Electrical – what is the EEPROM (28C64) output behavior between address change


I wrote all "1"s into a 28C64 EEPROM from address 0 to 15 (the rest address lines tied to GND).

Then I use a 4-bit counter (10HZ clock) to drive the address lines from 0 to 15 to read the 28C64. I noticed that there is a short voltage drop (output goes from high to nearly zero and back up to high, last about 160ns) when the address changes.

It happens somewhat randomly: the voltage drops may happen when address changes from 1 to 2, or from 5 to 6, etc. Then I tried two another same 28C64 chips, one does not have this voltage drop, and one does.

The data sheet used cross marks during address change. http://www.jameco.com/Jameco/Products/ProdDS/74827.pdf

I am quite confused, it is normal for the output to drop during address transition? thanks!

Here is the scope captured the voltage drop. The total time is about 240ns, the chip is -25.

enter image description here

Best Answer

Here is how a read cycle is sequenced (extract from the datasheet you linked):

enter image description here

You see that, when the address changes, the DATA I/O lines are hatched. This means the output is unspecified. It could be anything, until tAA has elapsed. This timing is your EEPROM access time, and, depending on the part number, ranges from 150ns to 250ns. So, what you see is consistent with the spec.

Basically, the output are guaranteed to be correct only after this access time has elapsed. In the meantime, the address decoding may still be settling within the chip, and output may be inconsistent in this period. And, yes, results may be different for each chip.