Help me understand SAM9260's clocks

Discussion in 'Embedded Systems and Microcontrollers' started by aleksazr, Jul 20, 2010.

  1. aleksazr

    Thread Starter New Member

    May 16, 2010
    14
    0
    I've started programming AT91SAM9260 and some
    things are not clear to me.

    At first, sam-ba was giving me problems
    (see http://embdev.net/topic/184418),
    so I created a small win32 app to mimic sam-ba.

    On reset, sam9260 sends "RomBOOT>" and then listens to commands.
    I then issue some commands to write to internal SRAM
    and then execute it. All that works.

    Speed is what interests me now, so I set the led,
    then clear it, in a loop. There are some 80 sets/resets
    and one jump, to minimize jump's execution time effect.

    The instruction are:
    str r3, [r2, #-203] ; 0xcb // clear
    str r3, [r2, #-207] ; 0xcf // set

    I have a 18.432 MHz clock, and this what I use for PLL:

    #define PLLA_SETTINGS 0x2060BF09

    or, to write it in a more readable way:

    #define DIVA 9
    #define PLLACOUNT 63
    #define OUTA 2
    #define MULA 96
    #define PLLA_SETTINGS (1 << 29 | MULA << 16 | OUTA << 14 | PLLACOUNT
    << 8 | DIVA)

    PLL's R1,C1,C2 are 1k, 10n and 1n, as in all schematics.

    Before I work with the PLLA, I first switch from PLLB
    (romboot was using it) to slow clock as the master clock.

    The fastest I got was 40ns per instruction.
    (i.e. 40ns per set, 40ns per clear)

    and that is only if MDIV field in PMC_MCKR is set to 00
    (master clock is processor clock).

    If MDIV is 01 (master clock is processor clock divided by 2)
    then I have 60ns per instruction. If it were 80ns, that
    would make more sense... but it isn't.

    First of all, I thought that master clock has nothing to
    do with CPU clock, with the execution of CPU instructions.
    I thought that Master clock is for SDRAM, peripherals,
    sampling the inputs, etc... but not for CPU instruction clock.

    Or, are instructions accessing peripherals executed
    differently than others?
     
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