A passive component tying a digital input, open-collector output, or communication bus to the positive supply rail (VCC). It ensures the signal remains at a valid logic high when no device is actively pulling it low. They are mandatory on shared buses like I2C where devices only sink current.
The I2C bus is locking up because the internal pull-up resistors on the micro are too weak for this cable length.
A standard pull-up value for general logic is 10k ohms, but fast buses like I2C often require stronger pull-ups like 4.7k or 2.2k to overcome line capacitance.
Also heard as
- pull-up
Do not mix it up with
- Pull-down resistor: A pull-up ties to VCC to default high, while a pull-down ties to ground to default low.
Related terms
Pull-down resistor
A resistor connected between a logic pin and ground to hold the line at a logic low state.
Logic high
The higher of two voltage states used to represent a binary one in a digital circuit.
I²C
A two-wire serial communication protocol used to connect low-speed peripherals to a processor.