So today I tried to build a simple manual EEPROM programmer to program the chips just by switches. I am able to read from the chips (all outputs are 1), but I am not able to write into them by bringing notWE low and than high again (while notOE is high). When I set it to output it outputs all ones at the IO pins as before no matter what I try to program into it. I have tried 3 different EEPROMs with the same results. I have measured the voltage on all pins few times and it all seems to be fine (voltage around 5,04V). I am using the AT28C256-15PU datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/doc0006.pdf
Has anyone experienced something similar? Any ideas why is this happening?

 

You did have (not)CE low while you were doing this?
One thought which occurs is that switches bounce. They do generate a clean logic transition. However, I think that would just result in writing the byte multiple times and so I don't think that is your problem.

 

Yes, I had notCE connected to ground the whole time and I tought about the switch bouncing, but that souldn't be a problem as you said. I am goind to connect some sort of electrically generated signal to the write pin just to be sure.

 

Yes, I had notCE connected to ground the whole time and I tought about the switch bouncing, but that souldn't be a problem as you said. I am goind to connect some sort of electrically generated signal to the write pin just to be sure.

Read section 4.2 in the datasheet.

Then make sure the devices haven't been write protected; by unlocking them.

 

Read section 4.2 in the datasheet.

Then make sure the devices haven't been write protected; by unlocking them.

I have read the section 4.2 and I understand it, but I dont know how to unlock it. I tried to go through the procedure described in section 20 (data protection disable algorithm) few times, but I still cannot write to any adress.

 

I have read the section 4.2 and I understand it

Draw diagrams for the timing of the signals you're using for write and read.

 

Draw diagrams for the timing of the signals you're using for write and read.

Well I actually suspect that I might have fake/not working EEPROMs, becouse I have ordered them from ebay for really low price
I have ordered 2 more chips from reputable local seller and I will test those and then report back.

 

Draw diagrams for the timing of the signals you're using for write and read.

So I have recieved the new EEPROMs today and I can read and write to all of them looks like I am not ordering any parts from ebay again, it is just a waste of time sometimes. However the seller agreed to resend me the chips, so maybe some of them will work.

 

So I have recieved the new EEPROMs today and I can read and write to all of them looks like I am not ordering any parts from ebay again, it is just a waste of time sometimes. However the seller agreed to resend me the chips, so maybe some of them will work.

eBay and Amazon are the source of a lot of counterfeit parts; especially if they come from China.

I won't buy electronic components from eBay unless it seems to be an odd lot. If it's something they routinely sell, I'm suspicious. The Sellers may also be victims themselves, but that's still no excuse for selling counterfeit parts.