Question

What must be done to interface TTL to CMOS?

(a) A dropping resistor must be used on the CMOS of 12 V supply to reduce it to 5 V for the TTL

(b) As long as the CMOS supply voltage is 5 V they can be interfaced (however, the fan-out of the TTL is limited to five CMOS gates)

(c) A 5 V zener diode must be placed across the inputs of the TTL gates in order to protect them from the higher output voltages of the CMOS gates

(d) A pull-up resistor must be used between the TTL output-CMOS input node and Vcc; the value of RP will depend on the number of CMOS gates connected to the node

I got this question in my homework.

My question comes from Digital Integrated Circuits in chapter Boolean Algebra and Minimization Techniques and Logic Gates of Digital Circuits

Answer 1

Right choice is (d) A pull-up resistor must be used between the TTL output-CMOS input node and Vcc; the value of RP will depend on the number of CMOS gates connected to the node

The best I can explain: To interface TTL to CMOS a pull-up resistor must be used between the TTL output-CMOS input node and Vcc. A pull-up resistor is used to avoid the floating state on the input node of the CMOS, thus using a small amount of current. The value of RP will depend on the number of CMOS gates connected to the node.