at first i would like to thank you Oznog and bmcculla
Oznog you have said
Are you sure you need 5v for the uC? It's common that they're spec'ed to operate down to 3v, although you may need to operate it at a reduced frequency
i am using atmel 89c51 , and i think it's working with 5v
the all story is, i have two circuits connected each other by serial bus ( full duplex)
the first circuit is a module from Motorola company
the data sheet of it says : the serial ports are not 5v logic compliant
for input signals, minimum input high voltage is 2v, and the maximum input high voltage is 3v. minumum input low voltage is 0v, and the maximum input low voltage is 0.8v,,
For output signals, minimum output high voltage is 2.4 and the maximum output low voltage is 0.5v
THE SECOND CIRCUIT IS ATMEL 89C51
so, this is all the story,
i can't risk to connect them now , i must make sure first that they are compatible each other ...
:?: :!: :!: