Advanced Parallel I/O - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 319

• The BiCMOS logic is not suitable for driving the HCS12 and other CMOS logic
chips at the same power supply level.
• The bipolar logic ICs are not suitable for driving the CMOS logic ICs or the HCS12
microcontroller at the same power supply level.

7.6.2 Current Drive Capability
A microcontroller needs to drive other peripheral I/O devices in an embedded system. The
second electrical compatibility issue is whether the microcontroller can supply (when the out-
put voltage is high, also called source) or sink (when the output voltage is low) the current
needed by the I/O devices that it interfaces with. Depending on the voltage level of an output
pin, the current may flow out from (supply the current) or into (sink the current) the pin. The
designer must make sure that the following two requirements are satisfied:
• Each I/O pin can supply (flowing out from the pin) and sink (flowing into the pin)
the current needed by the I/O devices that it interfaces with.
• The total current required to drive I/O devices does not exceed the maximum
current rating of the microcontroller.
Each logic chip has the following four currents that are involved in the current drive calculation:
• Input high current ( I IH ). This is the input current (flowing into the input pin) when
the input voltage is high.
• Input low current ( I IL ) . This is the input current (flowing out of the input pin) when
the input voltage is low.
• Output high current ( I OH ). This is the output current (flowing out of the output pin)
when the output voltage is high.
• Output low current ( I OL ) . This is the output current (flowing into the output pin)
when the output voltage is low.
The current capabilities of several common logic families and the HCS12 are listed in
Table 7.2. In the CMOS technology, the gate terminal (one of the three terminals in an N or
P transistor) draws a significant current only when they are charged up toward V CC or pulled

Logic Family V CC I IH I IL I OH I OL

HCS12 2,3
S
LS
AS
F
HC 3
HCT 3
ACT 3
ABT 3
BCT
FCT 3

5 V
5 V
5 V
5 V
5 V
5 V
5 V
5 V
5 V
5 V
5 V

2.5 μ A
50 μ A
20 μ A
20 μ A
20 μ A
1 μ A
1 μ A
1 μ A
1 μ A
20 μ A
1 μ A

2.5 μ A
1.0 mA
0.2 mA
0.5 mA
0.5 mA
1 μ A
1 μ A
1 μ A
1 μ A
1 mA
1 μ A

25 mA
1 mA
15 mA
15 mA
1 mA
25 mA
25 mA
24 mA
32 mA
15 mA
15 mA

25 mA
20 mA
24 mA
64 mA
20 mA
25 mA
25 mA
24 mA
64 mA
64 mA
64 mA

1 Values are based on the 74xx244 of Texas Instrument (xx is the technology name).
2 The total HCS12 supply current is 65 mA.
3 The values for I IH and I IL are input leakage currents.

Table 7.2 ■ Current capabilities of common logic families 1