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'how do you calculate the resistor for an open dra'
2003\02\13@142955 by Olin Lathrop

face picon face
> I am trying to hook up an I2C A/D converter,
> and the SDA and SCL lines show a 2K resistor
> on their example, but the example is using 2.7 to 3.6 vdc
> supply.
>
> So, this can't be what's needed for a 5 vdc supply, can it?

IIC specifies the maximum current a device must sink to drive a bus line
low.  If I remember right, this is 3mA.  The logic low threshold is
specified as 400mV (again, from memory), so the minimum pullup resistor to
5V would be

 R = V / A = (5V - 400mV) / 3mA = 4.6V / 3mA = 1533 ohms

Allowing for resistor tolerance, 1.8Kohms is the lowest allowable standard
5% value.  I would use at least 2Kohms unless you absolutely needed every
last bit of speed.


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