Claudio,

The 8251 is a very old part (an 8080 periph) and bit "finicky" in it's own way. However, if you have one and this is a hobby project then ...

Connect a clock source to the TXC/RXC inputs that is 1x,16x or 62x your desired bit rate. You must program the appropriate divide value in the mode instruction.

The data sheet is all you need but please note the "internal Reset on Power up" note as the 8251 can reset and think it is in sychronous mode and wants 2 sync chars so most folks write three 00H bytes to the control reg and then proceed to set the asynch mode.

regards,
p