Re: New ARM Cortex Microcontroller Product Family from STMicroelectronics


"rickman" <gnuarm@xxxxxxxxx> wrote in message news:1182540547.184518.91830@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Jun 22, 2:34 pm, Eric <englere_...@xxxxxxxxx> wrote:
On Jun 22, 12:40 am, "Bill Giovino" <conta...@xxxxxxxxxxxxxxxxxxx>
wrote:

However, if you are running out of Flash with the CPU at a higher speed than the Flash,
and so the Flash requires wait states while taking advantage of the Harvard
architecture

Any idea if the ST Cortex M3 can run without wait states from flash at
their rated speed? That would be quite impressive.

The data *** says it requires one wait state from 24 to 48 MHz and 2
wait states above 48 MHz. So compared to the Luminary parts running
at 50 MHz with *NO* wait states, I say the ST M3 parts are dogs.

It's not that bad. Cortex-M3 has a prefetch buffer and branch prediction. This
means that the cost of a single waitstate can be hidden for conditional branches,
ie. only indirect branches have a penalty. With 2 wait states the branch prediction
only works on unconditional branches, so you'll get a slowdown. However you can
change loops to use an unconditional branch at the end so they run at the speed
of zero-wait state memory.

The power consumption is not great either, at least not compared to
parts like Atmel SAM7. The advertisement says it gets "0.5 mA/MHz in
RUN mode from Flash", but this is not very accurate. The power curve
does not have a 0.5 mA/MHz slope. The STM32F103 data *** shows
higher current per MHz at low clock speeds with a Y intercept of about
9 mA. I think the lower mA/MHz at higher clock speeds reflects the
lower MIPS available due to the required wait states.

It is the flash power consumption. When you add wait states the power
consumption flash drops to 50% (1 wait state) or 33% (2 wait states). Ie.
the flash has identical power consumption at 24, 48 and 72MHz.

Of course the secondary effect of adding wait states is the core slows down
and so uses less power. Based on their numbers I estimate the slowdown is
between 10 and 15% - not too bad for 2 wait states.

Accounting for
that, the mA/MHz ranges from 0.54 at 24 MHz to 0.88 at 72 MHz. I
think this may be better than the Luminary Stellaris parts, but not as
good as the Atmel SAM7 parts which are claimed to be a true 0.5 mA/MHz
with very low static current in the uA range. I have not looked at
the newer Luminary parts in detail.

I calculate 40mA at 72MHz, so 0.56mA/MHz. Not quite 0.5, but close.
But I don't see where you get the idea they are worse than SAM7. I'm not sure
what part you were comparing with, but the SAM7A3 (also CAN and USB like
STM32F103) shows 70mA at 60MHz, or more than twice at the same frequency.

Now consider that an M3 runs twice as fast as a SAM7 at the same frequency,
so the MIPS/Watt is 4 times as good!

Actually, I guess a power factor would be required for the SAM7 parts
as well since they run with one wait state at their top speed. So
maybe the STM32 part do better on power than I realized!

If you're trying to compare MIPS/Watt don't forget that different cores running
at the same frequency do not run at the same speed.

Wilco


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