Q: IXP425 CPU (over) clock ? - Embedded

This is a discussion on Q: IXP425 CPU (over) clock ? - Embedded ; Hello, I am fiddling around with an NSLU2 from Linksys, which has a IXP425 operating at 266 MHz, if you remove a distinct resistor. Now I tried to overclock the CPU by replacing the 33 MHz OSC with a 36 ...

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Thread: Q: IXP425 CPU (over) clock ?

  1. Q: IXP425 CPU (over) clock ?

    Hello,

    I am fiddling around with an NSLU2 from Linksys, which has a IXP425
    operating at 266 MHz, if you remove a distinct resistor.

    Now I tried to overclock the CPU by replacing the 33 MHz OSC with a 36
    MHz Oscillator. But the reported CPU frequency is still 266 MHz.

    Does the CPU incorporate something like a clock speed limit?

    Does anybody know?

    I tried to get info by downloading several in depth whitepaper from
    Intel download website, but although they were in depth, they were not
    depp enough.

    In good hope for answers

    Heinz

  2. Re: Q: IXP425 CPU (over) clock ?

    > I am fiddling around with an NSLU2 from Linksys, which has a IXP425
    > operating at 266 MHz, if you remove a distinct resistor.
    >
    > Now I tried to overclock the CPU by replacing the 33 MHz OSC with a 36
    > MHz Oscillator. But the reported CPU frequency is still 266 MHz.


    How do you think the system should detect the changed speed ? If the
    single OSC is changed the system time will be faster together with the
    CPU and thus the speed test results in the same value, even though the
    working speed is faster. _Hopefully_ the Ethernet, serial port and USB
    clocks are not derived from that OSC, otherwise you will see
    communication problems.

    -Michael

  3. Re: Q: IXP425 CPU (over) clock ?

    Michael Schnell wrote:
    >> I am fiddling around with an NSLU2 from Linksys, which has a IXP425
    >> operating at 266 MHz, if you remove a distinct resistor.
    >>
    >> Now I tried to overclock the CPU by replacing the 33 MHz OSC with a 36
    >> MHz Oscillator. But the reported CPU frequency is still 266 MHz.

    >
    > How do you think the system should detect the changed speed ? If the
    > single OSC is changed the system time will be faster together with the
    > CPU and thus the speed test results in the same value, even though the
    > working speed is faster. _Hopefully_ the Ethernet, serial port and USB
    > clocks are not derived from that OSC, otherwise you will see
    > communication problems.
    >
    > -Michael


    Hello,

    I do not have any problems with ethernet or USB.

    The bogo mips with teh original settings is somewaht around 133, because
    a set of resistors forces the CPU to slow down to 1/2 of the nominal CPU
    speed.

    If you remove one resistor, you "de-inderclock" to the CPU frequency of
    266 MHz.

    I expected the CPU speed (bogo mips) to show more than 266 after the
    OSC replacement.

    I startet with just 3 MHz more, because I wanted to try it stepwise from
    slower to faster.

    As far as I know, the 266 MHz is derived from the OCS-freq. multiplied
    with a certain factor. The factor will be 266MHz / 33 MHz = 8,060606...

    I expected the CPU speed would be about 36MHz * 8,060606... = 290 MHz
    after the replacement. Furthermore, I expected that bogo mips will
    calculate the value, not just printing a preset value.

    At the moment, I wonder if the CPU is limiting the incoming osc
    frequency by itself. But I cannot find any clues, that this might be the
    reality.

    Bye,
    Heinz

  4. Re: Q: IXP425 CPU (over) clock ?

    Please re-read my post.

    A frequency, bogo-mips or whatever speed measurement needs to be made
    against a known clock. So the de-underclocking should show with the
    bogo-mips, as the timer used supposedly is running at the same speed as
    before. Unless there is a second OSC, the timer runs faster, too, if you
    change the OSC speed outside of the chip. So the measured CPU speed
    value will be the same even if the CPU runs faster.

    -Michael

  5. Re: Q: IXP425 CPU (over) clock ?

    Michael Schnell wrote:
    >> I am fiddling around with an NSLU2 from Linksys, which has a IXP425
    >> operating at 266 MHz, if you remove a distinct resistor.
    >>
    >> Now I tried to overclock the CPU by replacing the 33 MHz OSC with a 36
    >> MHz Oscillator. But the reported CPU frequency is still 266 MHz.

    >
    > How do you think the system should detect the changed speed ? If the
    > single OSC is changed the system time will be faster together with the
    > CPU and thus the speed test results in the same value, even though the
    > working speed is faster. _Hopefully_ the Ethernet, serial port and USB
    > clocks are not derived from that OSC, otherwise you will see
    > communication problems.
    >
    > -Michael


    Hello,

    I would say that the system is measuring against the RealTimeClock as an
    independant source.

    BTW, how do you think the system is detecting speed? In my opinion, it
    has to count cycles and compare it with a reference souce, e.g. the RTC.

    Am I wrong?

    Heinz

  6. Re: Q: IXP425 CPU (over) clock ?

    Michael Schnell wrote:
    > Please re-read my post.
    >
    > A frequency, bogo-mips or whatever speed measurement needs to be made
    > against a known clock. So the de-underclocking should show with the
    > bogo-mips, as the timer used supposedly is running at the same speed as
    > before. Unless there is a second OSC, the timer runs faster, too, if you
    > change the OSC speed outside of the chip. So the measured CPU speed
    > value will be the same even if the CPU runs faster.
    >
    > -Michael


    .... but does the CPU "know" about the external oscillators frequency?

    I cannot imagine, that the CPU knows that there has to be a 33MHz osc
    somewhere on the PCB and that by definiton the CPU speed has to be 266 MHz.

    As far as I understood the ixp425 manual, intel is manufacturing the
    CPUs in three different speeds.

    That brought me to the conclusion, that the cpu might be able to somehow
    reduce the clock that is entered into the clock in pin. But that is very
    vague.

    Bye,
    heinz


  7. Re: Q: IXP425 CPU (over) clock ?

    >
    > ... but does the CPU "know" about the external oscillators frequency?


    How should it know ? The OSC does not have a communication channel to
    tell the CPU at what speed it is supposed to run.

    >
    > I cannot imagine, that the CPU knows that there has to be a 33MHz osc
    > somewhere on the PCB and that by definiton the CPU speed has to be 266 MHz.


    You need to use software to set the internal clock speed generation
    (derived from the osc input) for the CPU and the hardware timers and the
    software timers of the OS according to the osc used, if you want that
    e.g. the jiffie ticker works as expected.

    >
    > As far as I understood the ixp425 manual, intel is manufacturing the
    > CPUs in three different speeds.


    That is just the max speed the chip is guaranteed to work with. It can
    work slower if using another osc and/or setting other parameters by
    software/hardware configuration. Now it will consume less power.

    >
    > That brought me to the conclusion, that the cpu might be able to somehow
    > reduce the clock that is entered into the clock in pin. But that is very
    > vague.
    >


    It can't. but it can set parameters for it's internal clock generating
    hardware and modify it's running speed by that.

    -Michael

  8. Re: Q: IXP425 CPU (over) clock ?

    >
    > I would say that the system is measuring against the RealTimeClock as an
    > independant source.


    Is there a separate crystal for the real time clock on the PCB ? If yes
    this is possible. You would need to look at the Kernel source to be sure.

    >
    > BTW, how do you think the system is detecting speed? In my opinion, it
    > has to count cycles and compare it with a reference souce, e.g. the RTC.
    >
    > Am I wrong?


    Of course you are right, but I suppose that there is a software module
    that calculated the speed and another module that creates some kind of
    ticker that is used by the first one. Both might depend on the board you
    are using by means of Kernel configuration and/or special source code.

    -Michael

  9. Re: Q: IXP425 CPU (over) clock ?

    Heinz Rohde wrote:

    > Michael Schnell wrote:
    >>> I am fiddling around with an NSLU2 from Linksys, which has a IXP425
    >>> operating at 266 MHz, if you remove a distinct resistor.
    >>>
    >>> Now I tried to overclock the CPU by replacing the 33 MHz OSC with a 36
    >>> MHz Oscillator. But the reported CPU frequency is still 266 MHz.

    >>
    >> How do you think the system should detect the changed speed ? If the
    >> single OSC is changed the system time will be faster together with the
    >> CPU and thus the speed test results in the same value, even though the
    >> working speed is faster. _Hopefully_ the Ethernet, serial port and USB
    >> clocks are not derived from that OSC, otherwise you will see
    >> communication problems.
    >>
    >> -Michael

    >
    > Hello,
    >
    > I would say that the system is measuring against the RealTimeClock as an
    > independant source.
    >
    > BTW, how do you think the system is detecting speed? In my opinion, it
    > has to count cycles and compare it with a reference souce, e.g. the RTC.
    >
    > Am I wrong?
    >
    > Heinz


    The RTC is usually only used to set a counter at system start. The
    counter is then run by a timer which, on a typical ARM ASIC is derived
    from the system clock.

    Kind regards,

    Iwo


  10. Re: Q: IXP425 CPU (over) clock ?

    Heinz Rohde a écrit :
    >
    > I would say that the system is measuring against the RealTimeClock as an
    > independant source.
    >
    > BTW, how do you think the system is detecting speed? In my opinion, it
    > has to count cycles and compare it with a reference souce, e.g. the RTC.
    >
    > Am I wrong?


    No you are right, but what if your RTC is using the same clock
    source as your chip?

    The IXP425 has internal timers, so if these are used to serve
    as a reference for BogoMIPS for instance, they will be wrong
    since their speed is derived from the external OSC. Cf section
    2.1.14 of http://download.intel.com/design/net...s/25247905.pdf
    which says:

    The IXP42X product line and IXC1100 control plane processors
    consists of four internal timers operating at 66.66 MHz
    (which is 2 * OSC_IN input pin.) to allow task scheduling and
    prevent software lock-ups. The device has four 32-bit counters:
    Watch-Dog Timer Timestamp Timer Two general-purpose timers

    If you want to be sure of what timer is used, take a look at the
    Linux kernel source.


    Laurent

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