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Discussion in 'Embedded' started by Artem, Mar 30, 2005.

  1. Artem

    Artem Guest

    Hi all.

    Could anyone explane me about ARM timing.
    I have a LPC2294 and external RAM (32, 0ws).

    volatile uint32_t* extRam = (uint32_t*)0x80000000;
    register uint32_t zero = 0;
    register uint32_t one = 0xffffffff;

    extRam[0] = zero;
    extRam[0] = one;
    extRam[0] = zero;
    extRam[0] = one;
    extRam[0] = zero;
    extRam[0] = one;

    497 mov r3, #-2147483648
    498 0348 FEFFFFEB mov r1, #0
    499 034c 0231A0E3 mvn r2, #0
    500 0350 0010A0E3 .L42:
    501 0354 0020E0E3 .LBE11:

    507 str r1, [r3, #0]
    508 .loc 1 337 0
    509 0358 001083E5 str r2, [r3, #0]
    510 .loc 1 338 0
    511 035c 002083E5 str r1, [r3, #0]
    512 .loc 1 339 0
    513 0360 001083E5 str r2, [r3, #0]
    514 .loc 1 340 0
    515 0364 002083E5 str r1, [r3, #0]
    516 .loc 1 341 0
    517 0368 001083E5 str r2, [r3, #0]
    518 .loc 1 342 0
    519 036c 002083E5 str r1, [r3, #0]
    520 .loc 1 343 0

    So i'ts software only set 4 bytes in external memory to from 0x000000
    to 0xffffff.
    If I understand right, str command take a one clock cycle, and writing
    to extRam another cycle.
    But when I check by oscilloscope, I have only 10mZh at data bus. I
    check BLS signal. I'ts 1 with two cycles and 0 (writing) one cycle.

    Why it's take 3 cycles?
    Artem, Mar 30, 2005
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  2. Artem

    tum_ Guest

    Try this question in comp.sys.arm, maybe you'll have better luck.
    tum_, Mar 31, 2005
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  3. The LPC external memory interface is rather slow, compared to the CPU.

    Examine your LPC user manual, look for the BCFG0 .. BCFG3 registers.

    There's may be at least one wait-state before the ARM will even start
    wiggling a CS line. (IDCY field)

    Then the actual access cycle takes at least 3 clocks (WST1 or WST2

    The reset defaults for these registers are all set to the longest
    delay possible.

    But even when setting these registers to the lowest values,
    you cannot go below 3 clocks per access.

    Anyway, you did start up the PLL, did you? If not your ARM
    will run with the XTAL clock only.

    =?ISO-8859-1?Q?=22O=2E_W=F6lfelschneider=22?=, Mar 31, 2005
  4. Artem,

    the benefits of LPC2000 device are in fast execution from internal
    memory as all higher end devices. External memory will always be slower
    because you need to go through bus bridges inside the device, needing
    to be synchronized and slowing down the access. This is why internal
    execution will be faster ( in the case of the LPC2000 also from Flash)
    than external execution even if your memory is 0 WS.

    An Schwob
    An Schwob in USA, Apr 1, 2005
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