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Erasable memory — Parallax Forums

Erasable memory

DarrenYDarrenY Posts: 61
edited 2008-07-30 04:34 in Propeller 1
Does anyone know of an IC that can be programmed/read from via I2C but will also erase its contents upon losing power?

I need to store a unique key in it and run it off a backup battery, but I want to link it in with a tamper switch so that if the device is interfered with (i.e. the switch opens and the IC loses power) then the key is erased.

The next time the Prop boots-up it checks to see if the key is present and if it isn't sets itself to 'unsecured' mode. (The Prop must also be capable of reprogramming the key when needed.)

Comments

  • RaymanRayman Posts: 14,824
    edited 2008-07-28 12:23
    The Real time clocks (RTC) generally all have a little bit of non-volatile memory...

    I'd recommend the DS1307.· It's needs 5V power, but can be connected directly the the Props I2C bus without resistors or anything...

    Oops.· I think I didn't read you message properly...· You want volatile memory...

    Post Edited (Rayman) : 7/28/2008 3:49:44 PM GMT
  • hippyhippy Posts: 1,981
    edited 2008-07-28 12:34
    There are also 256 x 8-bit I2C RAM chips but sometimes hard to find and often fairly expensive.

    Or use a PICmicro / AVR in I2C Slave mode.
  • 4Alex4Alex Posts: 119
    edited 2008-07-28 16:14
    Hi Darren,

    The 256x8bit I2C RAM referred to by Hippy (a good idea) is available at Digikey, regular supply from NXP (Philips), for about $2 USD @ QTY=1, much cheaper in larger qty.
    SOIC08: 568-1072-1-ND DIP08: 568-1071-5-ND

    I've used them, they are great.

    Cheers,

    Alex
  • DarrenYDarrenY Posts: 61
    edited 2008-07-29 10:15
    Yes! The NXP PCF8570 looks ideal, thanks smile.gif
  • mikestefoymikestefoy Posts: 84
    edited 2008-07-29 12:13
    what wrong with the I2C eeprom, and a timestamp written alongside, and if the timestamp number is out of date, then its invalid.
  • hippyhippy Posts: 1,981
    edited 2008-07-29 12:32
    @ mikestefoy : This is an easy to implement, low-current draw, hardware-based tamper-detect / self-destruct mechanism.

    Anything else requires some controller to be executing and monitoring the case-open tamper switch. This way the 'magic key' held in RAM is either there or not whenever the controller starts up.
  • DarrenYDarrenY Posts: 61
    edited 2008-07-29 14:11
    Exactly, the key is either there or not there, and the volatile memory is held up via a backup battery running through a tamper switch.
    Open the tamper switch, the memory loses power and 'poof' the key is gone smile.gif
  • hippyhippy Posts: 1,981
    edited 2008-07-29 14:42
    This is a slight aside but is to do with 'battery backup volatile memory' ...

    I've had reasonable proof-of-concept success with a PICmicro powered through an unused I/O pin from a capacitor / battery, the diode clamp to internal +V being enough to keep the RAM alive but the chip in reset when main power is removed.

    I cannot see anyway to do that with a Propeller, bootload overwrites all RAM, Cogs get stopped on Reset, overwritten when started. Can anyone see a way to pull the same trick ?
  • Cluso99Cluso99 Posts: 18,069
    edited 2008-07-30 04:34
    I wouldn't like to base a commercial design on this without an OK from the chip designers. Things can change between batches of chips. Sometimes slight changes to masks occur and the manufactures do not inform you of this. It's fine for a hobbyist, but not for commercial. Just my 2c worth.
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