Push-pull Output

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Read from a data sheet that the port can be configure as push-pull output or
open drain. What is push-pull output? Is it when I set it to high, output
appear high. Set it low, output appear low?



Re: Push-pull Output
On Mon, 24 May 2004 03:12:53 +0800, the renowned "bear12"

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Yes. It has active devices (transistors) driving the output high or
low.

Best regards,
Spehro Pefhany
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Re: Push-pull Output
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Open drain and open collector are pull-only, open emitter are push-only,
and push-pull is both combined. It means there is an active push and an
active pull.

Rene
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Re: Push-pull Output

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or

In theory, an open drain output means that there is an NPN transistor
between the output and ground, and you have to add a load between the output
and VCC. A push-pull output has an NPN between ground and the output, and a
PNP between VCC and the output.

In practice, the actual output circuits vary substantially from what I
described, but in spirit, this is how they operate.

Mike



Re: Push-pull Output
On Sun, 23 May 2004 12:50:40 -0700, "Mike Turco"

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Actually the term "open drain" means that the pull-down transistor is
some sort of FET.  If it used an NPN or other bipolar pull-down
transistor, it would be open collector.

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Jack Klein
Home: http://JK-Technology.Com
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Re: Push-pull Output
   I don't like any of the other three answers I just read, so I'm
writing this...

wrote:

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   The term push-pull comes from electronic amplifier circuits. Most
op-amps, audio power amps, and other electronic devices use two active
devices (transistor), one for each polarity/half of the output
waveform. This means the circuit can source current (send it out of
the port pin) when the output is high, and sink current (take current
into the pin) when the output is low. One transistor "pushes" current
out, the other "pulls" current in.
   When configured as "open drain" the pin will be disconnected when
an instruction sets the output to high, no current will flow, and the
pin can "float" to any voltage between the ground and power pins).
When set to low, the output will go low and "sink current" as it dues
in push-pull.
   The term Push-pull is usually used for analog amplifiers and such.
I've more often seen the term "totem-pole"  used for digital outputs
such as TTL or microcontroller output pins. This term comes from
schematic diagrams that draw the two output transistors one on top of
the other. When the top transistor turns on, the output voltage goes
high. When the bottom transisor turns on, the output goes low. Both
transistors are never turned on at the same time, as this would short
out the power supply and/or damage the transistors.
   In the "open drain" configuration, the top transistor is always
off, and the bottom transistor is turned on for a low output and off
for a high output. The drain refers to the connection on an FET
(Field-Effect Transistor). Open collector is a term that basically
means the same thing, but implies that the circuitry uses bipolar
transistors (as the old classic TTL chips did and do) instead of FET's
(as most logic and virtually all microcontrollers do).
   Open drain/open collector outputs can be used for "wired-or"
outputs - several outputs can be connected together, and a resistor
connected between the output and the power input. When any one of the
outputs is set to low, they will all go low (this is the
negative-logic meaning of "or" but that's how it was named). Open
drain/open collector outputs are also used for multiplexed outputs for
driving LED or gas-discharge displays. Some such outputs (generally
TTL drivers) have a voltage rating much higher than the VCC rating,

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   Totem-pole (er, push-pull) output yes, open drain or open
collector, maybe.

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   Yes.

   Ben's free hint of the day: You can simulate open drain/open
collector outputs on I/O pins by setting the output low and changing
the I/O select pin to input for "high/float" and output for "low."

   HTH. HAND. IANAL. TTFN.

-----
http://mindspring.com/~benbradley

Re: Push-pull Output
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One little quibble - "totem-pole" uses two NPN transistors. "Push-pull" uses
a complementary pair of NPN/PNP (or N-channel/P-channel FETs). In the bad
old days of TTL/NMOS, it was difficult to get both flavours of transistor on
the same dye. Less of an issue since CMOS ;).

Steve
http://www.sfdesign.co.uk
http://www.fivetrees.com



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