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5-1-a-why-sdk-enable-all-peripherals-by-default.org

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Why SDK enables all peripherals by default

How to provde that SDK enables all peripherals by default?

You’re able to read the Reset Control Register value and the Reset Done Register to confirm that:

  • If you read Reset Control Register value

    You should see the value is 0x00 bits: 00000000000000000000000000000000

    Which should be 0x01FFFFFF (by default) bits: 00000001111111111111111111111111

  • If you read the Reset Done Resgiter value

    You should see the value is 0x01FFFFFF bits: 00000001111111111111111111111111

    Which should be 0x00 (by default): bits: 00000000000000000000000000000000

It proves all peripherals are ready to be used when your main function is running!!!

That’s why, you should disable all unnecessary peripherals and only enable the ones you need manually to reduce power consumption.

How SDK enable all peripherals before main function?

When compiling your program, the compiled obj file will be linked to {PICO_SDK_PATH}/src/rp2_common/pico_standard_link/crt0.S at the link stage.

Inside the {PICO_SDK_PATH}/src/rp2_common/pico_standard_link/crt0.S, you can see the following content:

// Reset handler:
// - initialises .data
// - clears .bss
// - calls runtime_init
// - calls main
// - calls exit (which should eventually hang the processor via _exit)

That runtime_init is the secret you’re looking for, it’s inside {PICO_SDK_PATH}./src/rp2_common/pico_runtime/runtime.c:

void runtime_init(void) {
  // Reset all peripherals to put system into a known state,
  // - except for QSPI pads and the XIP IO bank, as this is fatal if running from flash
  // - and the PLLs, as this is fatal if clock muxing has not been reset on this boot
  // - and USB, syscfg, as this disturbs USB-to-SWD on core 1
  reset_block(~(
          RESETS_RESET_IO_QSPI_BITS |
          RESETS_RESET_PADS_QSPI_BITS |
          RESETS_RESET_PLL_USB_BITS |
          RESETS_RESET_USBCTRL_BITS |
          RESETS_RESET_SYSCFG_BITS |
          RESETS_RESET_PLL_SYS_BITS
  ));

  // Remove reset from peripherals which are clocked only by clk_sys and
  // clk_ref. Other peripherals stay in reset until we've configured clocks.
  unreset_block_wait(RESETS_RESET_BITS & ~(
          RESETS_RESET_ADC_BITS |
          RESETS_RESET_RTC_BITS |
          RESETS_RESET_SPI0_BITS |
          RESETS_RESET_SPI1_BITS |
          RESETS_RESET_UART0_BITS |
          RESETS_RESET_UART1_BITS |
          RESETS_RESET_USBCTRL_BITS
  ));

  // .... Ingore the middle part

  // After calling preinit we have enough runtime to do the exciting maths
  // in clocks_init
  clocks_init();

  // Peripheral clocks should now all be running
  unreset_block_wait(RESETS_RESET_BITS);

  // .... Ingore the rest part
}

As you can see, that’s very clear how it works:)