PCB Problems - why isn`t all running yet?

A project log for CalcHack

CalcHack is a project to add missing features to graphic calculators. It adds many exciting functions students wanted for a long time.

Sean Dylan Goff 08/19/2014 at 22:020 Comments

Just a quick one:

Many functions aren`t available yet (in fact most of them, the demos that already work are a very, very small part of what`s coming!) and this is why:

A few weeks ago I got the first batch of PCBs for CalcHack. Before that I only did basic testing with half of a link cable connecting a STM32F3, later an STM32F4 discovery board to the calculator. When I den soldered together my first CalcHack PCB it didn`t work at all, in fact it just pulled about 700mA without doing anything. I quickly realized, that there was a short on the output side of the switching regulator. Also, the regulator got quite hot. Because I designed this PCB completely myself, and haven`t done many switching regulator designs before I was worried that my design is bad. 

It took me very long to find the problem, and it was a very, very ugly one: Sometimes EAGLE (the layout editor I use) seems to make overlapping polygons. I haven`t found out why, but normally this isn`t a big problem because it generates DRC errors. But this time I missed this error, because I had known (ignorable) DRC errors nearby. In fact it was really just an 8mm (!) piece of copper connecting GND to 3V3 OUT.

After fixing this by cutting the trace I had another problem: the main CPU got hot. After some searching I found out, that it wasn`t the CPU but the MEMS oscillator a few millimeters besides it. I really just connected GND and VCC backwards...

I then fixed this too (by taking out the oscillator, it isn`t really needed except for precision and overclocking) but somewhere in this process the STM32F4 must have broken. I haven`t gotten the chance to replace it yet, and this is why all demos run on the coprocessor or externally on a discovery board.