I am at the very beginning in learning about T89C51RD2 processor and the way it is being programmed via UART. After many unsuccessful attempts, I saw that the problem lies in something I would never expect: the receive signal (when I send character "U") is not strong enough (or what ?). This signal comes from a standard HC00 gate, and with AT89S53 it is going literaly from 0 to 5V. With T89C51RD2 high level is at 5V, but "low" at 3.7 V ??? How could that be, it's the matter of pure internal HW, isn't it ? The Baud rate is 9600, the crystal is 11.0592 MHz.

I've found a post (2. Oct 2001) where it's mentioned that T89... requires a lower source resistance for RXD than Philips, but it can't be that much.

Regards, Slobodan