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FMC126 I2C question


I'm trying to use the VC707 with an FMC126 and was looking at the I2C control section in "fmc126_if.vhd" and unless I'm mistaken, it does not look like it is properly connected.

Between lines 685 to 703 (see below excerpt), the top-level pins i2c_scl and i2c_sda are connected to IOBUFs but the O inputs of the IOBUFs are not connected to what I would expect.  I would expect the O input on iobuf_scl to be connected to and_reduce(scl_o) and the O input on iobuf_sda to be connected to and_reduce(sda_o).  Instead, they are connected to scl_i and sda_i, respectively, which appear to not be assigned anywhere else.

Am I missing something?  Do I have the most recent version of the code?


(from fmc126_if.vhd)

oe_scl <= and_reduce(scl_oe);

iobuf_scl : iobuf
port map (
  I  => '0',
  O  => scl_i,
  IO => i2c_scl,
  T  => oe_scl

oe_sda <= and_reduce(sda_oe);

iobuf_sda : iobuf
port map (
  I  => '0',
  O  => sda_i,
  IO => i2c_sda,
  T  => oe_sda


It's a tri-state buffer. On the hardware, I2C lines are pulled up, which means I2C lines are high as default and become zero whenever output enable (oe_scl/oe_sda) is activated. "scl_i" and "sda_i" should be connected to "fmc126_i2c_master" and "fmc126_sc18is602b_ctrl" blocks. Are you sure they are not assigned to anywhere?

Thanks for your reply, Kyu.

I think I was misunderstanding how the BUFIO works in the FPGA.  Since there is a pull-up resistor connected to the "IO" port of the IOBUF and the input of the IOBUF ("I" port) is tied low '0', connecting the outgoing scl_en and sda_en to the "T" port of the IOBUF effectively results in the output going low for T = '0', and high for T = '1'.

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