I'm searching a suitable bus driver for a project. I found the Nexperia 74VHC244D. On the website they say the maximum frequency is 60MHz (here).
But how do I calculate the 60MHz? Because it's not mentioned in the datasheet. Is it just the inverse value of the propagation delay?
The manufacture states a cryptic Tpd = 15ns (Cl=50pf, Vdd=3.3V) as usable test values, but in addition the test is with a square wave of ~50 % duty cycle at 60 MHz at \$25 Deg^oC\$. Maximum frequency also depends on if the part is commercial, industrial or military grade, as they guarantee operation over a specific range of temperatures.
Be sure to read ALL of the datasheet as it often includes drift from maximum frequency based on ambient temperature. Look at the charts in the datasheet. If it MUST work at 60 MHz then look for all the reasons why it may not. Usually \$F_{max}\$ is obtained under ideal (but possible) conditions.
Looks like they're using Tpd = 15ns (Cl=50pf, Vdd=3.3V) as the figure to arrive at 60 MHz, with a little bit of margin.
If I had to guess why, it was the marketing droid badgering the tech writer for a ‘MHz’ figure, so they settled on 1/Tpd.
