Hi Matthias,

this is from the Dallas 89C420 data sheet, maybe, the T89C51RD2 works similiar:

AUTO-BAUD RATE DETECTION
The Serial Bootstrap Loader can automatically detect, within certain limits, the external baud rate and
configure itself to that speed. The Loader controls serial port 0 in mode 1 (asynchronous, one start bit,
eight data bits, no parity, one stop bit, full duplex), using timer1 in 8-bit auto-reload mode with the serial
port 0 doubler bit (PCON.7) set. For these settings, an equation to calculate possible serial loader baud
rates is provided below as a function of crystal frequency and timer reload value. The table below shows
baud rates generated using the equation.
) Re 256 ( * 192 _ load Timer
quency CrystalFre BaudRate er SerialLoad  =
** Timer Reload Values attempted by the Loader:
FF, FE, FD, FC, FB, FA, F8, F6, F5, F4, F3, F0, EC, EA, E8,
E6, E0, DD, D8, D4, D0, CC, C0, BA, B0, A8, A0, 98, 80, 60, 40
When communicating with a PC COM port having a standard 8250 / 16450 UART, one should attempt to
match the loader baud rate and PC COM port baud rate within 3% in order to maintain a reliable
communication channel. If baud rates cannot be matched exactly, it is suggested to configure the loader
to the faster baud rate to avoid the possibility of overflowing the DS89C420 serial input buffer.



Peter