Mystery EEPROM circuit

February 7, 2010 in idle by Ian | 4 comments

s3c is tantalizing us with this mystery EEPROM device. It’s a small I2C EEPROM data storage chip, and it appears to be powered parasitically from the I2C clock line.

Was bored today so I opened a dongle I had laying around. It only contains an I2C eeprom and a couple of passives but it’s pretty smart, it uses a diode and capacitor to power the eeprom from the eeprom clock line.

Anyone have an idea what the resistors R1 and R4 are for? I’m assuming PIN1 and PIN2 are connected to Vcc through external resistors and are connected as SDA and SCL for normal I2C operation.

We’ll send a free PCB to the poster with the most persuasive explanation of the circuit.

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  1. Travis on February 7, 2010 at 2:17 pm

    The diode D1 is to prevent a power supply being hooked up wrong, the D2 zener diode will create a voltage drop across it of a certain amount depending on what zener diode it is, but say it’s a 3.6 V, so then 3.6 volts goes to D1, which charges the R1 and C1 circuit which powers the eeprom. As long as constant data is flowing to this device it will work. The zener diode D2, is probably dropping a 5V supply down to a lower voltage because the eeprom can run on a lower voltage. R4 helps to create the 3.6V, because a zener diode can go into avalanche mode and destroy itself, the R4 will help control the current I believe, but I’m not positive on that part.

    This is a neat little trick to not have to power a sensor, I might keep this in my back pocket for something useful some day.

    Travis

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  2. Brad Gilbert on February 7, 2010 at 4:10 pm

    D2 and R4 are to make sure that the input voltage on Pin1 remains below some given threshold. R4 is mainly to reduce the current that D2 would otherwise have to maintain.

    D1 allows any positive voltage on Pin1 to go to Vcc. C1 helps to hold Vcc to a usable voltage. R1 is probably there to help bring the system to a “safe” state, after it is done being used.

    R2 and R3 are of course there to reduce the current to SCL and SDA.

    There would be a problem if Pin1 was connected directly to a voltage source that was higher than D2 would normally allow. So I would assume that Pin1 might be connected through a resistor on the other side of the connector.

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  3. x893 on February 7, 2010 at 8:02 pm

    I was shocked by the question about R1 and R4

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  4. spiffed on February 10, 2010 at 12:28 pm

    While it doesn’t provide any insight, this is a defacto standard circuit for toner cartridge page counters.

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