What is Pull up/down?

Thread Starter

System/360

Joined Feb 1, 2020
2
What does pull up mean? In circuit examples, for example for a ESP8266 chip, pull up is connecting to Vcc through a 10kΩ resistor. Why 10k? How is this value derived?

I have seen two separate cases of pull down. One connects to a 10kΩ to ground while another connects directly to ground.

Hope someone can edify me.

Thank you.
 

ericgibbs

Joined Jan 29, 2010
9,582
hi 360,
I guess you know you should not leave a CMOS, high impedance input disconnected ie; 'floating'
If open it can assume any value, so a MCU program when reading that input pin would be an indeterminate state.
So a PU or PD resistor is added to the Input pin, this sets the non active/rest state of the pin.
The PU resistor can be from 1k thru 47k, it depends upon the application.

If a Input is not being used on a device, its either tied directly High or Low depending what state the device needs to 'see' in order to function.

Note: many devices including MCU have internal pull ups approx 50k , they can be enabled by the program.
E

Footnote:
There are instances where PU's are added to Output pins in order to create faster high rising pulse edges.
eg: I2C connections
Typical PU are 4k7 for 400kHz and 2k2 for higher clock rates.
 
Last edited:

Delta prime

Joined Nov 15, 2019
171
notwithstanding mister Ericgibbs superb explanation ;) there are some disadvantages of pull-up resistors are the extra power consumed when current is drawn through the resistor and the reduced speed of a pull-up compared to an active current source. Certain logic families are susceptible to power supplie transients introduced into logic inputs through pull-up resistors, which may force the use of a separate filtered power source for the pull-ups.
 

MrChips

Joined Oct 2, 2009
20,372
In order to drive an input pin high you need to supply current to the input pin.
Read the datasheet of the device to find the required voltage and current for that pin.

For example, if the pin needs >2.5V @ 10μA from a 3V supply, the max pull-up value is
R =( Vcc - Vih ) / Iih = (3V - 2.5V) / 10μA = 50kΩ
Use a value less than thiat such as 10kΩ

You can do the same analysis for a pull-down resistor.
You need to pull current (sink current) from the input pin for low input.
If the pin needs <0.25V @ 10μA the max pull-down value is
R = 0.25V / 10μA = 25kΩ
Again a 10kΩ pull-down resistor is suitable.

Of course the above is for unused pins only or if the device does not already have internal p/u or p/d.
Some ICs will have "weak" internal p/u p/d resistors of about 30kΩ.

If the circuit driving the input pin can meet the above requirements then no additional p/u or p/d resistors are required.

Outputs using open collector, open emitter, open source, or open drain circuits will also need suitable p/u or p/d resistors.
 

MrSoftware

Joined Oct 29, 2013
1,678
Pull up and pull down are exactly what they sound like, pulling the voltage up or down. Small circuits typically run on 2.8, 3.3, or 5v and 10k ohm just happens to be a value that works well for many cases. Pulling up or down with just a wire, not a resistor, is actually a short or shunt, I don't think the term pull is accurate in that case.

You may see the terms strong or weak pull up or down, this refers to how much current is available, and it can be relative the application. A smaller resistor value (less ohms) allows more current and therefore is a stronger pull, while a larger resistor allows less current and is a weaker pull. For example I was working with an SPI circuit the other day that required pull-ups. The resistor keeps the circuit voltage high, the chip pulls-down to transmit data. When the chip releases, the voltage rises back up due to the pull from the resistor. With this specific chip, for slow clock speeds a relatively weak pull-up of 4.7k worked fine, but for higher clock speeds a stronger pull-up of 2k was required. Why? Because circuits have capacitance. So pulling the voltage up requires current to fill that capacitance and allow the voltage to rise. The faster the clock, the less time there is for the voltage to change before the next tick, so the more current you need to fill the capacitance and pull the voltage up in the available time. This is just one example, but hopefully you get the general idea. When in doubt, look at the data sheets for the parts that you are using. Datasheets are like the bible for each part.

If you're using a pull-up for more general stuff, such as a button on your own project, then values like 10k are common because they work, but many other values would work too. It just happens that 10k will flow enough current to pull up or down in a reasonable time, but doesn't flow so much current that it will kill your batteries, etc..
 

BobaMosfet

Joined Jul 1, 2009
1,012
What does pull up mean? In circuit examples, for example for a ESP8266 chip, pull up is connecting to Vcc through a 10kΩ resistor. Why 10k? How is this value derived?

I have seen two separate cases of pull down. One connects to a 10kΩ to ground while another connects directly to ground.

Hope someone can edify me.

Thank you.
Pull-ups and pull-downs, and the ever present question - how to choose the size of a resistor. Thank you for wondering why, this is the beginning of your truly wanting to understand.

The resistor size is easy- You size a resistor according to how much current you want a conductive pathway to have. That quantity is based on Ohm's law.

5V circuit, with a 10K Ohm resistor, means I = E/R which means I = 5 / 10K which means current is at 500uA (0.5mA). 5 ten-thousandths of an Amp. So it's not much, and is easily overcome if needbe. Pull-ups/pull-downs are only there to 'influence' the main pathway in absense of a overriding signal.

You tie the pull-up to Vcc, because you want the conductor influenced towards a positive voltage. You tie the pull-up to Ground, because you want the conductor influenced towards a ground (zero) voltage.

Believe it or not, all of electronics is based on this fundamental interaction. KVL, KCL, and Thevenin explain this phenomenon in various ways.
 

dl324

Joined Mar 30, 2015
10,072
I have seen two separate cases of pull down. One connects to a 10kΩ to ground while another connects directly to ground.
In the first case, you have the option of setting that input to a logic HIGH; in the latter, you don't.

The resistor value depends on the current for the input, how many inputs are connected, and the driver capability. A 10k resistor is commonly used for CMOS, regardless of the supply voltage. Smaller values are required for TTL. Whether the input sinks or sources current also determines whether a pull down resistor should even be considered.
 
Top