SDRAM+SRAM board (only concept for now)

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Sorgelig
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SDRAM+SRAM board (only concept for now)

Postby Sorgelig » Sat Jan 06, 2018 3:10 pm

Because of talks here and there about SRAM module, i've decided to try to combine both SDRAM and SRAM.

Good news - it works!
Not so good news - max speed i can achieve is 65MHz instead of theoretical 100MHz. 65MHz is still not bad for pure random access. SDRAM allows only 1/8 speed of its clock, so with 133MHz of clock only 16MHz effective speed can be achieved.

Currently it's just concept. It's unclear what core can get advantage from this SRAM. May be super-turbo mode for Minimig when(if) it will get hybrid emulation. 2MB of SRAM can perfectly emulate ChipRAM.

Tested with 8bit 2MB SRAM. SDRAM is the same, but max speed has been reduced to 140MHz.

Another cons is impossibility to make it in extra slim format since each chip occupies its own side. It uses classic SDRAM module size increasing either vertical or horizontal dimension.

A good usage i can see from such module is porting the cores requiring SRAM originally. So can port it in stages while using SRAM at beginning.

Sorgelig
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Re: SDRAM+SRAM board (only concept for now)

Postby Sorgelig » Sat Jan 06, 2018 3:14 pm

Chips cannot work at the same time. CKE/CS signal chooses the chip. Theoretically it's possible to use both by switching between them, and even retain the content of SDRAM. But performance will be very bad. It's better to use DDR3 as an additional memory than SDRAM.

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Re: SDRAM+SRAM board (only concept for now)

Postby ijor » Sat Jan 06, 2018 5:12 pm

Sorgelig wrote:Because of talks here and there about SRAM module, i've decided to try to combine both SDRAM and SRAM.
Good news - it works!
Not so good news - max speed i can achieve is 65MHz instead of theoretical 100MHz. 65MHz is still not bad for pure random access. SDRAM allows only 1/8 speed of its clock, so with 133MHz of clock only 16MHz effective speed can be achieved.


SRAM addition is nice :)

If you want to achieve the highest speed, SSRAM is IMHO much better than old async SRAM. But you will still get, at least, one cycle lattency. This FPGA is too slow for reading on the very same cycle at very high frequencies.

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Re: SDRAM+SRAM board (only concept for now)

Postby ex68k » Sat Jan 06, 2018 6:44 pm

ijor wrote:If you want to achieve the highest speed, SSRAM is IMHO much better than old async SRAM. But you will still get, at least, one cycle lattency. This FPGA is too slow for reading on the very same cycle at very high frequencies.


But it is one cycle latency on a memory controller, which runs 150-200 MHz. Your CPU runs much slower, so it should notice that one cycle ...

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Re: SDRAM+SRAM board (only concept for now)

Postby Sorgelig » Sat Jan 06, 2018 11:43 pm

I also know many smart words :)
The difference between knowing some terms, and making a real hardware is huge. Good luck to find SSRAM in TSOP or similar soldering iron friendly package and make it run at 200MHz on MiSTer module ;)
I even don't say about price.

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Re: SDRAM+SRAM board (only concept for now)

Postby alfishe » Sun Jan 07, 2018 7:21 am

Sorgelig wrote:I also know many smart words :)
The difference between knowing some terms, and making a real hardware is huge. Good luck to find SSRAM in TSOP or similar soldering iron friendly package and make it run at 200MHz on MiSTer module ;)
I even don't say about price.


Well... technically... 167MHz and TQFP chips exist. But... 18 bits data bus and large amount of other signals... http://www.cypress.com/file/226451/download

TQFP100 is hand-solderable... well... with microscope inspection =)
$30 USD per chip - still acceptable. https://www.digikey.com/product-detail/ ... ND/5982755

3 clock cycles for read latency, 1 for write

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Re: SDRAM+SRAM board (only concept for now)

Postby ijor » Sun Jan 07, 2018 1:23 pm

Sorgelig wrote:Good luck to find SSRAM in TSOP or similar soldering iron friendly package and make it run at 200MHz on MiSTer module ;)


200 MHz perhaps not, but technically there is no reason why it shouldn't run, at least, as fast as SDRAM.

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Re: SDRAM+SRAM board (only concept for now)

Postby ex68k » Sun Jan 07, 2018 4:14 pm

alfishe wrote:
Sorgelig wrote:I also know many smart words :)
The difference between knowing some terms, and making a real hardware is huge. Good luck to find SSRAM in TSOP or similar soldering iron friendly package and make it run at 200MHz on MiSTer module ;)
I even don't say about price.

Well... technically... 167MHz and TQFP chips exist. But... 18 bits data bus and large amount of other signals... http://www.cypress.com/file/226451/download
TQFP100 is hand-solderable... well... with microscope inspection =)
$30 USD per chip - still acceptable. https://www.digikey.com/product-detail/ ... ND/5982755
3 clock cycles for read latency, 1 for write

I used the cy7c1460/cy7c1470 before, and liked them a lot. But, not cheap :(

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Re: SDRAM+SRAM board (only concept for now)

Postby Sorgelig » Sun Jan 07, 2018 5:13 pm

65MHz*3 = 195MHz.. Haha, better than Async SRAM )))))

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Re: SDRAM+SRAM board (only concept for now)

Postby alfishe » Sun Jan 07, 2018 6:09 pm

Sorgelig wrote:65MHz*3 = 195MHz.. Haha, better than Async SRAM )))))


Interesting thing - 167, 200 and even 250MHz chips cost same price (near $30).
https://www.digikey.com/products/en?key ... 7C1380KV33

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Re: SDRAM+SRAM board (only concept for now)

Postby Sorgelig » Sun Jan 07, 2018 6:25 pm

There is no point to discuss about 100-pin chip. There are no such amount GPIO pins available.
Also, 300MHz is a marketing poo. Divide it to 3 latency cycles and you will get the same 100MHz as cheaper Async SRAM with 10ns.
Probably SSRAM is a little faster if you will get ZBT version, but it's definitely not deal breaker.

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Re: SDRAM+SRAM board (only concept for now)

Postby ijor » Sun Jan 07, 2018 7:37 pm

Sorgelig wrote:65MHz*3 = 195MHz.. Haha, better than Async SRAM )))))


It's 3 cycles only for the standard pipelined version, and only when switching bus direction. The flow through version has two cycles latency. And the NoBT/ZBT parts work at two cycles pipelined or single cycle flow through same as Async.

Flow through doesn't go at frequencies as high as pipelined though. But you can get 133 MHz for fully random access.

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Re: SDRAM+SRAM board (only concept for now)

Postby Sorgelig » Wed Jan 10, 2018 5:24 am

How the module looks from the back:
20180110_131931.jpg
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