I have a circuit where a 3.3V MCU which is not 5V compatible needs to drive an MCP23017. The MCP23017 will drive and read inputs and outputs from various 74HCT components. I'm thinking that based on the datasheet for the MCP23017, I can power it at 3.3V so it's communication to the MCP is 3.3V compatible. It would also put out logic HIGH at close to 3.3V but that is sufficient for the HCT parts to recognize as HIGH. I would power the HCT parts at 5V. Their outputs would be 5V for HIGH which, I believe, can still be read by the MCP23017 safely even if only driven at 3.3V. No bi-directional level shifter needed on the I2C lines?
Electronic – Conceptual Question regarding necessity to do level shifting
i2clevellevel-shifting
Related Solutions
The datasheet is very sparse, but all the information you need is plainly on it.
What you see here is a internal description of how it works, and the Mode Select pin. This is a 6 input to 6 output level shifter. It is not bi-directional, and you can only translate from one level to the other, no mixing and matching. The diagram shows that an input goes into the inverter (not gate), powered by VCC. It is then shifted before going into another inverter, powered by VDD, resulting in the output.
From this you can see that Vcc is the Voltage Supply for the Input level, and VDD is the Voltage Supply for the Output level.
The Mode select table tells you how the Mode pin should be handled. The TTL and Cmos does not refer to simply voltage levels, but how to interpret them.
And the Electrical Characteristics tell you the rest.
Here you can see what voltage, comparatively, will be interpreted. Cmos treats a Low and a High roughly equally. A Input Low (0) is treated as a little under half of the Voltage Supply level (Vcc), and a Input High (1) is treated as a little over half of the Vcc (When VCC = 5, anything under 1.5V, then up to 2.25v is logic 0, anything above 3.5V, then until 2.75V is logic 1).
TTL on the other hand, has a smaller, fixed range for logic low. An input (at VCC = 5v) between 1.5 and below is Logic 0, and 1.5 and above is Logic 1.
You need to look at the STM32's datasheet to figure out if it uses TTL or cmos level output.
If you are restricted to DIP packages, go for 74HCT125. The HCT family has TTL input levels (0-0.8 V low, 2-5 V high). No other components are required, other than a decoupling capacitor.
For real PCBs something smaller is suitable, like 74AHCT1G125 which is basically the same in a single-gate package.
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Best Answer
The I2C pins do not need a level shifter, as long as the MCP23017 and the MCU are both run at 3.3V, no level shifting is required since they are both operating at the same levels.
You will need to level-shift the inputs to the MCP23017 that are 5V though, since the 23017 can only handle VDD +0.6V, or a maximum of 3.9V. To do this, you can use a simple resistor-divider. You should size this such that the input is 3.3V, not 3.9V since 3.9V is an "absolute maximum" rating and may damage the device or severely reduce its lifetime.