Hi Andy,

That's it exactly, well, almost, except that isn't it at all.

(And you thought I was confusing before!?!?)

Okay, seriously. Yes, the top of a pullup resistor normally ties to a positive supply, unless it's biasing a pnp transistor. In any case, I was trying to describe things such that they would relate to the schematic image in Karthik's mind.

Now in the case of the pullup resistor analogy that describes the internal circuitry of a µ-controller port pin, I described it as not being tied to a positive voltage until the µ-controller writes a 1 (a positive voltage) to that pin, thereby applying a bias voltage to the top of that pullup transfer-resistor. I know that this is just turning on the base of a transistor. But in either case the collector (the pin) becomes positive unless something external grounds it. In the end, though lacking technical correctness, this seems the simplest analogy to explain why the datasheet says to write a one to a pin in order to treat it as input.