Another hint:

On using the DS89C420 there is a big danger to be too fast for many other peripherals.

So to get a similiar safe timing as the standard 8051 at 12MHz I would suggest to use the DS89C420 on a 1.8432MHz crystal without clock multiplier enabled.

Then if it works you can try to increase the speed.

I have currently detect a similiar problem on using the P89C668 at 20MHz in an older design. and nothing works. After reducing the crystal down to 7,3728MHz the device was working again.


Peter