I designed a circuit which have some components with different voltage ratings.
There is an ESP8266 which should connect to 3.3V.
There are more components such as PCF8574 (Multiplexer which is reading active-low buttons), DS1307 (RTC), 24L08 (eeprom), ... which should connect to 5V.
There are some pull-up resistors needed in this circuit. Pullup for I2C bus, PCF8574's interrupt, buttons, reset and program pins of ESP8266, ...
My Question is: To which voltage rail should I connect these pull-ups? For example, PCF8574 is connected to 5V and its I2C bus is connected to ESP8266 which is 3.3V tolerant. How should I determine its rail?
I2C bus is not my problem. I just want to know: Is it important which rail to pull up to?
Another example to clarify: Can I use 5V to pull up ESP8266's reset pin and calculate a suitable value for its resistor?
Is this really important which rail?