Eprom question

[Wawavoun]

Hello there !

I just experienced something very strange with an eprom.

The chip is a SGS M2716F1 eprom.

After 6 months of storage i get troubles with my Elektor Junior Computer and I check everything and mainly the eproms (I know it's a sensitive point).

On my eprom burner Vcc is 5 V.
During reading the vpp pin is connected to Vcc (in fact 4.56 V = 5 V less a shottky diode direct voltage). There is also a pull down 10 KR resistor from vpp pin to gnd.

After 20 minutes into the uv eraser if I read the eprom with Vpp pin high the eprom look empty ($FF everywhere).
But If I read it with Vpp pin to gnd via the resistor some datas are still there.

Then I put the eprom again in the uv eraser by two minutes steps and progressively (2 or 3 steps) all the datas were erased (always reading with Vpp pin to gnd)...
Of course with Vpp pin high the eprom also look empty...

Then I program the eprom and everything went fine.

The question is if this eprom has some defaults or not and if I can use it safely (in the destination assembly Vpp is always high) ?
Because the datasheet say Vpp high during read operations what can explain these observations ?

Thanks for help and advises.

Regards.
Philippe 

[rob77]

datasheet says clearly:

"VPP must be at 5V for all operating modes and can be maintained at 25V for all programming modes."

VPP pulled down to ground is not a defined state and therefore you can't expect consistent behavior.