I'd like to inform you (olimex) that you did not include the VCORE buffer cap (470nF recommended) on your header board. You buffered VCC with 5x100nF but the core voltage buffering is simply missing.
Even though I can add that to my PCBs it's simply nonsense due to the long connection lines, i.e. the inductance. You should think about a revision which includes that caps.
Also, AVCC could be buffered via separate caps and not connected to the digital VCC (or at least separate caps plus a ferrite bead).
Something else: The ratio between VCC-caps and VCORE-caps shall be 10, so the system should be modified as follows:
VCC-cap 4.7uF
VCORE-cap 470nF
AVCC should either be connected directly to VCC or via a ferrite-bead with separate blocking caps.
Hey Jürgen,
Thank you for the excellent information and the suggestions and corrections. Some of your ideas have been adopted for the next revision of the H5438.
Best regards,
Lub/OLIMEX