Something about using ram in FPGA

[Altera_Forum]

Hi everybaby: 

When I use ram in FPGA greater than %75,Quartus show me that something wrong with it !It means ram is not enough!     :confused: Thanks for somebaby who know how to do it!

[Altera_Forum]

Can you show the complete error messages? It's not always possible to use 100% of the memory due to lack of other resources on the FPGA (routing, as an example).

[Altera_Forum]

It could be that you are using your memories inneficiently. 

 For example - in stratix devices you have M4K blocks. On mode allows them to run with 256 x 16 bit words. If you only need 12 bits per word, thats 25% of the memory wasted that nothing else can now use, unless theres some other word with exactly the same data flow as the 12 bit word. This memory is now dead and is a 25% loss.

[Altera_Forum]

As Daixiwen already mentioned, it is mostly impossible to use 100% of the internal ram. 

As an example if you are using a cyclone II with M4K memory blocks. according to the device datasheet such a M4K has a total of 4608 bit you can implement a 512 x 8 memory out of such a single M4K but 512 x 8 is 4096 bit. in that case you have 4608 - 4096 = 512 bit unused and you can not use them for other puspose as this M4K block is used. so if you do not use all bits from your M4K memory blocks you can't reach 100%. for this example 4096 from 4608 bit means you have used only 87%.. if you want to know how much is realy used, look into the compilation report -> Fitter -> Resource Section -> resource usage summary. their you can see the truth, the compilation report -> flow summary what is displayed directly after the compilation is for sales guys ;-)

[Altera_Forum]

Thanks for your help! 

In my project I use the device ep1c6q240c8 ,the whole project include a sopc_builder system, just a example hello_world! When I set on_chip_memory more than 6k,during compile,error_information will tell me ram is not enough.

[Altera_Forum]

Ep1C6 has 20 M4K Blocks 4608 x 20 = 92160 Bits 

If you want 6Kbyte OnChip Memory you will need 12 M4K Blocks for this. you should check where you instatiate the other memory blocks. Look at the JTAG Uart or your nios settings (cache)