How flexible is the Block RAM found in Spartan-3 devices?

[davorin]

How flexible is the Block RAM found in Spartan-3 devices?

Can I use all Block RAM for individual dual-port/single-port RAM and/or ROM functions? No hidden limitations?

 

[Git]

Re: Spartan-3 Block RAM

You can use it more or less how you like. ISE will map your usage and it will initialise 'rom' values for download. It's very likely that any coregen or pre-defined xilinx blocks you use will want some bram, but the usuage is well documented. Be aware the sizes xilinx quote include parity bits.

Git

 

[Big Boy]

Re: Spartan-3 Block RAM

I think I saw someone having problem with using block RAM and multipliers. Not sure, but he couldn't use the multiplier and block RAM independently. He could use block RAM alone, and multipliers alone though.

For example, his design had used all multipliers, and also wanted some block RAM (independently of the multiplier), but couldn't. You CAN use block-RAM alone, or multipliers alone, but if you use a multiplier and a block RAM at the same time, independently, that will use 2 block RAM/multiplier resources, which can't be combined using the same block RAM/multiplier pair.

So, for example, if you have 6 block RAM, and 6 multiplier, you could use 4 block RAM, and 2 multipliers... or 3 of each. Unless the output of the block RAM feed directly into the multiplier (as I understood).

This is because they share resources

 

[davorin]

Re: Spartan-3 Block RAM

So actually the 18k blocks can be seen as normal 16kbit blocks for generic RAM/ROM usage?

 

[Big Boy]

Re: Spartan-3 Block RAM

You have those choices:

16384 x 1
8192 x 2
4096 x 4
2048 x 9
1024 x 18
512 x 36

Multiple BRAM can be used in parallel too, to get deeper or wider memory.

 

[davorin]

Re: Spartan-3 Block RAM

Multiple BRAM can be used in parallel too, to get deeper or wider memory.

I'm aware of that..since it's analog to Altera's devices...just looking for a new FPGA with large BRAM architecture which is affordable...need up to 512kBit ROM blocks in cases where external time-multiplex flash access can't be done anymore...like in old Arcade games (o;