Hi sommer
15 out of 75 parts is really a high percentage of failures. I am sorry that you are experiencing such issues but I will help you along the way in resolving this. Like dBC mentioned, for very low or floating voltage inputs, noise might trigger spurious zero-crossing interrupts. You can use the SAG detection feature to identify if the voltage has dropped below a particular value and then turn off the zero-crossing interrupt in the INTERRUPT MASK register. In that way, you avoid the problem of MCU spending most of its time servicing faulty interrupts.
1. What is the actual difference between good and bad units? Is it that the bad units provide zero crossing interrupts at zero input conditions while the good ones don't? Is the behavior different when higher amplitude voltage signals are used?
2. When you say you get increased jitter in interrupts at low voltage inputs, do you just mean that you get more random ZX interrupts when the voltage signals starts to get lower? If not, kindly elaborate.
Once we get to a point of clearly establishing why certain parts are faulty and what facts we have to back it up, we can try to get some failure analysis done on these parts to see if they reveal something.
Regards
hmani