Honored Contributor II
07-19-2018 02:52 PM
I've written a module to control the operation of an I[SUP]2[/SUP]C bus where the SDA line is a bidirectional port. I've attempted to constrain it with these SDC commands:
set_output_delay -clock i2c_clk -min 0.0 set_output_delay -clock i2c_clk -max 0.0 set_input_delay -clock i2c_clk -max 0.0 set_input_delay -clock i2c_clk -min 0.0where i2c_clk is a generated clock:
create_generated_clock -source -divide_by 126 -name i2c_clkCLOCK_50 is the 50MHz input clock to the design, i2c:i2c_inst|i2c_clk is used because I instantiate the I[SUP]2[/SUP]C module in a top level module. The output delay seems to work fine, but I'm getting issues with the input delay. I'm getting the error messages "FPGA_I2C_SDAT could not be matched with a port" (even though it works fine for output) and "Argument <targets> is an empty collection". Additionally, when I check my unconstrained input ports TimeQuest gives "FPGA_I2C_SDAT" (i.e. exactly as I have it in the constraint). I thought it might be an error caused by specifying output and input delays on the same port, but if I comment out the output delay it still gives the same error for the input delay. I noticed in Ryan Scoville's TimeQuest User Guide on page 114 that internal clocks should not be used to constrain IO ports. However, this clock is exported (or at least a combinational derivative of it that accounts for when SCL should be active and when it shouldn't be active according to the I[SUP]2[/SUP]C standard) and synchronizes the components outside the FPGA. Furthermore, if I replace this with a virtual clock of the same period I get the same error, so it doesn't look like that's the issue. If any part of this question is unclear I'd be happy to clarify/post more code. Thanks in advance.