Does anyone still make VRAM?

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I would like to use VRAM to interface to a 640x460 monochrome LCD display
without using an LCD controller. The trouble is, I can't seem to find any
VRAM manufacturer left. Not even NEC. Does someone know of a manufacturer? I
am looking for the type of memory which has a DRAM port on one side and a
serial data port on the other, generally x4 or x8 or maybe x16. Density need
not be very much. WRAM might be OK too.

Thanks



Re: Does anyone still make VRAM?

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I assume you have some form of timing circuit (or FPGA/PLD) that is going to
drive the timing and signals to the LCD ?

If so you already have half of the circuitry to perform dual porting of the
memory and any x4, x8, x16 memory could be interfaced and the suitable
size shift register added to the FPGA/PLD to give the output. The output
timing of the LCD is going to be faster than a bus access which if necessary
can be held off with wait states if an interleave is due in less then the
bus cycle time (or time it takes for PLD dual porting to access the RAM).

Just ensure you use reasonably fast cycle time RAM (DRAM/SRAM).

The size of memory is only 640 x 480 BITS (307200bits), so any RAM device
that is 32KBytes or larger will do the job if the access time is fast
enough.

Most 640 x 480 LCDs use a pixel/dot clock of around 25.18MHz (typical),
which is approxiamately 40ns per pixel, on an 8 bit shift register that
means 320ns to shift 8 pixels out, so you need to interleave memory so that
at least one shift register load cycle and one host cycle can be fitted in.
Considering that it easy to get 128KB (and larger) SRAMs with 12ns or faster
access times I don't see much problem in a 50ns window to load shift
register and remainder of time for host access. With 128KB RAM you can
do page switching.

I have even used 128KB RAM and PLD to do a crude frame grabber and the same
RAM to read out at dot rates for video work.

You would not want to contemplate some early (pre PLD) video memory banks
I designed with 96 x 64K x 1 DRAMs, that could be accessed as colour 3bit,
bank by 8bit or 16bit and 96bit shift register load. The absolute worst
case power spike was estimated at 10A during a shift register load!

--
Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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Re: Does anyone still make VRAM?
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manufacturer? I
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need
to
the
necessary

For a prototype or one-off system, you can do a lot worse than scrounging
the cache memory from an old '386 or '486 -- typically 10-25 ns access time!
I've gotten them as dense as 256K bits with 1x, 8, and 9x organizations.

    Norm


Re: Does anyone still make VRAM?
On Saturday, in article

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....

Given the cost of even one offs for 128KB SRAM 12ns from places like
Farnell in the UK is around 3 UKP, why bother, if he is really interested
I have some 5V 128KB SRAMs in a tube that cost a bit less than that.

Only bother doing pulls, if this is a one off hobby item, and your time
does not matter and you have more available to pull than you need.

--
Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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Re: Does anyone still make VRAM?
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display
any
and a
Density
going
device
time!
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I've told everyone at work not to scrap any '486 computers without letting
me pull the components I find useful.  I don't even have to go looking for
them, they come to me about once a month.  Just about every hobbyist has a
'386 '486 computer or motherboard laying around gathering dust.

    Norm


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