Saving the location of Watchdog timeout

[Gauthier]

I am trying to save the address of program counter when a watchdog reset occurs. In order to do that I use gcc's __builtin_return_address(0) from within the SDK's nrf_drv_wdt.c:WDT_IRQHandler().

What this returns is 0xfffffff9. If I interpret the ARM spec correctly, this address is in the area reserved for the chip vendor.

• What is at that address?

• Can I extract the PC before WD reset using the gcc builtin? Another function?

• What is the recommended way of getting the address when the watchdog timed out? I've seen people manually unpacking the stack, I'd be glad if I could do it in a less hacky way.

[Sigurd]

Hi,

That the __builtin_return_address() function is returning 0xfffffff9 is here expected. See the chapter “Exception entry and return” in the ARM Cortex-M4 manual, and read about when the EXC_RETURN(0xFFFFFFF9) value is loaded into the program counter.

One way to do this, is to dump the stack in the watchdog timeout handler, and read back the program counter when the program starts up again after the reset. Here is one way you can do it based on the watchdog example in the SDK:

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/** @file
 * @defgroup nrf_dev_wdt_example_main main.c
 * @{
 * @ingroup nrf_dev_wdt_example
 * @brief WDT Example Application main file.
 *
 * This file contains the source code for a sample application using WDT.
 *
 */


#include <stdbool.h>
#include <stdint.h>

#include "nrf.h"
#include "bsp.h"
#include "app_timer.h"
#include "app_error.h"
#include "nrf_drv_wdt.h"
#include "nrf_drv_clock.h"
#include "nrf_delay.h"
#include "app_util_platform.h"

#include <string.h>
#include "sdk_common.h"
#include "crc16.h"



#define FEED_BUTTON_ID          0                           /**< Button for feeding the dog. */

nrf_drv_wdt_channel_id m_channel_id;

typedef struct stack
{
    uint32_t r0;  ///< R0 register.
    uint32_t r1;  ///< R1 register.
    uint32_t r2;  ///< R2 register.
    uint32_t r3;  ///< R3 register.
    uint32_t r12; ///< R12 register.
    uint32_t lr;  ///< Link register.
    uint32_t pc;  ///< Program counter.
    uint32_t psr; ///< Program status register.
} stack_t;

static stack_t stack __attribute__((section(".non_init")));
static uint16_t crc __attribute__((section(".non_init")));

bool wdt_debug(stack_t * p_stack);
void dump_stack(stack_t * p_stack);



//NOTE: The max amount of time we can spend in WDT interrupt is two cycles of 32768[Hz] clock - after that, reset occurs
void wdt_timeout_handler()
{
 /* Needed to unwind original stack frame from WDT_IRQHandler when code optimization is disabled. 
    Try to remove "add	r0, #8" line if compiling with -O3*/
__ASM volatile(        
                 "   mrs r0, msp                             \n"
                 "   add	r0, #8                       \n"
                 "   ldr r3, =dump_stack                     \n"
                 "   bx r3                                   \n"
              ); 
} 




bool wdt_debug(stack_t * p_stack)
{

    if ((NRF_POWER->RESETREAS & POWER_RESETREAS_DOG_Msk) == POWER_RESETREAS_DOG_Msk)
    {
        memcpy(p_stack, &stack, sizeof(stack_t));
        if (crc != crc16_compute((const uint8_t *)p_stack, sizeof(stack_t), NULL))
        {
            return false; //invalid data
        }
        crc = 0;
        NRF_POWER->RESETREAS = 0;
        return true;
    }
    else
    {
        return false;
    }
}


void dump_stack(stack_t * p_stack)
{
    memcpy(&stack, p_stack, sizeof(stack_t));
    /*Note: RAM is not guaranteed to be retained retained through WD reset:
      http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.ps.v1.1/power.html?cp=2_1_0_17_7#unique_832471788.
      CRC may be used to verify the data integrity*/
    crc = crc16_compute((const uint8_t *)p_stack, sizeof(stack_t), NULL);
}



/**
 * @brief Assert callback.
 *
 * @param[in] id    Fault identifier. See @ref NRF_FAULT_IDS.
 * @param[in] pc    The program counter of the instruction that triggered the fault, or 0 if
 *                  unavailable.
 * @param[in] info  Optional additional information regarding the fault. Refer to each fault
 *                  identifier for details.
 */
void app_error_fault_handler(uint32_t id, uint32_t pc, uint32_t info)
{
    bsp_board_leds_off();
    while (1);
}

/**
 * @brief BSP events callback.
 */
void bsp_event_callback(bsp_event_t event)
{
    switch (event)
    {
        case BSP_EVENT_KEY_0:
            nrf_drv_wdt_channel_feed(m_channel_id);
            break;

        default :
            //Do nothing.
            break;
    }
}


/**
 * @brief Function for main application entry.
 */
int main(void)
{
    uint32_t err_code = NRF_SUCCESS;

    //BSP configuration for button support: button pushing will feed the dog.
    err_code = nrf_drv_clock_init();
    APP_ERROR_CHECK(err_code);
    nrf_drv_clock_lfclk_request(NULL);


    stack_t old_stack;

    if(wdt_debug(&old_stack)){
    // program counter from when the watchdog reset occurred can now be found.
    uint32_t watchdog_reset_pc;
    watchdog_reset_pc = old_stack.pc;

    }

    err_code = app_timer_init();
    APP_ERROR_CHECK(err_code);

    err_code = bsp_init(BSP_INIT_BUTTONS, bsp_event_callback);
    APP_ERROR_CHECK(err_code);

    //Configure all LEDs on board.
    bsp_board_leds_init();

    //Configure WDT.
    nrf_drv_wdt_config_t config = NRF_DRV_WDT_DEAFULT_CONFIG;
    err_code = nrf_drv_wdt_init(&config, wdt_timeout_handler);
    APP_ERROR_CHECK(err_code);
    err_code = nrf_drv_wdt_channel_alloc(&m_channel_id);
    APP_ERROR_CHECK(err_code);
    nrf_drv_wdt_enable();

    //Indicate program start on LEDs.
    for (uint32_t i = 0; i < LEDS_NUMBER; i++)
    {   nrf_delay_ms(200);
        bsp_board_led_on(i);
    }
     err_code = bsp_buttons_enable();
     APP_ERROR_CHECK(err_code);

    while (1)
    {
        __SEV();
        __WFE();
        __WFE();
    }
}

/** @} */

Note: I tested this with GCC/Segger Embedded Studio, and with Optimization Level set to None.

[Valerii_Ie]

Hello, I just tried to build this code in IAR and it does not work. It seems that IAR's inline assembler has different syntax.  

__ASM volatile(        
                 "   mrs r0, msp                             \n"
                 "   add	r0, r0, #8                       \n"
                 "   ldr r3, =dump_stack                     \n" //error here
                 "   bx r3                                   \n"
              ); 

Error[Og006]: Error in inline assembly: "Error[401]: Operand syntax error" 
Did anybody overcome this?

[Sigurd]

Hi Valerii_Ie,

For IAR, you can replace the line you marked as 'error' with this one:

"B dump_stack                                 \n"

You will also need to declare the stack and crc variables like this:

__no_init static stack_t stack;
__no_init static uint16_t crc;