I pretty much finished my project.
After having replaced a defective crystal the DSO is booting correctly and all voltages are ok.
However now I see that any signal I put at the input of the DSO is overlapped with a strange kind of square wave. I investigated that in more detail using a triangle waveform and found out that
- the displayed signal stayes exactly the same if I shift the vertical position using VPOS on the DSO (except it is shifted vertically of course)
- the edges of the squares stay in the same vertical position on the screen if I change amplitude, frequency and / or DC offset of the input signal
- the period of the squares changes with amplitude as well as with frequency of the input signal
- the period of the squares does not change when changing only the DC offset of the input signal (that indicates that the period is related to the slope of the input signal)
- the squares are clearly synchronized to the displayed signal
The signal at the input of the ADC looks ok.
My only conclusion is that there is something wrong with the ADC (one ore more output bits corrupted) or the digitized signal is not properly processed by the MCU.
Anybody out there having a similar problem?