USB to I2C interface

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mainevent

Joined Dec 7, 2009
11
There are several peripherals connected on the I2C bus giving a total of 40pF capacitance, I am working with a maximum capacitance of 60pF, 10pF if my connection is external or 20pF if internal with an estimated wire length of 10cm. I am going to access an EEPROM while it isnt receiving power from its main unit. I need a real current source in order to allow the EEPROM voltage to go to 2.69V(0.81Vcc) in order to pass the (0.7Vcc) threshold to access the EEPROM. I dont really know how im going to connect the transistors on the bus lines....
 

Papabravo

Joined Feb 24, 2006
22,116
Whatever you do you cannot operate the chip from the bus lines alone. The EEPROM must receive its power on the VCC and GND pins from some source quite independant of the bus lines. If the chip tries to draw power through the bus lines there is a very good chance that you will get quite unexpected and unpredictable results. You may even damage the chip. Where did you get the idea that you could do this? You appear to be the victim of some pernicious cant.

Let us assume for the sake of argument that a bus line has a pullup of 330 Ohms and there is 100 pf of capacitance. Then 1 RC time constant is 330 * 100e-12 = 33 nanoseconds which is 1.3% of the 2.5 microsecond period of a 400 kHz. clock. Your own numbers tell me that ordinary pullups will be just fine for any I2C system in which all of the chips connected to the bus have the proper VCC during the period of intended operation. The 330 Ohm pullup will have 10 mA going through it when the clock line is low. This is well within the capability of a typical driver stage. You can of course use a smaller pullup to decrease rise and fall times further at the expense of additional drive curren requirements. I leave you to do the math on your own.
 
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