I'm trying to work out worst-case propagation delays for an address-decode circuit I'm designing. I'm looking at the datasheet for a 74HC00N, and I'm confused by the propagation delay (tpd) values on p6 of the datasheet:
Focusing on the Vcc=4.5 V section, there are three lines:
I understand that the first line is indicating values where the ambient temperature is 25˚C, and the the next two lines are values for the two IC versions covered by the datasheet (54xx and 74xx). But one of these things is not like the other. If I'm using the SN74HC00, the datasheet says that the max tpd is 23ns. So when does the 18ns tpd value apply? The only explanation I could think of is that at TA=25˚C, both the 54xx and the 74xx versions exhibit the 18ns max, but above 25˚C, the max values diverge to the other values listed. Is that the correct interpretation?