74XX IC Selection & Compatibility

[ztnel]

74XX IC Series

The 74XX series ICs is a family of logic IC's and will contain all the combinational logic required to build our digital circuitry. There are 3 'modern' and commercial ICs we can choose from (sourcing wikipedia):

• 74LS - Lower power Schottky (current standard)
• 74HC- High-speed CMOS, similar performance to 74LS
• 74HCT - High speed CMOS TTL-compatible

Properties and Selection

In general IC families are separated by the following parameters:

1. V_IL (max) - maximum ‘low’ input voltage
2. V_IH (min) - minimum ‘high’ input voltage
3. V_OL (max) - maximum ‘low’ output voltage
4. V_OH (min) - minimum ‘high’ output voltage

When mixing devices from different families you have to be cautious of the variant high signals, specifically if one devices VOH(min) is less than another devices VIH(min)

Parameter	74LS	74HC	74HCT
VIL	0.8 V	0.3*Vcc V	0.8 V
VIH	2.0 V	0.7*Vcc V	2.0 V
VOL	0.4 V	0.1 V	0.1 V
VOH	2.4 V	Vcc-0.5 V	Vcc-0.7 V

Assuming a Vcc @ 5 V & I @ 4 mA we can resolve the following CMOS values:

Parameter	74LS	74HC	74HCT
VIL	0.8 V	1.5 V	0.8 V
VIH	2.0 V	3.5 V	2.0 V
VOL	0.4 V	0.1 V	0.1 V
VOH	2.4 V	4.5 V	4.3 V

74HC/T are CMOS ICs. 74HC is a pure CMOS family which means the outputs are going to have better V_OL and V_OH values (closer to the 0V for V_OL and 5V for V_OH). 74HCT is a TTL compatible version of the 74HC series because of its lower V_IH. Overall 74HC/T can drive 74LS chips but 74LS chips cannot consistently drive 74HC chips due to 74LS’s relatively low V_OH.

For more information see this excellent YouTube explanation here