pull up and pull down resistors

Can anyone help me out in determining the resistance values of pull up and pull down resistors that are being used for input and output pins. What are all the considerations needed to be taken into account?

Friend, that's depend on your logic circuit IO profile (TTL or CMOS), the idea state (High or Low state), selection of resistor for the reuired power dispassion.

hi
10k is popular.
regards

it will largely depend on how much current and what voltage you are working on. But as hr_rezaee said 10k and 4.7k is popular for the purpose.

devil

electtron,
For input pins, the criterion depends on whether the input pin is being driven, or if it is an unused input that must be held at a known state.
.
If the input pin is unused, then the value is determined by noise considerations. To minimize power dissipation, use the highest value you can without getting excessive noise due to capacitive coupling from nearby noise sources.
.
If the input pin is used, and it is driven by an open drain or open collector device, then switching speed is the primary consideration. Determine the line capacitance C of the input path. Then the maximum value of R that you can use is determined by Vt= Vf(1-Exp(-t/RC)). The same considerations apply to output pullups or pulldowns. If you know the maximum allowable time t to reach a given threshold Vt, for a given steady-state (Final) voltage Vf, then R can be determined by solving the above equation for R:
.
R = -t/[C*ln((Vf-Vt)/Vf)]
.
Regards,
Kral

I know some type of MCU such as 89c51 ,or PIC .89c51 has 4 port P0,P1,P2 and P3 .
Port P0 is open colletor device .therefore it needs to have a pull up resistor .I think 10K is popular.

power consumption is an important issue

Friend, as I written in my last lost, one of the parameter to be considered is power rating. I suggest you should refer to the datasheet for the voltage-current rating of the IO pin (sinking, sourcing, high-state, low-state voltage and current rating). From there apply P=VI to determine the require power rating of the resistor value.

A pull-up resistor is used in the design of electronic logic circuits. Pull-up resistors may be found at inputs to logic systems so that a definite logic level is asserted if an external device is disconnected. Pull-up resistors may also be used at the interface between two different types of logic devices, possibly operating at different power supply voltages.
A circuit showing a pull-up resistor (R2) and a pull-down resistor (R1)
A circuit showing a pull-up resistor (R2) and a pull-down resistor (R1)

In bipolar logic families operating at 5 Vdc, a typical pull-up resistor value will be 1000-5000 Ω, based on the requirement to provide the required logic level current over the full operating range of temperature and supply voltage. For CMOS and MOS logic, much higher values of resistor can be used, several thousand to a million ohms, since the required leakage current at a logic input is small.

Pull-up resistors may be used at logic outputs where the logic device cannot source current, such as open-collector TTL logic devices. Such outputs are used for driving external devices, for a wire-OR function in combinatorial logic, or for a simple way of driving a logic bus with multiple devices connected to it. For example, the circuit shown on the right uses 5 V logic level inputs to actuate a relay. If the input is left unconnected, pulldown resistor R1 ensures that the input is pulled down to a logic low. The 7407 TTL device, an open collector buffer, simply outputs whatever it receives as input, but as an open collector device, the output is left effectively unconnected when outputting a "1". Pullup resistor R2 thus pulls the output all the way up to 12 V when the buffer outputs a "1", providing enough voltage to turn the power MOSFET all the way on and actuate the relay.

Pull-up resistors may be discrete devices mounted on the same circuit board as the logic devices. Many microcontrollers intended for embedded control applications have internal, programmable pull-up resistors for logic inputs so that minimal external components are needed.

Some disadvantages of pull-up resistors are the extra power consumed when inputs change state, and the reduced speed of a pull-up compared to an active current source. Certain logic families are susceptible to power supply transients introduced into logic inputs through pull-up resistors, which may force the use of a separate filtered power source for the pull-ups.



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Bauer, a very good and detail explanation to the question posted. May I add some to your answer? One of the major problem with pull up/down resistor with open-collector I/O is high switching noise and long propagation delay.

Hi
Typically in most of systems
PULLUP = around 10K
PULLDOWN = around 1K

bauer said:

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I maybe stupid - but I missing free wheel diode over the relay - or FET can act as diode in this case?

/xxargs

hi
free wheel is not essential.
regards

For pull ups, look at the leakage current of the pin.
For example, the MCLR pin of a pic micro is typically 5uA. Too high a pull up value will result in a voltage drop which might make the micro reset with a little noise on the line. The highest recommended value in this case is 33K.

hr_rezaee said:

hi
free wheel is not essential.
regards

Click to expand...

Why? I can't see any DC path for inductance current from relay in case transitors break up DC-path.

In practical design you need free wheel diode or snubber circurits on all realys or coil or you will crash driver-transitor sooner or later depend of high voltage peak from inductance in all case transistor/switch break up relays current flow very fast.

(older car using this methode to make ignition voltage to spark plugs...)

For trustfully work under long time, you need free wheel diode or snubber circurits over the relay coil and IMHO essential part in design.

The values of pull up and pull down depends on the fan out values.