Why does I2C use pull-up resistors?
A pull up resistor is used to provide a default state for a signal line or general purpose input/ouput (GPIO) pin. Typically they are of high resistance thousands or tens of thousands of ohms of resistance.
What pull-up resistor do I need?
Pull-up resistors are used to solve the dilemma for the microcontroller by pulling the value to a logical high state, as seen in the follow figure. Without the pull-up resistor, the MCU’s input would be floating when the switch is open and pulled down to a logical low only when the switch is closed.
What is the difference between a pull-up and pull-down resistor?
A pull-up resistor connects unused input pins (AND and NAND gates) to the dc supply voltage, (Vcc) to keep the given input HIGH. A pull-down resistor connects unused input pins (OR and NOR gates) to ground, (0V) to keep the given input LOW.
Which is the lowest possible pull up resistor for I2C?
I OL1 = 3mA. 0.4V. So, in this case the calculation is (5.0V – 0.4V) / .003A = 1533Ω. That represents the lowest possible pullup resistor value, commonly designated R P you can use, because anything less would burn out your I2C pins.
What do you need to know about pull up resistors?
Understand what is meant by a pull up resistor. Understand what pull up resistors do in an I2C circuit. Be able to calculate the lowest R P using V OL and I OL. Understand the concept of bus rise time. Understand what is meant by bus capacitance and how pull up resistors interact with it.
Why does the PCB capacitance affect the I2C bus?
So if your I2C bus, with all the PCB capacitance and pin capacitance added together, winds up with a capacitance large enough, it will work to prevent the rapid changing of a voltage level on that bus, and suddenly you can’t make those quick snaps from logic low to logic high as cleanly as necessary. And how does this funky capacitor get charged?
How are pull up resistors used in VCC?
From Vcc through your pull up resistors! Your circuit and your pull up resistor form a big RC network (resistor / capacitor not remote control) that governs the charge and discharge rate of your capacitor. Now, mind you, this is a teeny, teeny capacitor.