bi-directional bus

[sjweinberg]

Hi everyone. Please try to bear with because I am very new to electronics in general.

I am modifying someone's VHDL project to add a bi-directional line (SDA) for use in an I2C temperature detector. The FPGA is a Spartan-3 on an Opal Kelly XEM3010 board.

The relevant ports are defined as follows:

SDA_P	: inout std_logic; 
SDA_N	: inout std_logic;

signal SDA : std_logic;

obufds_SDA : OBUFDS
   port map (
      I         => SDA,
      O       => SDA_P,        
      OB     => SDA_N         
);

and in the ucf file:

NET "SDA_P"      LOC = "F17"  | IOSTANDARD = LVDS_25;
NET "SDA_N"      LOC = "G18"  | IOSTANDARD = LVDS_25;

With this setup, I can assign values to SDA which seems to properly output values to SDA_P and SDA_N. However, I don't have any idea how to read input from the SDA line. I tried making a signal SDA_IN and adding

ibufds_SDA_IN : IBUFDS
   port map (
      O       => SDA_IN,
      I       => SDA_P,        
      IB      => SDA_N         
);

but that returned an error.

Thanks for reading!