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ROM using altsyncram: timing question

Altera_Forum
Honored Contributor II
901 Views

Hi, 

 

Another newbie question. 

 

I have a simple ROM attached to my CPU, courtesy of altsyncram, and initialised using a .HEX file. All works, both in simulator and in the chip. 

 

My VHDL defaults the ROM address to 0 at powerup. But the simulation suggests that it takes a few clock cycles before q is loaded with the contents of address 0. Is this correct, or am I misunderstanding the simulation? 

 

Thanks, 

 

Mark
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4 Replies
Altera_Forum
Honored Contributor II
85 Views

The altsyncram component has input and output registers. This means that when you assert an address, the output data (q) does not update for two clocks. 

 

Cheers, 

Dave
Altera_Forum
Honored Contributor II
85 Views

Dave, Thank you for the fast response.  

 

Whilst I greatly appreciate this forum where should I be looking to find out things like this, so that I can reduce my newbie questions? 

 

Regards, 

 

Mark
Altera_Forum
Honored Contributor II
85 Views

 

--- Quote Start ---  

 

Whilst I greatly appreciate this forum where should I be looking to find out things like this, so that I can reduce my newbie questions? 

 

--- Quote End ---  

 

 

There's not really one definitive source. The MegaWizard usually has descriptions - although somewhat terse. The MegaWizard sometimes links to documentation. The Quartus help also may contain information. 

 

The Modelsim simulator is also useful for answering these types of questions. 

 

Cheers, 

Dave
Altera_Forum
Honored Contributor II
85 Views

 

--- Quote Start ---  

 

Whilst I greatly appreciate this forum where should I be looking to find out things like this, so that I can reduce my newbie questions? 

 

--- Quote End ---  

 

 

Overall, I have found the Altera documentation "pretty good". In general, you can read the manual for the megafunction you are using (in this case, http://www.altera.com/literature/ug/ug_ram_rom.pdf). For something device-specific, you have to go to the device handbook (in this case for a Cyclone IV M9K, http://www.altera.com/literature/hb/cyclone-iv/cyiv-51003.pdf). Unfortunately, I'm not immediately seeing where either of those documents are useful and spell-out your observed behavior. Other than the parameter controlling input/output registering, for example I don't see any waveform that might be obvious.
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