Need help with info about EEPROM - Page 1
Forum section: electronics / general

author: Viral
author_rank: Newbie
timestamp: February 13, 2018, 08:29:31 pm
content: Dear Friends,I work at an electronics store and some of our equipment is card readers and door locks, the ones that you're finding at building entrances. You show your card ID and the door opens.Some of our customers say that sometimes the card reader loses memory, basically some of the cards, or even all the cards, don't work. Quick fix is to reset power and maybe reprogram those missing card TAG's. One of my colleague, says there is nothing wrong with the card reader. He says the problem is with the power supply, when the person shows the tag and the door opens, the door lock draws to much current and the power supply loses voltage. At that point, if the card reader and door lock share the same power supply, because of the voltage drop, the card reader is no longer powered, and after couple of times doing that, the memory gets corrupted and loses data.To be honest, this seems total BS. The door lock usually draws no more than 1 A plus the card reader 500mA at worst, and we normally recommend a 3 A power supply with back-up. Even with higher rated power supplies and low amp door locks still happens the same.The memory on device is PIC 24LC256. I think it's used only for storage of card ID's.I am a an engineer but i'm just at the beginning. This type of problem is hard for me to debug so that why i'm asking for help.In summary:1 - Could sudden drop in voltage of power supply cause data corruption in memory ?2 - What else could cause these data corruptions even if the equipment is few months old and the power supply is working good ?Best Regards

author: calexanian
author_rank: Super Contributor
timestamp: February 13, 2018, 10:19:23 pm
content: Couple thoughts.I would be curious to see the power supply voltage during lock operation. If its an older system the filter caps in the supply may be weak and this could indeed lead to a failure of the logic circuit from supply ripple voltage. Door lock solenoids are also notorious for burning out over time and the impedance may have gone down from shorted winding giving you a problem. Generally CMOS ic's and Eprom memories are pretty rock solid over the years. There are of course exceptions but other causes are usually to be suspected first. Generally those locks are 24VDC. I would start there.

author: BradC
author_rank: Super Contributor
timestamp: February 14, 2018, 12:36:26 am
content: It has been standard practice in access control to have locks on a separate power supply. It was less to do with inrush current (although that can be fairly significant, especially when you start getting up to custodial locks) and more to do with huge back emf spikes.That might be something worth looking into also.

author: @rt
author_rank: Super Contributor
timestamp: February 14, 2018, 04:12:35 am
content: There’s no doubt a solenoid with a kickback pulse that needs taking care of. Were you around for sat TV hacking in the early 2000s? Power supply and clock signal glitches were the only way to erase all but the mask ROM area of an original card so they could be rewritten. I made boxes with 555s and relays to do this. Never particularly aiming at stock EEPROMS though, because there’s simply no reason to try any fancy method to erase them.

author: Viral
author_rank: Newbie
timestamp: February 14, 2018, 05:56:15 am
content: Thank you all for your responses.I'm in my late 20' now so i was very young during that period.Forgot to mention that our systems mainly works at 12Vdc. The power supply can be linear or SMPS. And the door locks that are we using most of the time have a main board with a stepper motor driven by ULN2004a chip. Back emf spikes are low or almost non-existent. Happened to brand new devices, maybe 1 month old, the caps still looks "shiny".Or maybe those customers are using door locks with solenoids without flyback diodes connected. I will make sure to get more information about the installation in the future.

author: donotdespisethesnake
author_rank: Super Contributor
timestamp: February 14, 2018, 09:18:36 am
content: If there is code in the firmware to write to EEPROM, then it can also be called accidentally. There are two cases where this can happen, a bug in the code, or brownout condition. During brownout, the CPU can execute incorrectly, including jumping to random addresses. Most of the time random execution has no discernible effect, the only time you could notice is if it writes something to non-volatile memory like EEPROM or Flash.True story : some 3D printers lost firmware during shipping. Turned out that movement generated enough back-EMF form the stepper motors to power the CPU briefly, in some cases it jumps to the bootloader function that erases the application code.Lesson : always take appropriate action on brownout conditions.