quote:
Originally posted by brianl703:
That's very interesting about the ROM module being known to die.
That reminded me that certain type of ROM, called an EPROM (Electrically Programmable Read Only Memory) has a finite lifespan--I've heard 10 years is what was guaranteed on those EPROMs that specified a lifespan.
If it is an EPROM, there will usually be a sticker on the ROM chip which, if removed, will reveal a window through which you can see the actual chip. Exposing this window to UV light of the correct wavelength and intensity will erase the chip. EPROMs without the window are available to save costs, so lack of a window isn't a guarantee that it's not an EPROM.
EPROMs have a limited lifetime because the bits are stored as charges in an insulated layer and given enough time, the charge leaks away, causing the EPROM to revert to it's unprogrammed state. When the chip is exposed to UV light, the UV light makes the charge leak away in a few minutes.
The alternative to EPROMs, mask-programmed ROMs, do not have this issue because they are built with the correct programming inside of them--there is no charge to leak away over a period of years. Some automakers used mask-programmed ROMs and some used EPROMs. I don't know which, although I believe I read that Ford used mask-programmed ROMs. Mask-programmed ROMs are more economical for higher-volume applications, with EPROMs being more economical for lower-volume applications.
This is all very interesting because, back about 10 years ago, I was kidding around with one of my friends telling him that his fuel-injected car will eventually lose the contents of it's EPROM chips. (At the time I drove a carbureted car..)