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joancab
Advisor
Advisor
348 Views
Registered: ‎05-11-2015

FIFO generator won't fall first word through

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I'm using FIFO generator 13.2 on Vivado 2018.3

My FIFO is Native, Independent clocks distributed RAM, 8 bits wide, 16 words deep.

I expected after a first write when empty that value to propagate to the output but that doesn't happen.

joancab_0-1612994341732.png

I also noticed in the above (sim after synthesis) that empty doesn't go low after the first write.

 

 

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pthakare
Moderator
Moderator
306 Views
Registered: ‎08-08-2017

Hi @joancab 

Here the DOUT is not expected in your case

(The First-Word Fall-Through (FWFT) feature provides the ability to look ahead to the next word available from the FIFO without issuing a read operation. When data is available in the FIFO, the first word falls
through the FIFO and appears automatically on the output data bus DOUT)

To me this seems to be related to testbench or either FWFT is not selected in IP.

Can you please share with us FIFO gen IP configuration file (.XCI) file.?

In addition to this you can check if simulation for example design. For example design creation , After generating a core, right click on the generated core and click Open IP Example
Design

https://www.xilinx.com/support/documentation/ip_documentation/fifo_generator/v13_2/pg057-fifo-generator.pdf

Page 188 onwards.

 

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joancab
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Registered: ‎05-11-2015

This is just a testbench with a FIFO. I write a single value (55) when the FIFO is empty after reset. Two clocks later, empty goes low as expected, but why isn't dout 55?

joancab_0-1613029452918.png

And this is the FIFO configuration, did I misunderstand first-word-fall-through?

joancab_1-1613029543298.png

 

 

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pthakare
Moderator
Moderator
307 Views
Registered: ‎08-08-2017

Hi @joancab 

Here the DOUT is not expected in your case

(The First-Word Fall-Through (FWFT) feature provides the ability to look ahead to the next word available from the FIFO without issuing a read operation. When data is available in the FIFO, the first word falls
through the FIFO and appears automatically on the output data bus DOUT)

To me this seems to be related to testbench or either FWFT is not selected in IP.

Can you please share with us FIFO gen IP configuration file (.XCI) file.?

In addition to this you can check if simulation for example design. For example design creation , After generating a core, right click on the generated core and click Open IP Example
Design

https://www.xilinx.com/support/documentation/ip_documentation/fifo_generator/v13_2/pg057-fifo-generator.pdf

Page 188 onwards.

 

-------------------------------------------------------------------------------------------------------------------------------
Reply if you have any queries, give kudos and accept as solution
-------------------------------------------------------------------------------------------------------------------------------

View solution in original post

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joancab
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Advisor
306 Views
Registered: ‎05-11-2015

The only way I found to propagate an initial value to the output is by writing once and reading once, is that what 'first-word-falls-through' means?

joancab_2-1613030707519.png

 

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pthakare
Moderator
Moderator
300 Views
Registered: ‎08-08-2017

Hi @joancab 

Nope , As mentioned in previous reply 

The First-Word Fall-Through (FWFT) feature provides the ability to look ahead to the next word available from the FIFO without issuing a read operation. When data is available in the FIFO, the first word falls
through the FIFO and appears automatically on the output data bus DOUT)

pthakare_0-1613031162792.png

 

-------------------------------------------------------------------------------------------------------------------------------
Reply if you have any queries, give kudos and accept as solution
-------------------------------------------------------------------------------------------------------------------------------
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joancab
Advisor
Advisor
285 Views
Registered: ‎05-11-2015

Gotcha, FWFT is selectable in the second tab.... I just missed it

joancab_0-1613032455529.png

 

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