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i.MX35 suspend CPU and DDR2 from IRAM

Question asked by richardk on Feb 14, 2014
Latest reply on Apr 9, 2014 by Yixing Kong



I have to put my device into a very deep low power mode from Linux 2.6.38 and therefore, it's necessary to suspend all components, including CPU und DDR2.

What I found out so far is that I have to copy the core assembler function into the processor's internal memory and execute it from there. Basically, it looks like this:


cpaddr = iram_alloc(SZ_1K, &iram_addr);

if(!cpaddr) return -ENOMEM;

suspend_iram_base = __arm_ioremap(iram_addr, SZ_1K, MT_HIGH_VECTORS);

memcpy(suspend_iram_base, cpu_v6_sdram_off, SZ_1K);

flush_icache_range(suspend_iram_base, suspend_iram_base + SZ_1K);



__asm__ __volatile__(

  "ldr r0, %0\n"

  "ldr r1, %1\n"

  "ldr r2, %2\n"

  "blx r2\n"


  : : "m" (esdctl_addr),

      "m" (csd0_addr),

      "m" (suspend_iram_base));


So far everything works as expected, I can verify code execution from internal memory (in virtual address space) with the JTAG debugger.


If I understand it all correctly, I have to do the following in the IRAM function:

- disable interrupts and caches

- set the SDRAM controller into precharge power down mode

- execute a precharge all command and access memory with A10 high (e.g. 0x400) to effectively close all banks

- put the CPU into standby by executing a WFI instruction

- re-enable everything afterwards (left out in the source code below)



  @ r0: esdctl base address

  @ r1: csd0 address with a10 high


  cpsid   if


  @ disable I and D cache

  mrc     p15, 0, r2, c1, c0, 0

  bic     r2, r2, #0x00001000  @ disable I cache

  bic     r2, r2, #0x00000004  @ disable D cache

  mcr     p15, 0, r2, c1, c0, 0


  @ invalidate I cache

  mov     r2, #0

  mcr     p15, 0, r2, c7, c5, 0


  @ clear and invalidate D cache

  mov     r2, #0

  mcr     p15, 0, r2, c7, c14, 0


  @ precharge power down mode

  ldr     r2, [r0]

  bic     r2, r2, #0xc00

  orr     r2, r2, #0x400

  str     r2, [r0]


  @ precharge all command

  mov     r2, #0x92

  lsl     r2, #24

  orr     r2, r2, #0x228000

  orr     r2, r2, #0x0400

  str     r2, [r0]

  mov     r2, #0x12

  lsl     r2, #24

  orr     r2, r2, #0x340000

  orr     r2, r2, #0x5600

  orr     r2, r2, #0x78

  str     r2, [r1] @ dummy write access


  @ execute wait for interrupt

  mov     r1, #0

  mcr     p15, 0, r1, c7, c10, 4

  mcr     p15, 0, r1, c7, c0, 4


  cpsie   if

  bx      lr



The problem is at the point where the RAM is accessed with a dummy write. It simply results in a data abort exception and then the CPU gets lost.

If I leave this part out, the DDR2 doesn't seem to be put into low power mode, because the current consumption doesn't go down.


Now I'm totally stuck and out of ideas here. Could someone please give me a hint what I'm doing wrong or what I'm missing here?

Or is there any documentation or source code available demonstrating the whole procedure for the i.MX35 on Linux?