rbme,
Let's review...
The SDA line is being held low (0.55v). This is the case even if the 8451 SDA and SCL are not connected to the HMC6343.
That eliminates the 8451 from the suspect list.
With the 8451 SDA and SCL disconnected from the circuit, then we only have the following:
HMC6343 SDA pin is connected to a resistor (in the range of 2.2k to 10k). The other end of that resistor is connected to VDD (3.3v). Nothing else is connected to the SDA pin of the HMC6343, yet it is held low.
This means that the HMC6343 must be the culprit. Now, what might cause the HMC6343 to pull the SDA low?
In my opinion, the only suspect is the VDD to the device.
The VDD supply should be between 2.7v and 3.3v.
When you apply power to the chip, it should be a clean transition from zero to VDD. In other words, if you use a mechanical switch to apply the power, there should be some capacitance near the switch to reduce bounce or noise during the switch transition. Don't apply power just by inserting the wire connection into the screw terminal of the 8451. That will cause so much chatter on the power supply input that anything can happen.
The spec for the Power-on rate is inconsistent. It says minimum rise 'time', but it shows a minimum 'voltage change' per time. My interpretation of the spec would be that your supply connection to the device should go from 0v to 3.3v in less than 66 mSec and it should get there without noise.
Beyond that, I have no explanation as to why the SDA line would be held low.
Good luck,
Tobin
