Re: Linux x uClinux for ColdFires: what is the best for what?

Bob Furber wrote:
> While on this topic, can we talk you into reducing the minimum FlexBus 
> supply to 1.5v or less, to support the newer DDR3 memories and lower 
> voltage memories in the pipeline?

That's not really possible.  The JEDEC standard for DDR3 specifies a 
minimum operating frequency of 300 MHz, which means a data rate of 600 
MHz, and that entails a PLL running at 1.2 GHz.  That's just not 
realistic for the kinds of devices we're doing and for which customers 
are asking.

Besides, at those frequencies we cannot combine the FlexBus and the 
SDRAM bus.  They have to be split, as we've done on the 547x, 548x, and 
5445x, so the FlexBus supply no longer depends on the SDRAM voltage.  
These same devices power the FlexBus from the 3.3V digital I/O rail, 
but, in the future, I'm looking to give the FlexBus its own supply rail 
in order to support things like flashes and FPGAs/CPLDs with 1.8V I/Os.

> And, perhaps even talk you into reducing core voltages?

Core voltage is entirely dependent on process technology, when we change 
technology, we inevitably change core VDD.

> This would address two issues:
>
> 1. Memory availability: Yesterday's 3.3v SDRAM will be increasingly 
> harder to source. DDR will follow. So, for a new product, we really 
> want to design with tomorrow's memories in the hopes that they will be 
> around a little longer. If the product is any good, a life cycle of 10 
> to 15 years is not unreasonable. Try sourcing SDRAM 15 years hence ;o)

Well, the funny thing is that several DRAM vendors we spoke to 5 years 
ago, when we first started planning to support DDR, told us to drop SDR 
support in a hurry, because it wouldn't be around any longer.

Fast forward 5 years, and you can still buy SDR, and, in my experience, 
it's what most customers are still using despite our support for newer 
SDRAM options like mobile SDR (1.8V and 2.5V), DDR1, and LPDDR (mobile 
DDR)...

> 2. As circuits become smaller, power dissipation becomes a significant 
> issue. I believe power consumption is roughly proportional to V 
> squared. So, a small drop in supply voltage means a big drop in power 
> consumption. Smaller circuits mean shorter traces which translate into 
> less problems with high frequency signals (or, they make it possible 
> to run at higher frequency). Also, PCBs are priced by the square inch.

Again, technology mandates core VDD.  Our MPU devices are currently down 
to 1.5V for the core logic, and as we move to 90 nm, that will drop to 
1.2V for the low-power process and 0.9V for high-performance devices.  
That's not backwards either.  Low-power 90 nm processes generally run at 
1.2V in order to reduce leakage, while the higher performance bulk 90 nm 
process is specified at 0.9V in order to maximize frequency at the 
expense of leakage.

> My experience with the MCF5208 was that the 3 power supplies (1.5v 
> core, 1.8v DDR & flash and 3.3v I/O) were an irritant. But the board 
> was a dream in terms of low power consumption and heat dissipation for 
> significant performance. In hindsight, the power supply chips could 
> have been much smaller (less of an irritant).

Yes, but you wouldn't want us to regulator that 1.5V core on-chip, right :-)

Thank you for confirming what I've said about mobile DDR all along.  We 
often hear low-power used incorrectly.  People say low-power when they 
really mean they want low current draw.  But that's not always true.  We 
deal with numerous "industrial" customers who build things in small 
metal boxes with no ambient air flow.  For them, low-power really means 
reduced heat dissipation.  Lower current draw often comes with that, too.

> And, oh, yes, I should mention that FlexBus with DDR is addictive. 
> Once tasted you can never go back. It makes for cleaner, more compact, 
> quieter board layouts.

Thank you for your kind words about what I've always called the 
pseudo-split bus.  You've just confirmed what I've always wanted 
customers to experience when using it.  Clean, compact, quiet, and 
low-power.

BTW, I've seen your MCF5208 board, of course, and it is "elegant" to use 
the word in its engineering form :-)

Best regards,

John Bodnar
Systems Engineer
Microcontroller Solutions Group
Freescale Semiconductor, Inc.

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