remains a black box sine it has not binding entity

Started by ajellis 3 years ago7 replieslatest reply 3 years ago1169 views


I'm working on a design where I have implemented an I2C module which I'm trying to use as a component in another piece of VHDL.

The problem I've got is that when I try and synthesize my design, I get the following warning about the I2C component, "<i2c_master_src> remains a black-box since it has no binding entity." This is the prevents me from implementing the design. I have searched for some information on this to find a solution to this issue, but without success. Can someone please explain what is causing this warning, as I have managed to use components in other VHDL designs without this issue.

The code for the I2C master is in I2C_master_src.vhd, and the code which instantiates the I2C component is in I2C_Main.vhd. The code for I2C main is only in early stages, I haven't completed it yet because of my current problem.

Thanks in advance




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Reply by adouvilleAugust 28, 2018


Your code is a bit tricky...

You generate a 'data_clk' signal on a first clocked process and this signal is used as a clock later.

Secondly, 'scl_ena' is generated at both rising and falling edges of the clock.

You use a 'BUFFER' type: some synthesizer does not like this (it is preferable using a 'ack_error_int' signal internally and then perform a 'ack_error <= ack_error_int'.

I recommend that we re-read your code and/or code with a hardware spirit in mind. Otherwise, if you are not familiar with RTL, you can search through internet (you can find several I2C modules expecially on


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Reply by ajellisAugust 28, 2018

Hi Arnaud

Thank you for your reply. I'll take a look at my code again, and re consider / rework the use of buffer.

The code I've used was taken from here 


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Reply by adouvilleAugust 29, 2018


Why don't you use the last version proposed by digikey website? v2.2 code is better coded (no gated clock, no both clock edges and Xilinx accepts BUFFER so it should be OK using ISE)...

I start an implementation using Vivado 2016.2. It is OK with v2.2 while it is KO with your version...

Moreover, reading the error message: are you sure you indicate 'I2C_Master_src.vhd' as an input file into ISE... It is like ISE does not catch the file???


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Reply by rajkeerthy18August 27, 2018

I assume i2c_master_src is complete.If the whole module is optimised out by the tool there is a chance of missing entity. If possible could you share the messages spit out by the tool ?

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Reply by ajellisAugust 28, 2018

Hi Rajkeerthy18

Thanks for your answer. The message I get from my tool is pasted at the bottom of this post.

I should have said that I'm using Xilinx ISE 14.7, and the device is a xc6slx45t-3fgg484 on a SP605 board.

Kind regards


ERROR:NgdBuild:604 - logical block 'i2c_port' with type 'I2C_Master_src' could
   not be resolved. A pin name misspelling can cause this, a missing edif or ngc
   file, case mismatch between the block name and the edif or ngc file name, or
   the misspelling of a type name. Symbol 'I2C_Master_src' is not supported in
   target 'spartan6'.

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Reply by rajkeerthy18August 29, 2018

This is the message by builder. You may want to look at Synthesis report and look for warnings. Code seems mixed up with state assignments and output assignments, may have to have separate procedural blocks of each logic. 

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Reply by ajellisAugust 30, 2018


I think that part of my problem was due to test bench file in the simulation tab also accessing the file I2C_master_src.vhd In the implementation view the hierachy was showing a question mark for the I2C_master_src.vhd file, removing the test bench file in the simulation view has fixed the "black box" problem.

The synthesis is able to find the file, but there are other issues with the code which I need to work through.