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

David Brown wrote:
> With higher frequency and higher performance devices, you have to take 
> more care with your memories and buses, and you have more worries 
> about your power supplies and decoupling.  You often need more voltage 
> levels (the v4 has 1.5V for the core, 1.8/2.5V for the memory buses, 
> and 3.3V for IO - the 5234 with which I am most familiar has 1.5V core 
> and everything else at 3.3V).  It is typically easier on slower 
> devices to attach other peripherals to the external buses without glue 
> logic, level converters, etc.

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.

> I haven't used any v4 cores, so my rough comments are only based on my 
> reading of datasheets and application notes.  But our move from the 
> older, slower MC68832 to the MCF5234 required a bit step in board 
> complexity and quality - I would plan for another step up, though 
> perhaps not as large, in doing a v4 board.

I think we've made the MCF5445x family quite friendly for board design, 
especially on the 256-ball MAPBGA variants.  We're trying to take the 
lessons we've learned from doing the package and substrate design for 
those devices and apply them to new higher end parts going forward.

> I you want to tell me that I'm wrong, and that the MCF5445x is as easy 
> to use as the MCF5234 (baring the lack of a TPU), then go ahead - I'll 
> listen!  I like the ColdFire architecture, and if the v4 will give me 
> a lot more processing power for little more complexity, then it's 
> definitely good to know.

I think I mentioned it before, but I am hardly an expert on board 
design, so I could be wrong on all counts :-(  That said, I had plenty 
of help from some real experts on the MCF5445x, so I think we've made 
that family one of our better examples of how to pin out and package a 
device.  Of course, by saying as much, I'm now setting myself up for pot 
shots from the peanut gallery, so let the shooting commence :-)

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

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