What can this rounding of edge be due to ?

Have a look at this application note:

http://www.maxim-ic.com/appnotes....number/551

Here, noise on ALE pulse is discussed a bit.

Is the edge rounding of ALE pulse worse than that of your /WR and /RD pulses? If so, then there's something wrong. Maybe there's too much capacitive loading on port pins.

Dangerous for the DS1232 would be noise spikes superimposed to ALE signal. These could make DS1232 hang-up by lacth-up. That's why I asked, whether there's some ringing on ALE pulse. Signal integrity of ALE pulse is degraded by capacitve loading on address and data buses. And if there's some inductivity on ground connections heavy noise spikes can be superimposed to ALE signal possibly causing latch-up of DS1232.

Kai