Y6 Y5 Y4 Y3 Y2 Y1 Y0
74HC244
I6 I5 I4 I3 I2 I1 I0
74HC244
PP7
PP6
I7
Y7
I6
I5
I4
I3
I2
I1
I0
Y6
Y5
Y4
Y3
Y2
Y1
Y0
PP5
PP4
PP3
PP2
PP1
HCS12
PP0
Figure 7.24b
■
HCS12 seven-segment display circuit 2
#7
#6
#0
560
a
b
c
d
e
g
a
a
a
f
b
f
b
f
b
g
g
g
e
c
e
c
e
c
d
d
d
Y6 Y5 Y4 Y3 Y2 Y1 Y0
74HC244
I6 I5 I4 I3 I2 I1 I0
74ABT244
PP7
PP6
I7
Y7
I6
I5
I4
I3
I2
I1
I0
Y6
Y5
Y4
Y3
Y2
Y1
Y0
PP5
PP4
PP3
PP2
PP1
HCS12
PP0
Figure 7.24c
■
HCS12 seven-segment display circuit 3
LED segment is (4.9 2 1.8 2 0.1) 4 560 5 5.36 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 37.5 mA, which is within the total drive capability (70 mA) of the
74HC244 from TI. However, each digit select pin (connected to a single 74HC244 pin) needs to
sink about 37.5 mA current, which is above the pin current sink capability of the SN74HC244
(35 mA) from TI. Therefore, the current drive compatibility problem exists. However, this prob-
lem can be solved by using a slightly larger current-limiting resistor (for example, 680
).
In Figure 7.24c, the
V
OH
of the 74HC244 is 5 V with 5-V power supply. The
V
OL
of 74ABT244
is 0.55 V. Assume the voltage drop of the LED is 1.8 V, then the current flowing through each
Ω
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