Xscale PXA255 versus PXA250

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Hi.

I was considering either Intel's Xscale PXA255 and Motorola's
Dragonball MX1 for a new design based on Linux/RTAI. As Dragonball's
LCD controller doesn't support double panel STN LCDs I decided to go
with Xscale. Besides, Linux support seems to be more mature for Xscale
than for Dragonball. I have a question though: the aforementioned
support seems to be for PXA250. From data sheets and other sources of
information, I learned PXA255 is mostly compatible with PXA250, with
several bugs fixes and newer silicon technology. So, am I right to
assume that I won't have much trouble running Linux on PXA255?

TIA.

Elder.

Re: Xscale PXA255 versus PXA250
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I can't tell you whether you'll have any trouble with Linux on PXA255, but I
think I'm right in saying that PXA210/250 are discontinued in any case.
PXA255 has all of the fixes that accumulated up to stepping C0 of PXA250,
but there are still 'errata'.



Re: Xscale PXA255 versus PXA250
The Familiar project has ported linux to (at least) an iPAQ 39XX running a
PXA255.  See http://familiar.handhelds.org/familiar ; it's an excellent example
of open-source development at its best.

Also, here is the Intel info page on the differences.  They appear to be binary
compatible.

http://www.intel.com/design/support/faq/PXA_FAQ.htm

Also, I understand the internal bus on the PX255 is twice as fast as the
PXA250.


On Wed, 27 Aug 2003 16:51:08 +0200, Tim Clacy wrote:

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Re: Xscale PXA255 versus PXA250
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Hmm, that sounds good if you read it fast. In actual fact, the PXA255 has
one clock configuration that has a slightly faster internal bus clock than
the original PXA210/250... but the flip side is that only a fraction of the
original PXA210/250 range of clock configurations are left as valid and
working.



Re: Xscale PXA255 versus PXA250

Do not use pxa250! It has LOOOOOTS of hw bugs!
I haven't seen the fixed version (pxa255) but pxa250 on 400MHz is slower
than the 206MHz StrongARM 1110.

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Best Regards,
--
Alexander Popov                  ProSyst Bulgaria Inc.
RTOS Leader                      48 Vladajska Str.
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Re: Xscale PXA255 versus PXA250
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PXA255 is not a fixed version of PXA250; a lot of the errata remain but have
been converted to documentation and/or specification changes. It's true that
some errata are fixed; those that accumulated up to stepping C0 of PXA250.

Despite much PXAxxx-induced hairloss, I have to defend the speed issue.
XScale at 400MHz is almost 2 x faster than StrongARM at 206MHz.... but only
for register operations (internal). The fact that you don't see any benefit
from this speed demonstrates the folly of equating MHz with application
performance under an OS; you can still brew a pot cup of coffee in the time
it takes for Windows XP to get it's act together after power up... despite a
few GHz.

MHz mean nothing unless you have a single-register intensive thread of
execution; An OSs running on a 400MHz CPU will waste four times as much time
context switching than on a 100Hz CPU. Just about every thing that the
microprocessor architects do to speed up execution of instruction sequences
makes context swithing more expensive. The same is true of compiler writers.



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