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Re: Linux x uClinux for ColdFires: what is the best for what?
by Bob Furber
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John Bodnar wrote:
> Well, therein lies the rub. As soon as you start supporting DDR of any
> kind, you get away from being able to produce a device with only two
> voltage rails. The MCF523x family only supports SDR, so it never had a
> reason to support other than just I/O and VDD supplies.
>
> The MCF5445x still maintains a 3.3V general-purpose memory bus (FlexBus)
> for connecting up everything else but your SDRAM. In the future, I'm
> looking to move the FlexBus to its own dedicated supply rail in order to
> support flashes and FPGAs/CPLDs with 1.8V I/Os as a means of reducing
> I/O power. This would up the number of supply rails to four (core,
> SDRAM, FlexBus, and digital I/O), but you could still run the FlexBus at
> 3.3V, if desired.
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? And, perhaps even talk you into
reducing core voltages?
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)
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.
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).
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.
Thanks,
Bob Furber
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John Bodnar wrote:
> Well, therein lies the rub. As soon as you start supporting DDR of any
> kind, you get away from being able to produce a device with only two
> voltage rails. The MCF523x family only supports SDR, so it never had a
> reason to support other than just I/O and VDD supplies.
>
> The MCF5445x still maintains a 3.3V general-purpose memory bus (FlexBus)
> for connecting up everything else but your SDRAM. In the future, I'm
> looking to move the FlexBus to its own dedicated supply rail in order to
> support flashes and FPGAs/CPLDs with 1.8V I/Os as a means of reducing
> I/O power. This would up the number of supply rails to four (core,
> SDRAM, FlexBus, and digital I/O), but you could still run the FlexBus at
> 3.3V, if desired.
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? And, perhaps even talk you into
reducing core voltages?
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)
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.
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).
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.
Thanks,
Bob Furber
---
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