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There are
instances wherein the output of a TTL logic gate needs to be used for
driving the input of a CMOS gate. Since the voltage-current
characteristics and requirements of a TTL gate differ from those of a
CMOS gate, it is good practice to use proper interfacial components
between them when connecting them to each other. Below are some
common techniques used in connecting a TTL gate to a CMOS gate.
Figure 1.
Interfacing any TTL gate to any CMOS gate using the same power supply
(5V)
When the CMOS
gate that the TTL gate will drive also uses the same 5-V supply used by
the TTL gate, the simple interfacing technique shown in Figure 1 may be
employed. Here, a pull-up resistor is just placed between the TTL output
and the
5-V supply.
Figure 2.
Interfacing
an Open-Collector TTL gate to any CMOS gate using different power
supplies
When the CMOS
gate that the TTL gate will drive has a supply voltage that's different
from the
5-V supply used by the TTL gate and if the TTL gate has an open
collector, the simple interfacing technique shown in Figure 2 may be
employed. Here, a 10-K pull-up resistor is just placed between the TTL
output and the
CMOS gate's supply.
Figure 3.
Interfacing
any TTL gate to any CMOS gate using
different
power supplies
When the CMOS
gate that the TTL gate will drive has a supply voltage that's different
from the
5-V supply used by the TTL gate and if the TTL gate does not have an
open collector, it would be good to use an NPN transistor to translate
the TTL output voltage level to a correct CMOS input voltage level as
shown in Figure 3 so as not to overstress the TTL gate.
See Also:
CMOS-to-TTL Interfacing Techniques;
Logic Gates;
TTL
See
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