Tool/software:
We have a PCB with an electronic circuit containing a 74HC14NSR supplied at 5V and decoupled with 100nF (X7R, 50V). The input of this circuit is a slowly changing signal (5V to about 0V in about 200ms). The HC14 output must switch when this input reaches the VT- threshold. In our last production batch of this PCB, this circuit does not behave like before. In all older batches the HC14 switched correctly when the input reached the Vt- level. In our last batch the switching moment of the HC14 is influenced by the switching of other logic signals: When the input comes closer to the Vt- threshold, the output of the HC14 already switches when other logic is switched. We do not know if the path of influence is via the supply lines or via switching of an other logic gate in the same HC14 part.
The strange thing is that when we dismount the 74HC14NSR from a PCB from our last batch and mount an "old" 74HC14NSR , the HC14 in the corresponding circuit switches correctly.
From this we conclude that the "new" 74HC14NSR behaves different compared to an "old" 74HC14NSR. Comparing old and new datasheets of the 74HC14 does not show changes that can explain in this different behavior.
We have investigated several 74HC14NSR with several production codes. All with the TI logo and marked "HC14".
Next production codes have the described problem:
41C5X7K G4
41C5Y8K G4
3AC4C1K
Next production codes behave correctly:
74CV60K
12CT1GK
79CTH4K
86ARG4K
Questions:
* Has the 74HC14NSR been changed over time?
* What was the change and what is the impact at behavior and parameters?
