Electrical Engineering Stack Exchange
Electrical Engineering Stack Exchange

Questions tagged [pullup]

A Pullup Resistor brings a line up to a voltage (Typically VCC), while allowing the line to be pulled low.

A Pullup Resistor brings a line up to a voltage (Typically Vcc), while allowing the line to be pulled low. Encountered in Open-Drain or Open-Collector systems, including I2C.

Threads with useful reading

A question on pull up resistors (switch with a pull-up resistor)
How do I calculate the required value for a pull-up resistor?
Is there a correct resistance value for I2C pull-up resistors?

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How do I calculate the required value for a pull-up resistor?

There are many tutorials that use a pull-up or pull-down resistor in conjunction with a switch to avoid a floating ground, e.g. http://www.arduino.cc/en/Tutorial/button Many of these projects use a 10K resistor, merely remarking that it is a good…
Mark Harrison
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Is there a correct resistance value for I2C pull-up resistors?

The datasheet of the 24LC256 EEPROM states that: The SDA bus requires a pull-up resistor to VCC (typical 10 kΩ for 100 kHz, 2 kΩ for 400 kHz and 1 MHz). I thought that any resistor with a kΩ value would do the job (and it seems that my EEPROM…
JonathanD
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When to use pull-down vs. pull-up resistors

After learning and experimenting with microcontrollers, I've understood the concept of pull-up- and pull-down resistors. I now understand when and how to use them, and how they work. I've mainly used pull-ups because I was taught to, but it has…
osvein
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What happens if I omit the pullup resistors on I2C lines?

Just now I realized that the I2C data and clock lines (SDA and SCL) must have pullup resistors. Well, I've built a couple of clocks using the DS1307 RTC (see datasheet) according to the schematic below. Notice that I have omitted both pullup…
Ricardo
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Heuristic explanation of the usage of a pull-up (or pull-down) resistor needed

First, I acknowledge that there are several questions regarding this topic in the forum, however, the answers assume too much background knowledge of electronics to be of use to a true beginner (like myself). That being said, if you choose to…
Zak
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Why I2C designed to work with pull-up resistors and not pull-down ones?

I understand that in I2C, SCL and SDA lines use pull-up resistors and the pin drivers are open collector NPN devices which can drive pins to ground. This gives I2C an advantage that the same bus now can be shared with multiple slaves, and even if…
Ashutosh
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SPI bus termination and pull-up resistors

My microcontroller (ATMEL series) needs to communicate to a slave device using SPI protocol. I am designing the hardware. The maximum SPI bus speed is 1 MHz. The distance between the master and slave is less than 50mm. Q1) Do I need to add…
Bharav
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Pull-up and Pull-down Resistor Usage on Input or Output MCU Pins

Are pull-up/down resistors (whether internal or external) only needed for MCU INPUT pins? In contrast, an MCU pin configured as an OUTPUT "knows what level it's at" because it does the driving - a "floating" MCU OUTPUT pin tied to some input of…
The_Ders
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Why do some PCB designers put pull-up resistors on pins where there is already an internal pull-up?

I'm going to use a DP83848I Ethernet PHY chip and I have noticed that a lot of people put pull-up resistors on these pins: LED_CFG PWR_DOWN MDC RESET_N Example: Why? According to the datasheet, there are already internal pull-ups on these pins.…
euraad
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Why do we need external pulling resistors when microcontrollers have internal pulling resistors?

Microcontrollers do have internal pull up-pull down resistors yet most of the circuits have external pulling resistors. I looked on Google for answers and a few sites said that those resistors are not that strong but I thought they were good enough…
Gunjan Gangwani
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Considerations when using internal pull-up/down resistors

I am working with STM32 micro-controllers. These micro-controllers have the option to set the input pin as pull up or pull down: Earlier I used to give external pull up or pull down resistor wherever required but now I am wondering whether I can do…
Whiskeyjack
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Recommendation for default settings for unused pins on an STM32 (ARM Cortex M3) - pull up/pull down?

We are currently using various variants of the STM32 microcontroller family. I would like to know the following: What are the recommended default settings for pins of the microcontrollers in general if you have the option to select both pull ups…
IntelliChick
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Why are pull up resistors more common than pull down resistors?

I noticed that pull up resistors are much more common than pull downs, why? For example, the Arduino's MCU has internal pull ups but these tend to invert the physical logic of things you are working with (such as working with switches) whereas a…
Faken
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Placement/location of pull-up/pull-down resistor?

I have connected a microcontroller GPIO pin, which is intended to be an output, to a DC-DC converter's active high Enable input pin. Since this pin is active high, and because I do not want this converter to be ON at power up or before it is…
LoveEnigma
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Why does I²C only have pull-up resistors (interview question)?

The interviewer wanted to know why pull-up resistors are used on SDA and SCL when the opposite logic can also be implemented. Is there an explanation for why pull-up resistor usage is the chosen design?
caveman
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