So, when I laid out the Wisp1 board, I was confused about whether the STM processor could provide integrated pull-up resistors for the I2C bus, or whether I would need to install external pull-ups. I set the issue aside as I was doing the schematics, and promptly forgot about it entirely. I went on to have the boards built, and oops, forgot about the pull-ups.
Well, not surprisingly, I was having problems with I2C not working. After reviewing the code a dozen times over, I finally realized my mistake. I kludged in a few pull-ups on the board, in a very snug (and not terribly pretty soldering job. I stood a few 4.7k resistors on end at the end of a pair of capacitors that were on a VCC trace. I ran the wires from the top of the resistors over to the I2C pins on the processor. It was tricky, but I got 'em in there!
Having added the pull-ups, I was still having problems talking to the Si5351 chip. It wasn't responding. I looked on the 5351 end of things, and found that he TCXO was making a nice 26Mhz waveform on the TCXO side of the decoupling capacitor, but on the 5351 side, it wasn't making any signal. I fooled with that for a while, thinking maybe the oscillator signal wasn't getting to the 5351 due to a cold solder joint or something. I tried swapping my 0.47uF cap with a 100pF cap. No joy. I swapped that for a 0 ohm resistor, and suddenly there was no oscillation on either side.
Hmm.... was something wrong with the 5351, which was screwing up the Oscillator? I went over to the 5351 and looked at things more closely. All of the solder joints looked fine. No shorts. Getting nervous now, I decided to consult the datasheet. Was I maybe putting the oscillator into the wrong pin?
It didn't take long for me to have an outright panic. All the pinouts were wrong on the board! After another moment I realized the problem, and did a serious face-palm. I had installed the 5351 chip 180 degrees around. The pins were out of order simply because the chip was on backwards. Doh!
With a quick puff of hot air and flux, I flipped the chip around. I fired up my debugger so that I could confirm the oscillator was now oscillating. Lo and behold, before I could even pick up my test probe, I saw diagnostic messages from my I2C code scrolling up the screen. The processor was talking to the chip!
I can now continue on with coding and debugging. I'll have to look at my debugs more closely, and look at the pinouts. I may have blown up the 5351 to some degree, so I'll be skeptical of things until I see it work. It'll be easy enough to swap if I need to.
In the meantime, I've already done a "RevB" spin of the board with the pull-ups in place, and a pull-down added to the 1v8 regulator, which I discovered was missing previously. I guess I just have a mental block about pull-ups and pull-downs! OSH Park has already whisked the boards off to China for manufacture. I didn't move too many parts around, so I'll just use my RevA stencils, and manually paste the (6) 0603 parts that were added/moved. I expect once I finalize the design, I'll be panelizing the boards and ordering them in 0.8mm, at which point I'll order a stainless steel stencil to use on my tooling block.