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eprom_dump [2017/05/29 16:44]
lord_nightmare add note about vcc vs vih voltage
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-Techniques for dumping bit-rotten and/or damaged EPROMs: 
-Bit-rotten roms are EPROMs/​EEPROMs/​flash where the floating gate charges have decayed from age/light exposure below the threshold to reliably read as the '​high'​ state. On most EPROM based devices, a high floating gate charge indicates a 0 bit, so this means the values will read as 0xFF instead of the proper value. 
-There are several techniques to try to recover bit-rotten data like this: 
-  * Change temperature. Ex: People have reported success using a hair dryer or heat-gun on parts. This seems to work better than freezing parts does, maybe by '​boosting'​ via thermal noise the floating gate charge readout value? 
-  * Lower reference voltage: Lowering the VCC/​Reference voltage from 5v to below 4.97v or so lowers the '​threshold'​ for reading back floating gate charges, which can recover some bits which are just below the threshold. Too low of a voltage may make the chip malfunction,​ too high will not have any effect. To prevent potential damage to the chip by having address/​control lines driven higher than VCC, the reference input '​high'​ (ViH) voltage for all pins should be lowered simultaneously,​ but this might not always be necessary. 
-  * Multiple reads and binary ANDing or '​voting'​ of the bits: dumping a chip several dozen/​hundred times and having each dump '​vote'​ for whether a given bit is 1 or 0, the most popular votes winning. Binary AND is simpler, but this causes problems if a bit should read as 1 and erratically reads as 0. 
-Combining 2 or more of these techniques is significantly more effective than using them separately, so people have had much better luck both heating and simultaneously under-volting chips than doing either one separately. 
-Damaged Chips: 
-There are more or less two classes of damage to an IC which will prevent it from reading: Damage to the leadframe and bond wires, and damage to the die itself. 
-Heat might help for leadframe/​bond wire damage (if it cannot be directly/​permanently repaired with conductive epoxy or solder, etc) as it may make the metal expand enough to make contact with the other side it was broken off of. 
eprom_dump.1496090694.txt.gz ยท Last modified: 2017/05/29 16:44 by lord_nightmare
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