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Altera_Forum's avatar
Altera_Forum
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15 years ago

alt_timestamp() question

Hi!

I am just wondering if my understanding of the alt_timestamp is totally wrong...

My oszillator clock is 50MHz which i can see in the sopc builder and which has never changed.

I configured a 32bit/1millisec interval timer in the sopc builder (i named it "TIMER_MSEC") for my nios ii and selected it to the timestamp timer in the BSP editor.

Here are the settings:# define ALT_MODULE_CLASS_Timer_Msec altera_avalon_timer# define TIMER_MSEC_ALWAYS_RUN 1# define TIMER_MSEC_BASE 0x0# define TIMER_MSEC_COUNTER_SIZE 32# define TIMER_MSEC_FIXED_PERIOD 0# define TIMER_MSEC_FREQ 50000000u# define TIMER_MSEC_IRQ 0# define TIMER_MSEC_IRQ_INTERRUPT_CONTROLLER_ID 0# define TIMER_MSEC_LOAD_VALUE 49999ULL# define TIMER_MSEC_MULT 0.0010# define TIMER_MSEC_NAME "/dev/Timer_Msec"# define TIMER_MSEC_PERIOD 1# define TIMER_MSEC_PERIOD_UNITS "ms"# define TIMER_MSEC_RESET_OUTPUT 0# define TIMER_MSEC_SNAPSHOT 1# define TIMER_MSEC_SPAN 32# define TIMER_MSEC_TICKS_PER_SEC 1000u# define TIMER_MSEC_TIMEOUT_PULSE_OUTPUT 0# define TIMER_MSEC_TYPE "altera_avalon_timer"

# define ALT_MAX_FD 4# define ALT_SYS_CLK none# define ALT_TIMESTAMP_CLK TIMER_MSEC

At program start i use alt_timestamp_start() once.

Afterwards i do multiple read outs of alt_timestamp().

Expected behaviour:

I though the value alt_timestamp() should increase every millisec by one.

That means i should reach the limits of the 32bit timer after 49 days.

Observed behaviour:

The value of alt_timestamp() reaches the limits of 32bit after approximately 90 seconds.

Do anybody have an explanation why alt_timestamp() is so fast?

Is my understanding of the interval timer totally wrong?

What perhaps is the best way to implement a 32bit millisec clock in the nios (i would like to have a clock that can be accessed like this in code: t = GetMillisecClock())?

Regards

FredFlintstone

13 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    Well 90 * 50000000 is 0x10c388d00 so what you have generated is a counter clocked by the 50MHz input clock that would generate an interrupt every millisecond.

    What you want to do is write a bit of vhld to divide sys_clk by 50, count those pulses, and make the 32bit value available to the bios cpu.

    When I looked at the altera supplied 'timer' blocks, none of them do that!

    In fact, IIRC, the timers are based on some 16bit vhdl so also require a significant number of instructions (and avalon accesses) to get a timestamp value.

    For really fast timestamps, the counter value could be made the result of a custom instruction - so be read in a single clock and without a data delay. The fastest avalon cycle is (probably) 3 clocks + the 2 clock data delay.
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    If your application structure allows it, you could use the 1ms interrupt from the timer to update a counter variable. Then your GetMillisecClock() would simply read that variable.

  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    @dsl:

    Hey dsl!

    Thanks for your fast reply.

    --- Quote Start ---

    What you want to do is write a bit of vhld to divide sys_clk by 50, count those pulses, and make the 32bit value available to the bios cpu.

    When I looked at the altera supplied 'timer' blocks, none of them do that!

    --- Quote End ---

    Grrr, and i thought creating a 32bit millisec clock would be a piece of cake and i could do it from sopc builder.

    I was already thinking about that what you suggested: creating my own 32bit millisec clock via vhld (ok, i don't like vhdl - i prefer the schematic mode). But how would you make the 32bit value available to the bios(?) cpu? Via pio? Or is there a better way?

    --- Quote Start ---

    Well 90 * 50000000 is 0x10c388d00 so what you have generated is a counter clocked by the 50MHz input clock that would generate an interrupt every millisecond.

    --- Quote End ---

    I would like to understand this quoted phrase, but i cannot figure out where the "90" comes from. Could you explain this to me - perhaps i am having a mental blackout.

    Thanks for sharing your experience

    FredFlintstone
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    @Cris:

    Hi Cris!

    Yes, doing the GetMillisecClock() thing via interrupt is a good idea. But i am still astonished, that this alt_timestamp() stuff does not deliver the results that i would expect. Perhaps i only have the wrong mental links to the word "timestamp".

    Best regards

    FredFlintstone
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    --- Quote Start ---

    @dsl:

    I would like to understand this quoted phrase, but i cannot figure out where the "90" comes from. Could you explain this to me - perhaps i am having a mental blackout.

    --- Quote End ---

    It's the 90 seconds you mentioned.

    This means that 50M increments/sec after 90 sec will overflow the 32bit timer counter.
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    @Cris:

    LOL. Ok, that was my mental blackout. I was only checking the nios defines for finding the "90". I should have checked my own text...

    Ok, now i understand the calculation by dsl:

    90sec * 50000000clocks/sec = 4500000000counts = 0x10C388D00 which does not fit into 32bit.

    Got it.

    Thanks again

    FredFlintstone
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    Possibly he easiest way to read the 32bit counter is from a custom instruction!

    But you'll probably need a simple avolon pio slave for something - and making it return the counter value is then fairly easy.

    It is rather surprising that none of the Altera timer blocks contain a scaled 32bit up-counter.
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    @dsl:

    --- Quote Start ---

    It is rather surprising that none of the Altera timer blocks contain a scaled 32bit up-counter.

    --- Quote End ---

    Yes, a scaled 32bit up-counter is exactly what i am missing in the sopc builder (is there some Altera wishlist?).

    Right now i am trying to abuse a performance counter for my GetMillisecClock() purpose. Hope this will work...

    Best regards

    FredFlintstone
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    The HAL functions for the 'performance' counters take a lot of clock cycles ...

    This may not matter for you, I got out HW chaps to put in 32bit counters for sys_clk itself and 1ms derived from it.

    (And anther derived from an asynchronous 8MHz network clock they they forgot to synchronise to sys_clk before counting.)
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    @dsl:

    My GetMillisecClock() from the performance counter works well. Ok, it may take a lot of clock cycles to do this, but at the moment this isn't crucial.

    --- Quote Start ---

    I got out HW chaps to put in 32bit counters for sys_clk itself and 1ms derived from it

    --- Quote End ---

    Could you explain this a little bit further to me? I couldn't follow that.

    Thanks and regards

    FredFlintstone