MCUmall EPROM BIOS Chip Burner Forum
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 True USB Willem Programmer (GQ-2X, GQ-3X, GQ-4X)
 Random EPROM readings. Possible to change read-acc

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x4e Posted - 03/28/2017 : 09:50:26
Hi All

I am trying to read an old (about 15/20 years old) 27C800 EPROM and I am experiencing something very similar to what is described here:

At each reading, a small percentage of data (about 20 bytes over 1Mb) differs from the previous reading. It seems that some bits are left at '1' when comparing mismatching bytes.

I checked the parameters available within the device configuration file under "AppData\Roaming\mcumall\GQ USB Programmer\devices.txt", but I could not spot anything to configure the read-access time.

Also, the very limited documentation on adapter ADP-054 v4 says "J5, ‘Chip Compatibility Jumper’ Only change it if the chip has inconsistent reading and/or writing."

I moved the J5 jumper from the original position (V3?) to the right (V4?) with no much better results.

Anyhow, my question is. Is it possible to change the read access time for this type of EPROM and programmer+adapter. If so, how?

Thanks in advance for your help!
12   L A T E S T    R E P L I E S    (Newest First)
supervizeur Posted - 04/25/2017 : 11:55:56
And good power.
anniel Posted - 04/15/2017 : 06:29:48
Yes, good pin contact is the first thing to look for.
ZLM Posted - 04/13/2017 : 21:40:29
make sure all pins are contacting good.

using a good USB cable.

if problem still same, then the chip may not be a good chip. The data in chip may be in marginal state due to the long time storage.
supervizeur Posted - 04/11/2017 : 17:57:15
Are you using a good USB cable? Try measuring with an external power supply.
x4e Posted - 04/05/2017 : 14:03:02
I measured the VCC voltage on the original devices. It is 5V spot on.

The programmer when nominally at VCC="5V",RVpp="5V" measures 4.85V. Is it normal? Is the programmer defective?

With VCC="5.5V",RVpp="5V", it measures 5.26V.
With VCC="3.6V",RVpp="5V", it measures 3.68V.

I have tried all above configurations when reading the EPROM but I still get random readings. Both 4.85V and 5.26V are within less than 10% tolerance from 5V.

Is there any other way to change the read-access time?

anniel Posted - 03/29/2017 : 02:41:28
Yes, old EPROM are quite resilient to voltage if you don't go overboard with extremes.

Some chips require VPP (at lower voltage) even for reading. You can also try that for your chip.

Probably different voltage as old power supplies go out of specs (you can easily mesure that and reproduce on the programmer) or a mix of the 3.
x4e Posted - 03/28/2017 : 22:44:56
Got it. So first try with lower voltages and if it does not work with higher voltages.

What is the devices.txt attribute for the read voltage? "RVPP"?

Also, how do you explain the fact that the device where the EPROM was installed is still capable of reading this EPROM? Different read timings, different voltage, or just luck?
anniel Posted - 03/28/2017 : 15:30:17
In some old chips the cells tend to slowly shift voltage values. It's called bit creep or bit rot.

When dealing with difficult chips I use an external variable lab power supply and isolate the voltage from the programmer to find the sweet spot.

To prevent further damage start by decrementing the voltage.

x4e Posted - 03/28/2017 : 13:15:13
And I am still getting random reads at Speed -2.

When you say "play with the voltages", are you suggesting to incrementally the voltage?

If so, what parameter is it within the devices.txt file? And is there a risk of damaging the EPROM?
x4e Posted - 03/28/2017 : 13:12:11
Thanks for the reply. I played with the "Speed" settings but it seems to only affect the write speed. The read to buffer always takes about 24 seconds regardless of the speed parameter. Is the "Speed" -2 to 2 really changing the read-access time?

By the way, my programmer and adapter are MCUmall GQ-4x4 and ADP-054 v4.
anniel Posted - 03/28/2017 : 12:40:08
And keep in mind after 10 years data rot sets in. You then need to play with the voltages.
anniel Posted - 03/28/2017 : 11:38:39

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