- Wed Sep 28, 2016 5:55 pm
#55841
Resistor selection varies with devices on the bus, but a good rule of thumb is to start with 4.7k and adjust down if necessary. I2C is a fairly robust protocol, and can be used with short runs of wire (2-3m). For long runs, or systems with lots of devices, smaller resistors are better.
Signal Levels
Since the devices on the bus don’t actually drive the signals high, I2C allows for some flexibility in connecting devices with different I/O voltages. In general, in a system where one device is at a higher voltage than another, it may be possible to connect the two devices via I2C without any level shifting circuitry in between them. The trick is to connect the pull-up resistors to the lower of the two voltages. This only works in some cases, where the lower of the two system voltages exceeds the high-level input voltage of the the higher voltage system–for example, a 5V Arduino and a 3.3V accelerometer.
If the voltage difference between the two systems is too great (say, 5V and 2.5V), SparkFun offers a simple I2C level shifter board. Since the board also includes an enable line, it can be used to disable communications to selected devices. This is useful in cases where more than one device with the same address is to be connected to a single master–Wii Nunchucks are a good example.
Source: https://learn.sparkfun.com/tutorials/i2c
Signal Levels
Since the devices on the bus don’t actually drive the signals high, I2C allows for some flexibility in connecting devices with different I/O voltages. In general, in a system where one device is at a higher voltage than another, it may be possible to connect the two devices via I2C without any level shifting circuitry in between them. The trick is to connect the pull-up resistors to the lower of the two voltages. This only works in some cases, where the lower of the two system voltages exceeds the high-level input voltage of the the higher voltage system–for example, a 5V Arduino and a 3.3V accelerometer.
If the voltage difference between the two systems is too great (say, 5V and 2.5V), SparkFun offers a simple I2C level shifter board. Since the board also includes an enable line, it can be used to disable communications to selected devices. This is useful in cases where more than one device with the same address is to be connected to a single master–Wii Nunchucks are a good example.
Source: https://learn.sparkfun.com/tutorials/i2c
