• Propagation delay time
• FAN-OUT factor
• Output characteristics
• Overview of basic types (TTL,TTL-OC,TTL-LS,CMOS,H-CMOS)

Propagation delay time

Propagation delay time is defined as time needed for d. device to change states of it's outputs after changing of states has occur on it's inputs. E.G. NOT-logic device converts logic "1" at input into logic "0" at output, it would be ideal if this conversion take place immediately, but it take some time and that is our delay time. Digital device with smaller p.d. time is better, because it can operate at higher frequencies which is essential in modern digital circuits.
Most common values of p.d. time in ordinary digital IC's are 10-20ns.

FAN-OUT factor

FO-factor is number which tells us how many TTL compatible devices we can connect to one output of digital device preserving voltage level of logic signals in determined range:
-0.8v max for logic "0"
-2.4v min for logic "1"
Common value is 10 TTL-LS (it means that max. 10 d. device of TTL-LS type can be connected on one output).

Output characteristics

Outputs of digital IC's can be divided into three groups:

• Bipolar outputs (TP-totem pole) are ordinary TTL outputs which can generate both logic "1" and "0".
• Unipolar outputs (OC-open collector) can generate only "0" and at logic "1" the transistor is turned off and acquired voltage level is obtained by resistor connected to power. These outputs are used when is needed to connect more logic outputs on same line (bus).
• Tri-state outputs (TS) have characteristics of bipolar outputs but in addition they have third state (state of high impedance) which allows them to be connected onto bus lines.

Overview of basic types of digital devices

TTL-technology (74xx) is most common technology in basic logic circuits, and it can be characterized by two basic factors:
-Delay time of aprox. 20ns per logic gate.
-Power consumption of cca. 15mA/logic gate

TTL-LS (low shottky: 74LSxx) is characterized by much lower power consumption then ordinary TTL preserving same operating speed:
-Delay time of aprox. 20ns per logic gate.
-Power consumption of cca. 4mA/logic gate

TTL-OC (there isn't special mark for those circuits but they can be identified by different number: 7400-four NAND gate, 7401-four NAND gate with open collector) these d.devices have similar properties as ordinary TTL but they have extended delay time (compared with TTL) when changing state from logic "0" to "1": cca. 50ns propagation delay time for changing from "0" to "1" and 20ns for changing from "1" to "0".

CMOS-(4xxx) technology makes possible to manufacture digital devices with very low power consumption which is main advantage of this technology:
Power consumption of cca. 0.7microA/logic gate

H-CMOS (high speed CMOS: 74HCTxx) allows digital circuits to operate at higher frequencies preserving low power consumption which is main characteristic of all CMOS devices:
Delay time of aprox. 8ns per logic gate.