I'm bringing up a design based on the i.mx53 QSB schematic but with a changed pcb layout to suit our application. The first example worked very well at maximum speed, with the CPU running at 1GHz and the DDR3 memory at 400MHz (i.e. 800MHz doubled edge clocked.)
Other boards in the same batch suffer from random freezes when running at full CPU clock rate. They all run fine at a CPU clock of 200MHz, and most will run OK at 800MHz. The obvious culprit is a memory problem, but they all pass various memory tests at the full memory speed (400/800MHz) and reduced CPU clock, only failing when the CPU clock is increased. A typical memory error at full clock rate shows the upper word in error, possibly because a previous write cycle failed.
I've tried minor changes in memory timing, and adjusted both CPU and memory voltages up and down a little, but these don't have any noticeable effect. I'm aware that there were some problems with the CPU that require it to operate at a higher voltage, so I tried this too in case it was related, but with no improvement.
The power rails are all stable, with very little noise, and I can't find anything else which looks amiss.
Has anyone else seen this, or a similar, effect which could indicate where to look?