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FPGA based processor vs. "hard" processor

Discussion in 'Embedded' started by fp, Aug 5, 2006.

  1. fp

    fp Guest

    I just finish reading the interesting thread of PIC vs AVR. I am not
    in the embedded area but have a related question. In recent Xilinx
    marketing stuff for Spartan-3 FPGA, it indicates that the 100K-gate
    FPGA now reaches $2 a piece, and the costs of softcore 8-bit processor
    (PicoBlaze) and 32-bit processor (MicroBlaze) are reduced to $0.10 and
    $0.48.

    Despite of the marketing hype, do you think that the FPGA-based
    softcore processor can really compete with the "hard" processor
    (such as PIC or AVR) in embedded application?

    Thanks in advance.

    S.C.
     
    fp, Aug 5, 2006
    #1
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  2. The fpga cpu gets you just that: the CPU.
    The $2 "CPU" parts will also have memory (Flash & RAM) & peripherals.
    The memory can be made using FPGA resources, but it's inefficient.
    Such peripherals as SPI, UART etc. can also be in FPGA. However, you
    won't get an ADC (which most MPU's now include).
     
    David R Brooks, Aug 5, 2006
    #2
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  3. As always, watch for the spin.

    a) FPGAs approach $2 only in 7 digit volumes, in 2007.
    Go via the Xilinx store, and you'll find $8.41/100 is a more
    real-world price. The next largest one is $12.54

    b) The core budgets are just that : core only, no CODE and no
    Peripherals. Usually they also ignore the Loader memory costs !!

    So it is classic stone soup - if you took only the cores in
    the PIC/AVR/80C51/RS08, they would also be in the 'some cents'

    Also, if ADDING the FPGA processor bumps you up a FPGA size, then
    the cost becomes a real +$4.13 in the above example, and that $4.13
    can buy quite a lot of microcontroller-system these days (not just a core)!

    If you are lucky, and have enough fabric, and BRAM, spare, doing
    nothing, then a FPGA core can be almost free.

    So it depends on the design.
    FPGA's will never touch the small microcontrollers.

    Where a FPGA processor makes sense is when
    a) You already have the FPGA doing other tasks, and have enough spare room
    b) You need a Processor to assist
    c) Good coupling between the Processor and FPGA is needed.
    d) Multiple cores help the design

    The weaknesses of FPGA are
    * Code memory handling.
    * A complete lack of Analog Peripherals
    * High Icc costs
    * Complex and narrow power supply ranges

    So, you should always 'reality check' your FPGA core, against
    something like a LPC2101, and make sure a smaller FPGA + Good uC is not
    a better solution.

    There are some interesting gaps :
    Most microcontrollers run direct from FLASH, and are tru single-chip
    solutions.
    However, FPGA cores keep edging up in speed, and are now over 200MHz,
    whilst microcontrollers are more flash-limited to 30-75MHz.
    That's the opposite of a few years ago, when the cores were slower,
    but more flexible.

    Something has to happen to non-volatile storage speeds, or the FPGAs
    will get something of a speed advantage.

    -jg
     
    Jim Granville, Aug 5, 2006
    #3
  4. fp

    steve Guest

    I think power consumption for an FPGA is not in the same league as the
    AVR or MSP430's.
     
    steve, Aug 5, 2006
    #4
  5. As soon as you need a fourier transform co-processor...

    Rene
     
    Rene Tschaggelar, Aug 5, 2006
    #5
  6. fp

    PeteS Guest

    There's an understatement. Typical idle currents for AVR are in the
    sub-milliamp range ( _well_ sub-milliamp), and (from the data sheet)
    about 3.5mA active, 4MHz, Vcc = 3V

    A small FPGA won't get below about 5mA idle (no internal clocks at
    all). Run current is tougher becaue it depends on how many cells are
    being toggled and I/O power, but a typical number to do the same as the
    AVR is 10-15mA (using the SWAG technique).

    The AVR in powerdown mode has a max Icc of 15 uA with the WDT and
    external interrupts enabled, which suits me just fine.

    There is a time and place for a soft processor, but even though I have
    enough room left in a Spartan 3 in a new design, I still use an AVR for
    system management (I power off the FPGA completely) and system startup
    / shutdown control simply because I can't afford the extra power
    consumption.


    Cheers

    PeteS
     
    PeteS, Aug 5, 2006
    #6
  7. FPGA's are the right choice for high performance applications, like video
    processing, specialized sd/ddr memory controllers, fast networking etc. etc.

    It really doesn't make sense to use an 8bit soft ip for a battery operated
    blood pressure tester or something similar.

    However an fpga based lcd/tft controller with on screen user interface would
    benefit from an 8bit ip.

    In applications like networking, the choice of doing tasks in hardware or
    software (on the same chip) leads to very high performance solutions at
    minimum consumption of resources. You can implement a fairly complex network
    controller @100MHz with a 16bit cpu in a small Spartan 3E chip for about 8$
    in single quantities.
    See:
    http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=49926&Row=18546&Site=US


    MIKE

    --
    www.oho-elektronik.de
    OHO-Elektronik
    Michael Randelzhofer
    FPGA und CPLD Mini Module
    Klein aber oho !
     
    M.Randelzhofer, Aug 5, 2006
    #7
  8. Never - because by the time that the bleeding-edge silicon and the
    buggy tools has yielded a working product, the available CPU's will
    have better performance and lower price than the FPGA "solution"! It
    is, I.M.O., not possible to beat the economics of the volume behind
    the generic processors.

    PS:

    I have personnally watched a large telecom equipment provider do this
    dance every 3 years or so when the pain from the last project
    "leading-edge" blowout becomes forgotten. They just started again ;-).
     
    Frithiof Andreas Jensen, Aug 9, 2006
    #8
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