Aren’t they ultimately dependent on battery power? What happens to the mileage reading when a car sits in a garage for a few years without a battery? Did the mechanical ones turn out to be so easy to roll back, that LCD is a superior choice?
I don’t know how they work. But my guess would be the mileage is stored in nonvolatile memory after every mile is driven.
I’m sure Mr. Goodwrench will be along shortly with the difinitive answer, but my guess is that it uses a small battery similar to what’s inside your computer to retain the memory in the event of an electrical system failure.
You don’t want the unit to reset when you disconnect the car’s 12 volt battery, either.
It also has to be relatively tamper-proof, as well.
Only the LCD screen is dependent on power to run/show. The digital odometer itself operates by computer chip and much the same way as your own PC’s hard-drive, loss of power doesn’t erase its memory. They are also most certainly NOT tamper-proof.
Because the manufacturers had to be able to set or reset them if they failed and were replaced, there are digital odometer calibrators that can be used to alter the mileage. A quick search on Yahoo! or Google of digital odometer calibrator will reveals a slew of these devices for sale online as well as garages that do legitimate (lawful) recalibrations. Some are handheld devices that plug into the car’s computer, but some are simply software that can be loaded onto a laptop’s hard-drive and connect to the car’s computer.
Purchasing a digital odometer calibrator for the purposes of tampering may require a more costly piece of software and/or hardware interface initially, a truly unscrupulous person can make their money back quickly. Here’s the skinny…Calibrators can be purchased for a grand or two (high end models are as high as $7,000). Each rolled back mile means a $0.10 difference in price. The auto industry says the average roll back is around 32K miles. That means one car rolled back 32K miles can be sold for $3,200 more after the rollback. Presto! Money back and then some.
And unlike the old mechanical odometers (which will show traces of tampering such as misaligned numbers or small scratches on the dials), it is virtually undetectable.
My '88 Cavalier Z24 had a digital dashboard with LCD odometer. It rather obviously did not depend solely on the car’s battery, as after the electrical system failed rather spectacularly on the interstate in Albany and the battery (and everything else) had to be replaced, the odometer was still correct.
I think it’s obvious that disconnecting the battery does not reset the odometer. But what exactly is the method used to retain the memory? Battery backed memory is one possibility, like the setup info on a PC, but an odometer would need to keep working for over 20 years. I don’t know if any battery lasts that long, no matter how small the current drain.
Rechargeable batteries are another possibility - drawing from the 12V supply to keep the battery topped off. Digital cameras often have built-in rechargeable batteries to keep the clock running. But those usually don’t last long, as any owner of such a digital camera knows.
So I’d guess it is some kind of non-volatile memory, like flash memory used in digital cameras. Does anyone know for sure?
http://www.autoed.com/resources/sampchaps/erjavec_ch20a.pdf
“The accumulated mileage value of the digital display odometer is stored in a nonvolatile memory (ROM) that retains the mileage value even if the battery is disconnected.”
Agree with the nonvolatile stuff, but not sure where they got “ROM” from, since plain ol’ ROM can only be written to one time…
I’ve never seen an LCD but I’ve seen odometers with LED when I’ve rented cars. My car is 10 years old and I’m probably going to have to buy a new one, soon. The thing I don’t like about all the LED ones I’ve seen is that there’s no tenth-of-a-mile. Is this standard for digital odometers? Does anyone make them with the manual anymore?
Also, I always use my resettable sub-odometer as my gas gauge (checking the MPGs while I’m at it). I can’t do that with the digital ones I’ve seen either. Do any car manufacturers have that feature in digital form, or has that gone the way of the dodo?
ArchiveGuy, I recently bought a Nissan Xterra. It has a digital odometer with the overall mileage and two trip odometers. The two trip odometers can be reset, and they both have tenths of a mile. The main odometer doesn’t have tenths, and I kind of miss them.
Yes, my car (2002 Mazda Protege 5) has 3 LCD odometers – 1 total miles, and two trip odometers that can be reset as needed.
The trip odometers go down to tenths of a mile, the cumulative one doesn’t.
I don’t know about the manual odometers though, but in the scheme of things, tenths of a mile are pretty insignificant.
Mine has a digital display, and a button I push to toggle to the tripometer. I’m not aware of any cars that have the digital display and don’t have the tripometer. Maybe I’m missing what you’re asking though.
Probably means its stored in an EEPROM.
For those who don’t speak geek:
An EEPROM (electrically erasable programmable read only memory) is a type of ROM chip. There are lots of different types of ROM (read only memory) chips. Typically, a PROM (programmable ROM) is a OTP device (one time programmable). An EPROM is an erasable PROM, and is usually erased with ultraviolet light (you remove the chip from the system, put it in a special “cooker” and let it go for about half an hour and it is erased). An EEPROM is an electrically erasable PROM. Because an EEPROM can be erased and reprogrammed in the circuit, it is the only type that could be used in this type of situation. EEPROMs are also often called flash ROMs. When you “flash” your BIOS on a computer, you are reprogramming the boot ROM (which is not a good idea unless you really really know what you are doing). Flash chips are often used in data logging devices in harsh environments because they don’t have any moving parts and thus don’t wear out (well, sorta, they do have a limited number of writes that can be done) and are much more immune to heat and vibration than many other devices that store data. It’s not a bad choice to use in a car odometer.
As you can tell, engineers love acronyms. 
I’m just going to keep quiet on why we still call it a ROM when you can obviously write to it. Suffice it to say that it’s just the way it is and we aren’t going to change it.
Any idea how radiation effects EEPROMS ?
Any idea what this limit is? And how often would the odometer update it’s non-volatile memory? Even it it only updated as part of a shutdown or startup routine it would conceiveably need tens or even hundreds of thousands of rewrites
adversely
In my pervious job I worked on electronic systems that went into satellites. I worked on the EEPROM board for the Chandra xray telescope satellite back when it was called AXAF. There were various models for radiation degrading the eeprom. One was total dose which basically meant that after a while radiation would degrade the circuits. If I remember correctly the main problem was radiation depositing charge in the on the gates of the transistors changing the threshold voltage of the transistors if it changes by enough the circuit would fail.
Another failure mode was that enough energy would be imparted by particle to flip the state of a node from 1 to 0 or 0 to 1. There were nodes in the EEPROMs that if fliped in this manor could cause a section of the EEPROM to be erased. This only would happen when the part was powered so to preserve the computer program we would turn off the EEPROMS when they were not being used.
As for limits on writing and rewriting. The limit in the old days used to be on the order of 10,000 to 100,000 erasures. I don’t really keep up with this stuff anymore.
They are claiming slighty higher numbers these days, but 100,000 is still in the right neighborhood for most parts. Industrial computer flash disks (a chip that looks like a hard drive to the computer but is really an EEPROM) gets around the limited write thing by spreading out the data all over the device. That way if you keep writing a bunch of stuff you don’t hit the same area of the chip over and over but instead the writes get mostly evenly distributed across the chip. I imagine that an odometer would do basically the same thing. You wouldn’t just store the current number in one particular location, but instead would have some algorithm for repeatedly storing it in different memory locations, so you could easily get a few million writes out of the thing.
Appreciate your answer. I am working on something where EEPROMs are subjected to radiation and hence wondered.
Can I please have references to the radiation models ? Only radiation which could deposit charge would be alpha-rays or beta rays. Usually these are not as penetrating as neutrons. So i donno if total radiation is a big factor.
Also, (I am not an electrical engineer so may be wrong here) do the EEPROMs function the same way as other memory ? I mean is their any redundancy to check if some bit flipped over by itself ?
Any help is appreciated
Sadly for this thread that was somebody else’s job and it was about 6 years ago so I don’t have any references for you. But NASA must have a lot of papers on the subject.
The EEPROMS themselves that I have seen don’t have redundancy built in. The redundancy would come from have more EEPROM than needed and use some kind of coding scheme to implement the redundancy. There may be special purpose EEPROMs with redundancy but I don’t think the market is big enough to justify making them so people mostly roll their own.