piclist 2001\10\13\195035a >
Thread: weird I/O problem
face BY : Kathy Quinlan email (remove spam text)

Hi all, Back out of hospital and just living till the next operation :o(

Ok Dennis,

I have not got any online examples at hand, but I can describe it easily.

I  assume that you are familiar with the totem poles of the North American
Indians (IIRC) where they carved heads in to the pole one on top of the
other. Now just look at a pair of heads, Imagine each head is a Transistor
or FET, The top Transistor / FET is tied to Vdd (+5V), and the bottom one is
tied to Vss (0V).the space between the two heads (transistors / FETs) is
where we take our output. Inside the chip behind out totem pole is some
logic to provide the signal for the output, but also a complimentary
(inverted) signal, the normal signal goes to the top device (Transistor /
FET) and the complimenty signal goes to the bottom device. A few parts
(resistors, diodes etc) are used to make sure that while the voltage changes
that BOTH devices are not switched on as that would short the +5v to 0V.

When the true signal is a logic 1 (+5V) the out put will be ~+5V (due to
losses in the junctions of the transistor), and the bottom device will be
switched off.

Now if the true signal is 0 the top device is off and the bottom device is
on , causing the output to be pulled low (0V).

As you can see a fair few parts are needed to drive the top device,
compliment the logic, and provide a dead crossover region.

It soon became clear when mixing logic devices, that a totem pole output was
no good, as maybe you needed more than +5V to signal logic high, A cmos
device for example may need 10V but our poor Pic sitting on +5V (and no we
can not give him more voltage in) can not drive the cmos pin to a logic 1
(we would only see 0V and an undefined voltage, both could possibly be
interpreted as a logic 0)

To over come the above problem open collector was designed, Instead of
pulling the output High, we let it float and use an EXTERNAL pull up
resistor to pull the signal to what ever level we like, all we need to watch
is that we do not overload the driving pin by pulling to much current, or
pushing a too higher voltage in.

If you have any further questions, pleas ask :o)



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See also: www.piclist.com/techref/microchip/ios.htm?key=i%2Fo
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