I2C pull up resistor?

lookind through the tutorials I noticed that when the DS1307 RTC is normally connected to the Arduino no pull up resistors are used on the SDA/SCL lines however the tutorial for the MCP23008 IO expander recomends 1.8k ohm to 4.7k ohm pull up resistors be used.

the Atmega data sheet says “In this mode, there is a spike filter on the pin to suppress
spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver with
slew-rate limitation.” and shows R1 and R2 pullup resistors.

min max
VCC – 0,4V 300-1000ns
Rp value is listed as ----------- --------------
3mA Cb

pretty sure I don’t know what that means! lol! What the hey is that comma doing in there?

I want to use both of these chips on my project. The RTC is close to the Mega but the IO expander is about 5 feet away. What values should I use?


What the hey is that comma doing in there?

It is a decimal point.

Use 4K7 pull ups. There is a built in pull up on the arduino but this is too high to be of any real use.

Thanks Mike!

Not the first time you saved my tail!