This disconnect feature is what allows multiple open drian (or open collector) outputs ot be connected in parallel, so that any one that turns on controlls the output voltage by pulling it toward the negative supply rail. In CMOS logic family that includes PIC microprocessors, those switches are either N-channel MOSFETs (which usually are connected to conduct between their drains (outputs) and the negative supply rail or P-channel MOSFETs (whichare usually connected to conduct between their drains and the positive supply rail.Īn open drain output is just an N-channel MOSFET that makes a connection between the output pin and the negative supply rail when it is on, or disconnects the output from everything (leaves it floating so some externa lsource of current can pull the output voltage up above the negative rail). Maybe i am misunderstanding "totem pole" relating to TTL technology (TTL is by default drawing current on digital inputs).Įverything digital is done with switches that are either on (conducting) or off (nonconducting). I am talking about RA4, which is open-drain.
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