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SN74HCT125: How to calculate the max input frequent

Dong Ba
Intellectual 825 points
Part Number: SN74HCT125
Other Parts Discussed in Thread: SN74HCT74

How to calculate the max input frequent

http://www.ti.com/lit/ds/symlink/sn74hct125.pdf

1.there are tpd, tt,but it doesn't say how to use the parameter to get the max input frequent

tt :Input transition (rise and fall) time

tpd: PROPAGATION DELAY TIMES

2.what's the relationship between tt and tpd, what does "tt" mean?

3. In the part selection tool, the data rate is shown"50Mbps", what's the relationship with input frequency?

  • 0
    Guru 275790 points

    The propagation delay (tpd) could be used to estimate the data rate, but it is actually possible to change the input even while the output is still switching, so it would only result in a lower bound.

    tt is not helpful for this question (it appears to be the output transistion rate, which is depended on the capacitive load).

    The data rate (measured in Mbps) is twice the maximum frequency (measured in MHz), because the full cycle of a square wave can transmit two bits.

    [FAQ] What is the maximum data rate (or operating frequency) for a logic gate or buffer? says:

    The maximum operating frequency can be estimated by finding a clocked device in the same logic family.

    The SN74HCT74 flip-flop is rated for 25 MHz, which corresponds to a data rate of 50 Mbps.