If you attempt to interface to other 5-volt I2C devices you need to ensure that none of them have pull-ups that would bring the logic levels up to 5-volts. Because of this, the Arduino logic levels on its I2C bus will be set to the levels of the pull-ups, which in this arrangement are in the Raspberry Pi. You should also note that Arduino has open-collector outputs. As long as you don’t connect any devices that pull the levels up to 5-volts you will be OK. The Raspberry Pi has internal pull-up resistors on the I2C lines, which pul the bus up to 3.3-volts. Any pull-up connected to 5-volts will raise the logic level, possibly destroying the 3.3-volt device(s). It is critical that in this arrangement the pull-ups are connected to the 3.3-volt reference. The most important thing to pay attention to when interfacing 3.3-volt logic to 5-volt logic is the arrangement of the pull-up resistors. However, you need to be careful when doing this. In this configuration, it is possible to connect the Raspberry Pi and Arduino directly together, as the Master is determining the logic levels. The most common scenario for interfacing a Raspberry Pi with an Arduino is to have the Raspberry Pi assume the role of Master. This type of gate is often used to “clean up” a noisy logic line and will not be triggered by a 3.3-volt signal. This is because most 5-volt logic chips have a threshold of a bit less than 3-volts, in other words, a logic signal of 3-volts or more will be recognized as a valid signal.Īn exception to this is a Schmitt Trigger, a logic gate that has very narrow thresholds for zero and one. Interestingly, a 3.3-volt logic signal is capable of working properly when connected to a5-volt input. You may NOT connect a 5-volt output to a 3.3-volt input.You may connect a 3.3-volt output to a 5-volt input.The rule for connecting the two logic families together is pretty simple: The original microprocessors of the 1970s used this type of logic, as did the discrete CPU designs before them.ĭespite its age 5-volt logic is still very common, and it’s no coincidence that the standard USB voltage is 5-volts (although the newer USB-C can make use of multiple voltages).ģ.3-volt logic devices have also been around for many years, they are popular as they consume less current and are therefore ideal for battery-powered devices. Mixing Logic Levelsĥ-volt logic, also sometimes referred to as “TTL logic”, has been around for many decades. Keep that in mind when we start hooking up our Raspberry Pi and Arduino. This concept is very important to understand when interfacing devices with mixed logic levels.Īnother important concept is that it is the Master that determines the logic voltage level. These resistors pull the logic and clock levels up to the level of the VCC reference voltage. The arrangement of master and slaves(s) is illustrated below. In many arrangements, this voltage is also used to power the slave device.
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