Advanced Parallel I/O - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 318

The voltage-level compatibility issue arises because IC technologies differ in the following
four voltages:
• Input high voltage ( V IH ) . This is the voltage that will be treated as a logic 1 when
applied to the input of a digital circuit.
• Input low voltage ( V IL ) . This is the voltage that will be treated as a logic 0 when
applied to the input of a digital circuit.
• Output high voltage ( V OH ) . This is the voltage level when a digital circuit outputs
a logic 1.
• Output low voltage ( V OL ) . This is the voltage level when a digital circuit outputs
a logic 0.

In order for the digital circuit X to be able to drive circuit Y correctly, the following conditions
must be satisfi ed:
• V OHX $ V IHY (the output high voltage of circuit X must be higher than the input
high voltage of circuit Y). The difference between V OH and V IH of the same
technology is referred to as the noise margin high (NMH).
• V OLX # V ILY (the output low voltage of circuit X must be lower than the input low
voltage of circuit Y). The difference between V IL and V OL of the same technology is
referred to as the noise margin low (NML).

The input and output voltage levels of a few popular logic families are listed in Table 7.1,
from which one can draw the following conclusions:
• There is no problem using CMOS logic chips to drive bipolar logic chips at the
same power supply level.
• The HCS12 has no problem driving the CMOS logic chips and being driven by the
CMOS logic chips at the same power supply level.

Logic Family V DD V IH V OH V IL V OL

HCS12 3
S 4
LS 4
AS 4
F 4
HC 3
HCT 3
ACT 3
ABT 5
BCT 5
FCT 5

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

3.25 V
2 V
2 V
2 V
2 V
3.5 V
3.5 V
2 V
2 V
2 V
2 V

4.2 V
3.0 , 3.4 V 1
3.0 , 3.4 V 1
3.0 , 3.4 V 1
3.4 V
4.9 V
4.9 V
4.9 V
3 V
3.3 V
2.4 V

1.75 V
0.8 V
0.8 V
0.8 V
0.8 V
1.5 V
1.5 V
0.8 V
0.8 V
0.8 V
0.8 V

0.8 V
0.4 , 0.5 V 2
0.4 , 0.5 V 2
0.35 V
0.3 V
0.1 V
0.1 V
0.1 V
0.55 V
0.42 V
0.55 V

1 V OH value will get lower when output current is larger.
2 V OL value will get higher when output current is larger. The V OL values of
different logic gates are slightly different.
3 HCS12, HC, HCT, and ACT are based on the CMOS technology.
4 S, LS, AS, and F logic families are based on the bipolar technology.
5 ABT, BCT, and FCT use the BiCMOS technology.

Table 7.1 ■ Input and output voltage levels of common logic families