I stumbled, and made progress on my Simblee breakout shield. If you didn’t see the first installment, check it out first.
Since the first post, I built up a non-shorted board. It seemed mostly okay, except that it gave me 3.9 volts on the 3 volt side, and the same on the 5 volt side. The answer to the puzzle lead me to one of the downfalls of open source hardware (well, it’s only a downfall in combination with poor practices). That is, not knowing what the original designer had in mind.
I took a look at the Arduino 101, which uses a 3 volt Intel Curie module. The Arduino 101 uses a Texas Instruments LSF0108PWR line level converter chip. Works for them, so I designed it in. Folly me, I didn’t read the data sheet close enough. It’s an open drain device and needs pull-up resistors on the I/O lines.
The 101 didn’t use pull-up resistors. It gets by because most 5 volt devices are fine with the 3 volt “1”. They’re really just using the chip as a5 volt to 3 volt done converter and not worrying about going up to 5 volt line levels.
Why not push ups?