I notice that the ram capacity (ignore program flash for now, but it tends to basically be proportionate) of microcontrollers seems to grow fairly continuously (say in 2x jumps) from very small (a dozen or so bytes in an 8 bitter, lots of MSP430's in the 128 to 1k byte range, Cortex M0's with 4k, etc.), up to about 32k (Cortex M4). Above that there are a few chips with 64k or 128k, that are quite expensive, and above that not much is available til you get to external DRAM which on ready-made boards usually starts at 32 meg or even 64 meg (Olimex Olinuxino) or 512 meg (Raspberry Pi). So there is a big jump from 32k to 32 meg. It would be nice to have a low cost, single chip, 256k or 1 megabyte device but this doesn't seem to exist. Is there some technical reason for this, or is it just a market-determined thing? I know that desktop cpu's often have megabytes of sram cache, so it's certainly technologically feasible to do something similar with a smaller cpu. Thanks.

On-chip ran is usually SRAM, and that's usually at least a factor of six time less dense than DRAM, so large on-chip memories usually require fairly large dies. And it's worse in practice since most microcontrollers are not implemented in the latest processes, and DRAM process are highly optimized for density, both of which multiply the overhead. Things like eDRAM are possible, but require considerable extra processing in the fab, so are largely impossible from a cost perspective for low cost devices. Since external DRAMs are (mostly) commodity items, the price pressure on the manufacturers are severe, leading to excellent price per bit. Smaller external DRAMs are certainly possible, but there's not much of a price break below 32MB or so. I suspect we'll see stacked dies before too long, which would provide the large capacity without the hassle of an external DRAM.

large microprocessors?

Feb 03, 2013 41 Replies

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Jukka Marin 13 years ago

It's been postponed, probably till 2014. Sad.

-jm

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Przemek Klosowski 13 years ago

OK, but how broad was your sample? I mean, I wouldn't be surprised if you had access to a wide selection of code samples (automotive, medical, data acquisition, white goods, audio, etc.). Was it as varied as that?

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