Push-pull Output

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?

Reply to
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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Reply to
Spehro Pefhany

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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Reply to
Rene Tschaggelar

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

Reply to
Mike Turco

I don't like any of the other three answers I just read, so I'm writing this...

On Mon, 24 May 2004 03:12:53 +0800, "bear12" wrote:

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,

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

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.

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Reply to
Ben Bradley

On Sun, 23 May 2004 12:50:40 -0700, "Mike Turco" wrote in comp.arch.embedded:

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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Reply to
Jack Klein

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

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Steve at fivetrees

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