Hi Roberto,

1. Listen Roger's advice and try access the I/O's with /RD. Set PDCE1 and PDCE2 only.
Keep PDCE2=0 in order to use MOVX and access enabled to 74HC373 and 74HC244 with /PCE2 and /RD, /WR.
You will feel the need of streching memory cycles in I/O space, when you will increase the clock speed.
Since the stretch cycle affect external MOVX memory operations only, and not MOVC.
It's better to listen to more conventional guru's thoughts. They will not teach us wrong.
2. The external capacitor reset is for other MCS51 systems. Not for 80C390 !.
Please respect the designers effort and leave the internal precision band-gap voltage reference
and comparator circuits to provide a power-on and power-fail reset function. Trust them. You will not be disapointed.
In addition use /RSTOL to inhibit unexpected writes to external RAM. Even if DS1270 is well protected.
3. How do you plan to load the external RAM used for code program ?.
Pulling out from socket since DS1270 is battery back up internal ?

I still didn't understand why you choose a system with large amount of code/data and weak peripherals.
Only for demultiplexed feature ?
Sorry, no offence. But it is more suitable to get used with 805X micro starting with more ordinary system.
Anyway, congratulations for your efforts.

Best regards,
Silvio