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FPGA & Softcore Vs FPGA & MCU

Discussion in 'Embedded' started by ratemonotonic, Sep 11, 2007.

  1. Hi all,

    I have designed (on paper ) a radio system with the following
    components -

    1) ATmega1280 - Responsible for talking to 3 uarts , the FPGA and
    Ethernet controller.
    2) A Xilinx FPGA interfaced with the Atmega - responsible for digital
    modulation , demodulation of radio signals.
    It also interfaces with the Ethernet controller for high bit rate.

    the decision behind the ATmega was the low power requirements.

    Now I am having second thoughts about this design and am thinking of
    FPGA Devices with Soft / Hardcore and scrapping the ATmega. I have
    never used a FPGA with CPU core and am not aware of the pros and cons
    of it.

    I would be grateful if some experienced person could enlighten me
    about it.

    ratemonotonic, Sep 11, 2007
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  2. You would be better of with the AT32UC3A0128/256/512 AVR32
    which is low power and has an integrated Ethernet MAC.
    It also has a significant amount of SRAM on board.
    Ulf Samuelsson, Sep 11, 2007
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  3. ratemonotonic

    linnix Guest

    Doesn't sound like you need absolutely lowest power, if you have
    ethernet anyway.
    I suggest looking into the Actel ProASIC3 with ARM core. $1.5 @250K
    sound good. Although we wouldn't be buying 250K, it should be
    reasonable for lower qty. They have ethernet core for the FPGA as
    well, so you just need the phys.
    linnix, Sep 11, 2007
  4. ratemonotonic

    Tim Wescott Guest

    No matter what the FPGA vendors tell you, a dedicated processor core is
    going to take up less silicon per MIP than a soft core. That's how I'd go
    unless the computational load were small -- and then I'd consider a
    separate processor as you're doing.

    Look at your $/MIP with the soft core processor, and $/MIP with a
    dedicated processor, and use that to guide your decision.

    Tim Wescott
    Control systems and communications consulting

    Need to learn how to apply control theory in your embedded system?
    "Applied Control Theory for Embedded Systems" by Tim Wescott
    Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
    Tim Wescott, Sep 12, 2007
  5. ratemonotonic

    Jack Klein Guest

    linnix already pointed out the Actel FPGA with soft ARM cores. I am
    actually surprised that this has not come up on this group.

    Both Xilinx and Altera would love for you to use their soft cores,
    Microblaze and Nios, respectively. Because the soft core license only
    allows you to use it on THEIR parts. It's a form of lock-in. If you
    want to change to a competitor's parts, not only do you need new
    development tools, but you have to change processor cores, too,
    meaning extra firmware changes.

    ARM has announced soft core availability for appropriate parts from
    Xilinx, Altera, and Actel. Actel gives you the ARM IP license free to
    use in their parts, because they are the little guy using every
    advantage they can.

    Neither Xilinx or Altera even mentions the soft core ARM on their web
    sites. They don't promote it, or even tell you it exists. Why?
    Because if you use a soft core ARM instead of Microblaze or Nios, it
    is that much easier and cheaper to move your design to another
    manufacturer's parts.

    Jack Klein
    Home: http://JK-Technology.Com
    FAQs for
    comp.lang.c http://c-faq.com/
    comp.lang.c++ http://www.parashift.com/c++-faq-lite/
    Jack Klein, Sep 12, 2007
  6. That makes more sense, than Atmega+Ethernet - unless chip-count
    and cost do not matter to you ?

    To decide between FPGA + uC, or FPGA+SoftCPU, you do the
    simple maths,

    The uC has peripherals - do you need ADC ?
    If yes, add that as external device to FPGA.

    The uC has peripherals - do you need UART ?
    If yes, add that Gate count to FPGA.

    The uC has FLASH included, and a means to PGM it
    Add a external FLASH device to your FPGA, or a SRAM,
    and a larger boot load device.

    You now have an external memory BUS, so might need
    more EMC mitigation/more layers. Allow for those costs.

    The uC has Brownout detect - do you need that ?
    If yes, add that as external device to FPGA.

    Add the SoftCPU gate count, and all the peripherals gate
    counts, and then compare the price delta of the
    two FPGAs indicated. Add the Config Device Delta,
    PCB area/layers, assembly costs, etc,
    and you have your comparisons.

    Normally, if you can find a std, volume production, uC
    that WILL do the job, it makes sense to use that. If you HAVE to have
    external memory on the FPGA anyway, then it is a closer call.

    Jim Granville, Sep 12, 2007
  7. Hi Jim ,

    A really helpful suggestion. The only thing thats not clear to me is
    the 'Config Device Delta' , do you mean the MCU used to boot the
    bitstream to the FPGA?

    ratemonotonic, Sep 12, 2007
  8. Sorry a bit brief : - with a typical SoftCPU setup, the config device
    ( usually a serial Flash device, as they are cheapest ) needs to be
    larger - to store the CPU itself, and also the code
    that is then loaded into RAM to run.

    So, the Config device needs to be larger by two factors, and that
    will be another cost.
    Jim Granville, Sep 12, 2007
  9. ratemonotonic

    linnix Guest

    ProASIC3 is flash based, so you don't need to store the config bits.
    However, you do need external program and working memories. You can
    also build a CF/SD boot loader directly from IP cores.
    linnix, Sep 12, 2007

  10. Another issue is Debugging.
    Listened to an Actel presentation on Cortex-M1 in FPGA.
    They can fit a Cortex into a $4 FPGA (large volume), but if you need
    then the core size increases so much that you need almost 2 x size,
    so I guess the CPU core is about $8 if you need debugging.
    Ulf Samuelsson, Sep 12, 2007
  11. Actel told us yesterday that the ARM was not really suitable for inclusion
    in FPGA...
    Frequency limited to 29 MHz, and would hardly fit into their smallest FPGA
    wihtout peripherals.

    They believe more in their Cortex-M1, but personally I see few customers
    where 512 kB flash applications only need a few kB of SRAM.
    In practice you will need an external SRAM.

    No way that is going to compete with a std ARM micro for price, performance
    and power.
    You will need that customization urge, to be interested.

    Any volume business can be handled by products like the AT91CAP stuff
    which will allow custom chips in realistic distribution volumes (15-25
    Ulf Samuelsson, Sep 12, 2007
  12. Of course, it is not so much an Engineering Solution, as a FPGA Sales
    Solution - the drive is to sell more/bigger FPGAs, not to solve a
    pressing design problem (in fact, you create a few : higher Power,
    and much worse EMC, but the market-spin does no mention those! )

    For apps where you CANNOT fit the code into a Microcontroller, the
    playing field levels a little, but the speed comes in a distant
    second to the 200MHz+ alternatives.

    Of course, Microcontrollers keep getting bigger: 4MByte is one
    data point.
    Do Atmel have any plans to do some point-mask solutions based on this ?

    Jim Granville, Sep 13, 2007
  13. ratemonotonic

    Jack Klein Guest

    Actually, I personally do not have much interest in using a soft core
    CPU in an FPGA. In fact, I don't have any interest at all.

    It is a push from some (not all) of our EEs, who think the best way to
    do anything and everything is to put everything in an FPGA. The
    biggest pusher is a Xilinx groupie, so of course everybody should do
    things his way and use Microblaze.

    Personally, I picked ARM as our next generation 32-bit architecture
    some years ago, replacing 486. We've been using Atmel's AT91RM9200
    since just about when it first shipped.

    But the "Forces of Evil"(tm) keep pushing for soft cores just because
    it's "new" technology.

    One of the reasons that I picked ARM was the wide range of parts from
    a large number of vendors with all sorts of price/performance options,
    and all sorts of on-chip peripherals. And one development tool set
    for all of them.

    What I dislike most about soft cores from Altera and Xilinx is the
    lock in. You can use one of their soft cores only as long as you use
    their FPGA parts. If you switch from one to the other, you can't take
    the soft core with you. And you basically have only one gcc port and
    a very limited selection of off-the-shelf OS/RTOS choices.

    That is why, if I have to use a soft core in an FPGA, it would be nice
    to have a standard soft core that you could use on any brand of FPGA,
    and it would also be nice if there was a wide choice of development
    tools and OS/RTOS options for the core.

    Which means, if I get forced into using a soft core in an FPGA, having
    an ARM soft core would seem to be the best of all possible choices.

    But don't worry, Ulf, I am still resisting the "Forces of Evil"(tm)
    with all my might, and our second product with a new board using
    AT91RM9200 will be shipping by the end of the year.

    Jack Klein
    Home: http://JK-Technology.Com
    FAQs for
    comp.lang.c http://c-faq.com/
    comp.lang.c++ http://www.parashift.com/c++-faq-lite/
    Jack Klein, Sep 13, 2007
  14. The AT91CAP products was conceived as a way to cost effectively bring out
    customized AT91 circuits. Now the focus has shifted slightly, but this is
    still a possibility.

    The current focus is on winning all designs incorporating
    a combination of an MCU and an FPGA.

    The dedicated interface between the parts allow the FPGA access to the
    internal of the MCU (It can become an AHB master),
    so it is decidedly superior to a solution putting the FPGA on the memory

    The idea is that if you win all those designs in general, you
    also win all high volume designs in particular which can then
    migrate to a mask version,losing the cost of the FPGA.

    The NRE to do this is less than 20% of the NRE of a standard cell ASIC
    and the design will be much simpler, so it is expected that the volumes
    mentioned above will really be the starting point.

    Then again, the price for the custom version is lower than the price for
    the version bonding out the dedicated FPGA interface (due to less pins),
    so Atmels revenue will be higher if they don't do the conversion :)
    Ulf Samuelsson, Sep 13, 2007
  15. ratemonotonic

    linnix Guest

    Can you make some guess on 100K prices and NRE for such, using
    QFN 32?
    linnix, Sep 13, 2007
  16. Do Atmel have any plans to do some point-mask solutions based on this ?

    NRE's are in the range of $150k, but I doubt that they will fit into QFN32
    due to the memory blocks.

    You will be hard pressed to find *any* 32 bit in QFN-32,
    but I guess they will come.
    Ulf Samuelsson, Sep 18, 2007
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