Electrical Engineering Stack Exchange
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Questions tagged [open-drain]

Questions about Open-Drain devices and protocols. Open-Drain connections have two states, Open/Floating, or Drain to ground. Typically require Pullup resistors.

Questions about Open-Drain devices and protocols. Open-Drain connections have two states, Open/Floating, or Drain to ground. Typically require Pullup resistors.

Open-Drain devices typically have N-Channel Mosfets, and are common in LED Drivers, and is an essential part of protocols like I2C. The Drain in Open-Drain refers to the Drain of the Mosfet used.

They are similar to Open-Collector Devices, which use NPN Transistors instead of N-Channel Mosfets.

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Open Drain vs Open Collector

Is there a practical difference between Open Drain and Open Collector outputs, or are the terms used interchangably? If they are indeed different, what are the contexts where each is advantageous? My hunch is that they are functionally equivalent,…
vicatcu
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Why are open-drain outputs needed?

I have almost no knowledge in electronics and electricity. I've encountered the concept of open-drain output recently and I think I got the idea. What I have understood is that it is a transistor connected to an output pin, acting as a sink.…
anmomu
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Open-drain microcontoller ports

I want to understand the concept of open-drain ports like port P0 of the 8051 microcontroller. Why do we need to connect pull up resistors to the port P0? I have basic knowledge of MOSFETs and other electronic devices.
abkds
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Why do we need NMOS transistors for NAND gate?

I have a hard time understanding how gates are built from CMOS transistors. For example, I don't understand why do we need the NMOS transistors if the PMOS transistors will already produce the desired outputs? if X or Y on both PMOS transistors is 0…
Nulik
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How can I float a pin that otherwise should be low?

I want to enable/disable a TI TPS54302 buck regulator with a microcontroller. The enable (EN) pin should float to enable the device, or be tied low to disable it. I currently have the EN pin connected to a GPIO pin on the MCU. At startup, before the…
JYelton
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DS1307-breakout-board

I want to build an RTC clock for the Raspberry Pi. Adafruit has a guide to accomplish this with Dallas DS1307. On the schematic they put a 2.2K resistor between VCC and SCL and do the same for VCC and SDA. SQLW is just wired out. But on the IC's…
dash17291
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Why do I2C lines use open drain driver instead of tri-state drivers?

My understanding is that I2C lines use pull-up resistors to passively pull up the bus to logic high because the drivers used on the bus are active drivers, namely open collector/open-drain. Since open collector/open-drain drivers can drive the line…
athedcha
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Connecting Led to push-pull instead of open-drain

I am using the PCA10056 Nordic dev board which integrate a nRF52840. The following LEDs are wired to some of the MCU GPIOs. Currently, my MCU firmware configures those GPIOs as Push-pull output. But I wonder if open-drain mode could be a better…
The_Average_Engineer
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Difference between open drain and push-pull modes

The figures show diagrams of line signals: the first in the open-drain + pull up mode, the second push-pull + pull-down. There are several questions: Why does a push pull output have a faster slope than an open drain output? Why would a sawtooth…
1i10
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Driving a LED with an open drain digital output

I need to drive an LED from an STM32F217ZG. So I was thinking about a classic circuit with a BJT. But then I saw that GPIO outputs of the microcontroller are configurable as open drain. Would this circuit work? The LED has a 20mA forward current…
damien
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Identify this open collector serial protcol

I'm trying to analyze the serial link between two devices, respectively a motor controller board and a Li-Po battery manager, which battery powers both devices. The application is a battery-powered gardening tool. The two devices use the same…
clabacchio
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Power Consumption for Open Drain Communication

When using an open drain communication scheme, like I2C, it seems that the majority of power consumption comes from the sinking of current through the pull up resistor when a logic 0 is transmitted. My question is: is there any power consumption…
Jake Hertz
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When using an open-drain output to drive an NPN transistor, can the base resistor be replaced by a pull-up?

Assuming the value of R1 is the correct value for the base resistor for the given motor L1, is it okay to omit the additional base resistor, used in push-pull applications, for an open-collector output? To my understanding, when the open-collector…
Benedikt M.
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multiple Open Drain UART over long distance

I have 9 microcontrollers ATMEGA3208 and I have connected the RX and TX lines of 8 of them in parallel. My configuration is in master-slave. The master microcontroller send data to one of the 8 RX lines but only one of them can respond at a time.…
MrBit
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Open Collector/Drain vs Push pull

Given the situation where an LED is being used for a status indicator or similar, not toggling quickly. Which is the best configuration for the IO pin, push pull or open drain? As I see it either will work, and I don't see any benefits to one…
Colin
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