How do I probe the XPS_IIC tri-state signals with ChipScope?
Why is the _O signal tied to GND when I probe the tri-state signals inside the XPS IIC Core?
The IIC signals SDA and SCL are connected to an IOBUF. The input to the IOBUF is always tied to ground, so the signal on the I/O line is effectively controlled by the _T signal.
Following is the logic table for a Virtex-5 FPGA IOBUF:
Inputs Bidirectional Outputs
T I IO O
1 X Z X
0 1 1 1
0 0 0 0
When I is '0' and T is '0', the output is '0'.
When T is '1', outputis high 'Z'. But, since IIC signals are both pull-up, it is pulled to '1' on the line.