"Large" memory array in VHDL

My application needs a somewhat large memory array, for constant (every
clock cycle) sequential read/write access -- the size in question is
exactly 64KB (524288 bits).

Is a memory block of this size reasonable / possible to do in just
VHDL?  My FPGA in question is a Spartan-3E, and we have not yet decided
upon the exact package yet.

If this task is not feasible in VHDL alone, what external RAM device
might be recommended?  Having the VHDL for access to such an external
RAM available for reference would be a big boon here.

A side note on how this memory block will be used.  One "task" will be
iterating over the block repeatedly, with data trickling in replacing
old values -- so each incoming byte stored in the RAM will be accessed
many times before it is overwritten.

Any tips or links to materials regarding this would be greatly
appreciated.

Thank you.

Alex McHale

Alex wrote:
> My application needs a somewhat large memory array, for constant (every
> clock cycle) sequential read/write access -- the size in question is
> exactly 64KB (524288 bits).
>
> Is a memory block of this size reasonable / possible to do in just
> VHDL?  My FPGA in question is a Spartan-3E, and we have not yet decided
> upon the exact package yet.
>
> If this task is not feasible in VHDL alone, what external RAM device
> might be recommended?  Having the VHDL for access to such an external
> RAM available for reference would be a big boon here.
>
> A side note on how this memory block will be used.  One "task" will be
> iterating over the block repeatedly, with data trickling in replacing
> old values -- so each incoming byte stored in the RAM will be accessed
> many times before it is overwritten.
>
> Any tips or links to materials regarding this would be greatly
> appreciated.
>
> Thank you.
>
> Alex McHale

If you want to do it all internally, the only part big enough is the
XC3S1600E. However, you could simply plop on an external SRAM, have
enough RAM left over for growth, and get by with a MUCH smaller FPGA.
Even better would be an appropriately sized external dual-port RAM.
This would allow you to just implement your control logic and data
interface in the FPGA, which (depending on your application) could be a
much smaller device.

FPGA's make for some very expensive RAM. If you need RAM in bulk, it's
generally better to just add RAM unless you have some other constraint
that forbids it.

radarman wrote:
> If you want to do it all internally, the only part big enough is the
> XC3S1600E. However, you could simply plop on an external SRAM, have
> enough RAM left over for growth, and get by with a MUCH smaller FPGA.
> Even better would be an appropriately sized external dual-port RAM.
> This would allow you to just implement your control logic and data
> interface in the FPGA, which (depending on your application) could be a
> much smaller device.
>
> FPGA's make for some very expensive RAM. If you need RAM in bulk, it's
> generally better to just add RAM unless you have some other constraint
> that forbids it.

This is along the lines of what I expected.  Dual port ram is
definitely a possibility, do you have any suggestions as to a part?
Like I said, I'd be looking for a minimum of 64KB.  On one side of the
DP RAM would be the Spartan-3E, and on the other would be a NetBurner
module (a pretty average microcontroller).

Thank you!

Alex McHale

The XC3S1500 has enough (576 K bits) of dual-ported RAM inside the
FPGA. You decide whether that is economical or not. External SRAM need
not be dual-ported, can be single-ported, if you have enough
nanoseconds to time-multiplex. I believe external dual-ported RAM is
exotic and expensive. Internal BlockRAM is dual-ported "for free".
Peter Alfke, Xilinx
============
Alex wrote:
> My application needs a somewhat large memory array, for constant (every
> clock cycle) sequential read/write access -- the size in question is
> exactly 64KB (524288 bits).
>
> Is a memory block of this size reasonable / possible to do in just
> VHDL?  My FPGA in question is a Spartan-3E, and we have not yet decided
> upon the exact package yet.
>
> If this task is not feasible in VHDL alone, what external RAM device
> might be recommended?  Having the VHDL for access to such an external
> RAM available for reference would be a big boon here.
>
> A side note on how this memory block will be used.  One "task" will be
> iterating over the block repeatedly, with data trickling in replacing
> old values -- so each incoming byte stored in the RAM will be accessed
> many times before it is overwritten.
>
> Any tips or links to materials regarding this would be greatly
> appreciated.
> 
> Thank you.
> 
> Alex McHale

> This is along the lines of what I expected.  Dual port ram is
> definitely a possibility, do you have any suggestions as to a part?
> Like I said, I'd be looking for a minimum of 64KB.  On one side of the
> DP RAM would be the Spartan-3E, and on the other would be a NetBurner
> module (a pretty average microcontroller).


Alex, what sort of clock speed is the uC running at?

One solution is to pipeline the uC access to the ram through the FPGA
then alternate access between the uC interface and the FPGA process
that's doing the clever bits.

If the uC clock is slow this becomes much easier to implement.


Nial

http://www.google.com/search?hl=pt-BR&q=xilinx+using+block+ram

It depends on the size of your fpga.

Alex escreveu:

> My application needs a somewhat large memory array, for constant (every
> clock cycle) sequential read/write access -- the size in question is
> exactly 64KB (524288 bits).
>
> Is a memory block of this size reasonable / possible to do in just
> VHDL?  My FPGA in question is a Spartan-3E, and we have not yet decided
> upon the exact package yet.
>
> If this task is not feasible in VHDL alone, what external RAM device
> might be recommended?  Having the VHDL for access to such an external
> RAM available for reference would be a big boon here.
>
> A side note on how this memory block will be used.  One "task" will be
> iterating over the block repeatedly, with data trickling in replacing
> old values -- so each incoming byte stored in the RAM will be accessed
> many times before it is overwritten.
>
> Any tips or links to materials regarding this would be greatly
> appreciated.
> 
> Thank you.
> 
> Alex McHale

radarman wrote:

> 
> If you want to do it all internally, the only part big enough is the
> XC3S1600E. However, you could simply plop on an external SRAM, have
> enough RAM left over for growth, and get by with a MUCH smaller FPGA.
> Even better would be an appropriately sized external dual-port RAM.
> This would allow you to just implement your control logic and data
> interface in the FPGA, which (depending on your application) could be a
> much smaller device.
> 
> FPGA's make for some very expensive RAM. If you need RAM in bulk, it's
> generally better to just add RAM unless you have some other constraint
> that forbids it.
> 

I think you'll find that dual port RAM makes the FPGA look cheap. Dual 
port RAM devices are not generally commodity items, and the ones that 
are out there tend to dissappear into no longer available land pretty 
quickly.  Since you already have the FPGA in the system, use the FPGA as 
an interface to a standard single ported RAM and time-multiplex the 
access.  You'll need 2x the RAM bandwidth, so if you are limited in the 
clock period, double the width of the memory to make up for the slower 
time-multiplexed access.  Use a few of the dual ported BRAMs in the FPGA 
to buffer data into and out of the SRAM and for clock domain crossings.

Thanks to everyone who has replied.

Can anyone point me to a good resource on learning to interface the
FPGA with a RAM chip?  I'm using VHDL for everything in the FPGA.  The
more example VHDL I can see, the better.

This is a project that landed in my lap that I'm having to learn a lot
for as I go.  The more resources I have to learn how to use this FPGA
and VHDL, the better.

Thank you.

Alex McHale

The FPGA and RAM datasheet are very good starting points. The
interface, well, the IO pins are there just for that purpouse.

You may try google, opencores, hamburg hdl archive...

Regards,


Alex escreveu:

> Thanks to everyone who has replied.
>
> Can anyone point me to a good resource on learning to interface the
> FPGA with a RAM chip?  I'm using VHDL for everything in the FPGA.  The
> more example VHDL I can see, the better.
>
> This is a project that landed in my lap that I'm having to learn a lot
> for as I go.  The more resources I have to learn how to use this FPGA
> and VHDL, the better.
> 
> Thank you.
> 
> Alex McHale

"Alex" <alexmchale@gmail.com> wrote:

>Thanks to everyone who has replied.
>
>Can anyone point me to a good resource on learning to interface the
>FPGA with a RAM chip?  I'm using VHDL for everything in the FPGA.  The
>more example VHDL I can see, the better.
>
>This is a project that landed in my lap that I'm having to learn a lot
>for as I go.  The more resources I have to learn how to use this FPGA
>and VHDL, the better.

This may be a good start:
http://minila.sourceforge.net

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