Pull Up And Pull Down Resistors  Microcontroller Family Models :Atmel, NXP, Philips, 8051, 8052, 89C51, 89C52, 89S51, 89s52, 89C1051, 89C1051, 89C2051, AT89C4051, AT89S8252,l AT89C1051, AT89C2051, AT89C4051,P89C51RB+, P89C51RC+, P89C51RD+, P89C51RB2Hxx, P89C51RC2Hxx, P89C51RD2Hxx, P89C660, P89C662, P89C664, P89C668, P89C669, P89C51RA2xx, P89C51RB2xx, P89C51RC2xx, P89C51RD2xx, P89C60X2, P89C61X2,P89LV51RB2, P89LV51RC2, P89LV51RD2, P89V51RB2, P89V51RC2, P89V51RD2, P89V660, P89V662, P89V664.

Introduction Pull Up Resistors 89C51, 89C52 MicrocontrollerA pull up resistors circuit refers to the one of the common method of interfacing a switch to the input and output pins of 89C51, 89C52 microcontrollers. Many of the 89S51,89S52 microcontrollers needs the logic voltage be more than some threshold or below some threshold point for 8051 Microcontroller or its variant. If the microcontroller circuit is disconnected the input logic voltage will be allow to float and we may guess what type of the value of the input pin of microcontroller will have in the device.

When a switch is connected with AT89C51 or AT89C51 Microcontroller or its variant, it will make the input of the microcontroller to float. Depending on the Atmel microcontroller it will lead to unexplained operation of the input pin of microcontroller and oscillator pulses between various states or high power consumption circuit and even the physical damage of the microcontroller.

Port 0 Pull Up Resistor 8051 MicrocontrollersPort 0 of 8051 microcontroller has 8 pins, it is a 8 bit port and can be used for dual role for input and output. To use the pins of 89C51 or 89C51 Microcontroller, we have to connect 10 k ohm resistor as a pull up resistor externally. It is a Open Drain means having the same way as used in open collector.

When a switch of the microcontroller is in the open voltage on the input pin of 8051 microcontroller pin it will be much closer to VCC. The input pin has usually very high impedance, it means there is a small voltage runs through the circuit as a result it will have little voltage drop level across the resistor. If the switch is closed, the entire current will run from the resistor and the from switch to ground voltage. Now the drop of the voltage will be from resistor to VCC, so the input pin of microcontroller will detect the voltage, which is very near to the ground voltage. Internal pull up resistors of 89C51or 89C52 microcontroller are not available in one port, other ports have internal pull up and pull down resistors and can be enabled with the software.

Pull Up Resistor Or Pull Down Resistor Example

Let's take an example 89C51 Microcontroller with one of the pins is set as an input pin. If anything is not connected with the the same pin, it will be considered as a floating pin. In this situation 8051 MCU will have some difficulty to read the state or logic voltage on the input pin of microcontroller. Pull up or pull down resistor 89C51 or 89C52 microcontroller will hold the pin may be either a high (1) or low (0) voltage state when using the low amount of

the current.

Pull resistors are common and most used logic levels in two wire communication (I2C) lines. An application of a pull up or pull down resistor is connecting a button to the input pin of 8051 microcontroller. With the help of pull up resistor 89S51, 89S52 microcontroller the input pin of the microcontroller will read a high voltage when the button is open on the circuit. When the button is in closed state, it connects the input pin of 89C52 microcontroller to its ground so to read the low state of the signal.

What Is Pull Up Pull Down Resistor ValueValue of pull up resistor is 10 k Ohm for AT8051 AT89S51 AT89S52 AT89C51 At89C52 microcontrollers. In almost every circuit pull up resistor are used with the same type of value. 

Pull up resistor Hardware Connection with 8051 Microcontrollers