UART interface FPGA - uC

[Enrique Perez (Gast)]

Hi guys!

I am trying to interface a FPGA and PIC18FL4520 through an UART. Both 
chips has the same power supply: 3v3 and there is a direct connection 
between the FPGA and uc using 2 pcb tracks.

Is it necessary to place a level translataor or some kind of pull up 
resistor in these lines (TX and RX)? As I told previously, I don't have 
anything and by default the uC and the FPGA is driving to high level 
both lines. Then when the communication starts, the pulses are 
generated, but at the endof the comunication, one of the line (RX_in 
input to the FPGA) is driving to low level by the FPGA while th uC is 
driving to high level. It is like the FPGA pin is hang up...But the it 
is srtange because this interface has been used in other boards with the 
same FPGA and uC and exactly the same pin connections (2 direct tracks 
btwen chips) and it was working always...

Any suggestion?

Thanks.

[N. B. (saint1234)]

Hello,

most FPGAs have the option of internal pullups. Use them or insert some 
actual hardware pullups in your design on both lines.

Did you cross the RX and TX lines on the FPGA? Microcontroller RX goes 
to FPGA TX and Microcontroller TX goes to FPGA RX.

greetings,