Hardware Reference
In-Depth Information
down toward GND level. After that, the gate terminal draws only leakage currents. Bipolar
technology is different from the CMOS technology in that a DC current always flows into or
out of the base terminal of the transistor of a bipolar logic chip.
To determine whether a pin can supply and sink currents to all the peripheral pins that it
drives directly, the designer needs to check the following two requirements:
1. The
I
OH
of an I/O pin of the microcontroller is equal to or greater than the sum
of currents flowing into all peripheral pins that are connected directly to the
microcontroller I/O pins.
2. The
I
OL
of an I/O pin of the microcontroller is equal to or greater than the sum
of currents flowing out of all peripheral pins that are connected directly to the
microcontroller I/O pins.
In addition, the designer must also make sure that the total current needed to drive the periph-
eral signal pins does not exceed the total current that the microcontroller can supply.
One question that arises here is what should be done if an I/O pin cannot supply (or sink) the
current needed to drive the peripheral pins? A simple solution is to add buffer chips (for example,
74ABT244) that can supply enough current between the microcontroller and the peripheral chips.
This technique is widely used in microcontroller applications and is illustrated in Example 4.13.
Example 7.3
▼
You are given the three seven-segment display circuits shown in Figure 7.24a to 7.24c.
Perform an appropriate analysis to find out if any circuit has a current incompatibility problem.
#7
#6
#0
560
a
b
c
d
e
f
g
a
a
a
f
b
f
b
f
b
g
g
g
e
c
e
c
e
c
d
d
d
PP7
PP6
PP5
PP4
PP3
PP2
PP1
PP0
HCS12
Figure 7.24a
■
HCS12 seven-segment display circuit 1
Solution:
In Figure 7.24a, the
V
OH
of the 74HC244 is 5 V with 5-V power supply. The
V
OL
of HCS12
is 0.8 V. Assume the voltage drop of the LED is 1.8 V, then the current flowing into each segment is
(4.2 2 1.8 2 0.8) 4 560 5 2.86 mA. This value is within the current source capability of each HCS12
I/O pin. The total maximum current that the HCS12 must supply is 7 3 (4.2 2 1.8 2 0.8) 4 560 5
20 mA. This value is within the current-source capability of the HCS12 (25 mA). An HCS12 I/O pin
can also sink this amount of current. Therefore, there is no current drive incompatibility problem.
In Figure 7.24b, the
V
OH
of the 74HC244 is 5 V with 5-V power supply. The
V
OL
of 74HC244
is 0.1 V. Assume the voltage drop of the LED is 1.8 V, then the current flowing through each
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