RE: on-chip memory

[Jeff Fox <Jfox@xxxxxxxxx>]

Dear MISC readers:
Joe wrote:

>> Chuck has said that on chip memory is generally speaking expensive
>
>A conventional SRAM, as for a cache, is 6 transistors per bit. Add about
>20% overhead for addressing and output mux. Such a design is supported by
>all standard IC logic processes. Ignoring array packing efficiencies, a
>1024 bit memory block is then equal to about 7K logic transistors. 

So adding about 100 words of memory would double the size of F21.  That
might not sound expensive to industrial budgets.  To me that sounds
expensive.  It is just a matter of perspective. Now I realize there is
enough white space to fit 100 words on the current die size without
a problem but the suggestion was made for hundreds or thousands of times
that much memory on chip.

If you figure I was paying $10k for 15K transistors then at that rate
2048 bits, 102 cells would cost about $100 per word in low volume
prototype.  You could also think of it as only costing $4 per chip if
all 25 in a prototype batch worked. But $4 to $100 per word sounds
pretty expensive at the prototype level.

>Of course things are different when large amounts of memory are desired
>on-chip. DRAM based implementations are process specific, but currently
>some of the most advanced IC processes provide for logic and DRAM to meet
>system on a chip requirements. Today the cost effect to using embedded DRAM
>is about a 20% premium (bit cost) as compared to purchasing off-chip
>commodity DRAMs. The ability to embed DRAM is relatively standard in 0.25u,
>0.18u, 0.15u ... processes.

Yes.  Well once again I would remind people that Chuck was working with 1.2u
for a few years and then .8u for a few more before Mosis shut down the .8u
fab
line last year.  He has been talking about the jump to .5u or two jumps to
.35u for while.  

.15u process stuff is cool.  It would fun to do multigigahertz stuff.
Perhaps
for iTV real funding may make things appear cheaper to you than they do
at UT.  For me it is still blue sky.

>Hope things are going well for you,
>Joe

Pretty well thanks.  The same to you.  Best wishes,
Jeff Fox