I2C with two ATTiny 84s and a Mega - can't connect both

Projects Networking, Protocols, and Devices
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Hello,

I'm wondering if anyone has some insight here.

I have an Arduino Mega with an I2C bus. I have two connectors that attach to 2 ATTiny84 devices that connect to SDA/SCL, 5V, GND, and a digital pin so the Mega knows the precise time that they have connected.

I can connect one ATTiny and everything is fine. If I even connect the wires to the connector for the other ATTiny I2C stops - before the Tiny is even attached! I2C continues to be non-functional after the Tiny is attached. I have tried various ATTinies and connectors, I have tested connectivity everywhere, and the problem is definitely with the I2C. This behavior is consistent across several Megas.

The wires for the SDA/SCL are less than 10cm long. I am using pull up resistors on SCL and SDA on the Mega. 2.2k and 4.7k both work well for one device, but no value of pull up resistor seems to change the situation.

Does anyone have any idea what might be up with this? Thanks for any ideas, I'm really stumped!

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The wires for the SDA/SCL are less than 10cm long. I am using pull up resistors on SCL and SDA on the Mega. 2.2k and 4.7k both work well for one device, but no value of pull up resistor seems to change the situation.

The Mega has onboard pull-ups, so usually you don't need external pull-ups and if you do, 2k2 is to small, many I2C chips cannot sink that high currents.

I have an Arduino Mega with an I2C bus. I have two connectors that attach to 2 ATTiny84 devices that connect to SDA/SCL, 5V, GND, and a digital pin so the Mega knows the precise time that they have connected.

Provide schematics for that. My (wild) guess is, that you get a problem with the timing connection as that one is probably the only one that gets activated at the time the Mega freezes.