Grounding problem

Hello
I was communicating a 24C64A eeprom(from Atmel) with an MSP430F169 MCU. When I connect my DC power supply (3.3V) with ground connected to the ground, chip selects and write protect pins of the eeprom, the eeprom doesn't work. The I2C doesn't get ready for a read access, but it seems it can write data. However when I connect the reference voltage connector(or shall I say ground) of an oscilloscope everything works fine. I2C gets ready, writes and reads data as expected. But just after I disconnect the reference voltage connector of the oscilloscope, eeprom doesn't respond from I2C.
What might be the problem?
I'll be very appreciated if anyone could help me,
Taskin

hi,

If the MCU and EEP are in sockets, remove the two devices from the sockets.
Disconnect the scope from the pcb.

With a DVM on 'resistance' measure from the 0v pins on the sockets to the power supply ground [0v].

From your description , a ground connection is missing, so when you connect the scope, you are making the ground [0v] line thru the scope and power supply.

If you cannot unplug the MCU and EEP then, disconnect the scope, power up your pcb, measure from the psu 0v to the 0v pins on theMCU and EEP, also all the pins that are supposed to be connected to 0v.
If the DVM shows a voltage, that pin is not grounded.

Using a DVM on volts, I think you will discover one the 0v pins is 'floating'

EricG