MCU/DSP Combination

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Good Day,

I have an application where I have to run a 2048 or a 4096 points FFT
every 30 seconds.  I am wondering whether I can accomplish this with a
regular microcontroller since my timing requirements are not that
stringent.  It is not greatly important whether the FFT is integer or
floating point, the values are 16 bit A/D values.  I was considering
just using a plain old TI-MSP 430 with 10K or RAM.
Does anyone have a better suggestion as far as ease of implementation,
low power and price?

Thank you.

Re: MCU/DSP Combination
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Since that is at the top-end of MSP430, I'd also look at the bottom-end
of the many ARM Flash uC recently appearing.
Philips LPC2xxx family would be a good place to start, but you could
define more peripherals, and that would focus your choice more.
AnalogDevices, Atmel, Intel, Motorola, Philips, ST are all busy
announcing/ramping ARM single chip uC in various market segments.
-jg


Re: MCU/DSP Combination
Did you look into the PXA800F from Intel? It has a MCU and DSP built
into the same chip.

Sandeep
--
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Re: MCU/DSP Combination
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I suggest getting a portable C implementation and trying it on the
MSP430.  I haven't tried it, but if your processor has the built-in
multiply and you use a sine lookup table, I suspect it will do the
transform in a few seconds, under you 30 second allotment.

Thad

Re: MCU/DSP Combination
snipped-for-privacy@eng.umd.edu (fulan) writes:
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Your first problem with doing an FFT is data space.  A 4K
transform is going to need at least 16 Kbytes for even a
single-precision float form.  This won't fit well in 10
Kbytes of RAM.

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Re: MCU/DSP Combination
snipped-for-privacy@mojaveg.iwvisp.com (Everett M. Greene) wrote in message
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Hi,

using the LPC2105 or LPC2106 from Philips with 32 / 64k RAM would
definitely be an easier implementation than trying it with a 10k MSP
430. However, if stand-by current is an issue (battery driven?) you
might want to try the MSP430 as I consider it best in class for
stand-by. Looking at Mips/Watt I would think the ARM devices can
actually easily compete.

Cheers, Bob

Re: MCU/DSP Combination
Thank you for the many replies.

My specs dictate that I have a 14-16bit ADC converter, this can be
internal to the processor or external.  The requirement is the ability
to read at least 256 samples per second, ideally 512 samples per
second. We would repeat the reading for approximately 8 seconds and
perform some FFTs on the data.  The philips LPC2106 seems like a good
option however it lacks an internal A/D.  Furthermore, are their A/D
converters capable of acquiring that many points at that resolution
that quickly?(internal or external).

Moussa

snipped-for-privacy@aol.com (Schwob) wrote in message
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Re: MCU/DSP Combination
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There are plenty of 16-bit audio D/A converters that sample 48,000 times
a second. I don't know off hand of microcontrollers with internal ADCs
wider than 12 bits, but I don't know much. I don't know what you intend
to do with the data you collect, but be aware that many converter/-
anti-alias filter combinations impose delays of many sample periods. I
would call a sample rate below 1 KHz relatively slow.

What do you mean by repeating the reading? You could mean acquiring a
single string of 8 x 512 = 4096 contiguous samples and do one FFT, or
else acquire 8 strings of 256 samples each, perhaps with caps between
them, and doing 8 shorter FFTs, presumably averaging the results. Since
the FFT gives cosine and sine (or real and imaginary, as you please),
and averaging requires converting that to magnitude, the first way is
likely to need less computation.

Jerry
--
Engineering is the art of making what you want from things you can get.
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Re: MCU/DSP Combination
snipped-for-privacy@eng.umd.edu (fulan) wrote in message
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I'd recommend you have a look at Analog Devices' Microconverter family
of products.

From the website (www.analog.com/microconverter):

--------
MicroConverter products combine analog precision and digital
processing on a single chip.

ADC: 12-bit SAR at up to 1MSPS
or 24-bit sigma-delta
up to 16 input channels

DAC: 12-bit 5µs rail-to-rail output
up to four separate DAC outputs

MCU: ARM7TDMI at up to 45MIPS
or 8052 at up to 20MIPS
with up to 62kB Flash
--------

The price for a development board based on 8052 is just US$75!!!
Schematics and Gerbers are provided, too.

Nice, isn't it?

JaaC

Re: MCU/DSP Combination

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The TI MSP430 family of microprocessors has quite a few members with A/Ds
http://focus.ti.com/lit/ml/slab034g/slab034g.pdf

   Best wishes,
   --Phil Martel

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Re: MCU/DSP Combination
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This may sound silly bit I ask if you need a full FFT.  Do you want to
find the magnitudes of ALL frequencies in your range or just a few
frequencies of interest.  If it's the latter then you don't need FFT
just synchrronus demodulation and that's alot simpler.

I can point you int that direction if it would help.

George

Re: MCU/DSP Combination
Thanx for the reply George,

You are correct, the customer does not necessarily need a Full FFT, I
would be interested in finding out about synchronous demodulation.

Moussa.

P.S.: we will still give him the ability to perform a Full FFT,however
it should be interesting to finding out how that can be optmized.


snipped-for-privacy@att.net (George) wrote in message
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Re: MCU/DSP Combination

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   I guess it depends on how you define "combo" - Those listed above
and the lower-end TI, Motorola (and I presume AD) DSP's are basically
DSP cores with embedded peripherals added on-chip, and perhaps also
advertised with extra "embedded instructions" for fast bit twiddling.
There seem to be many of them around, so perhaps these should have a
different name to distinguish them from plain-vanilla high-end DSP's.
Maybe it should be called a "Digital Signal Microcontroller." or DSM
   A DSM has a MAC and instructions to use it, as well as the features
of microcontrollers (data RAM, program ROM,  I/O ports) so that it can
be made into an operational computer (with "DSP capabilities") with
only a small addition of external passive components.
   The distinction between microprocessor and microcontroller was made
many years (decades?) ago (who did it? Was it Motorola with the
68HC11, or maybe Intel's 8048?).  DSM's have grown in popularity
without being distinguished from the original DSP's.

   Zilog had (and may still have) a "true" combo of both a
microcontroller and a separate DSP on one piece of silicon, with
various ports so they can communicate with and (presumably) interrupt
each other. I haven't heard of any other such device.

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Re: MCU/DSP Combination
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Good point, Motorola even tries the strange label of 'hybrid', which
does not gel well in a industry that already knows what hybrid means.
I give dsPIC a higher name selection score.

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There are quite a few at the top-end, where DSP+ARM is now almost the
default for certain cell phone engines, tho these are not single-die
system solutions in the way a microcontroller is.
-jg


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