Although the data sheet identifies the recommended voltage of 0.85 VDD (4.25 V) as the VIH level, will the device work with the inputs operating at 3.3 volts maximum and 2.8 V minimum? From our experience and testing, it does. Is there any concern about operating the device in this way (long term reliability, loss or corruption of data, etc.)?
What were the conditions for your tests? Did you test for all the ranges of temperature and humidity? Did you test for hundreds of devices, to see what happens with those that are relatively far from production average parameters?
A logic device has a threshold. Its output is either low or high. If you push your working point behind the tested scenarios... you can get data corruption. Moreover, if the device input is very close to the threshold (wherever the threshold is), its output can start to oscillate, with the consequent noise generated to the rest of the circuit, and increased power consumption.
There is always concern when you operate a device out of its tested and guaranteed limits. In this case: data corruption, noise, increased power consumption. I don't think this will affect long term reliability, since you are not violating absolute maximum ratings.
Thank you Albert for your reply.
This device has been used in this manner for the last 4 years without any issues seen. It has been tested over the full thermal operating range and been tested to 95% humidity. Approximately 500 devices are used in this manner. We have not seen any evidence of data corruption or noise on the outputs when we have set them using the address bits and control bits over all environmental conditions. As state before, the Vih was a recommended operating condition in the data sheet. Is there a way to find what the absolute minimum operating Vih is? It seems to be much lower than the recommended.
Vih and Vil are the absolute max operating levels and are guarranteed across temp and process. anything beyond that will void warrantee. However as you suspect they are highly guardbanded.
Actual threshold of a CMOS device is 1/2 vcc and threshold of a TTL input 1.4V. They will shift with temp and process but usually not more than a couple hundred mv.
If the part has a schmitt trigger input then there is a specified max and min. These should be followed as stated.
So, in summery. Standard inputs Vih and Vil are heavily guardbandes and you will usually be safe if you cheat a little. However you do so at your own risk.