Turning off unused RAM to get minimum power consumption.

[Adam Cook]

I need to wake up the nrf52840 every 250ms so I am using the system on sleep mode and the RTC. I have only a few dozen bytes that I need to have persist between each wake up. I want to turn off all but one block of RAM for maximum power efficiency and store the bytes there.  I have modified the Ram Retention example in SDK17 using Segger to turn off all but one block but I am not getting lower power consumption. In fact, the lowest power consumption is full, followed by partial, and then none. I measured 3.06uA with no RAM retention, 3.00uA with partial, and 2.94uA with full using the PPK2. Is there anything else that needs to be done in order to turn off unused RAM blocks? Are there any other peripherals that I need to be aware of? I have attached the modified ram retention code.

int main(void)
{
    uint32_t * volatile p_ram_test = (uint32_t *)RAM_MEMORY_TEST_ADDRESS;
    uint32_t            loop_count = 0;

    bsp_board_init(BSP_INIT_LEDS);

    APP_ERROR_CHECK(NRF_LOG_INIT(NULL));
    NRF_LOG_DEFAULT_BACKENDS_INIT();

    // This pin is used for waking up from System OFF and is active low, enabling sense capabilities.
    nrf_gpio_cfg_sense_input(PIN_GPIO_WAKEUP, BUTTON_PULL, NRF_GPIO_PIN_SENSE_LOW);

    // Workaround for PAN_028 rev1.1 anomaly 22 - System: Issues with disable System OFF mechanism
    nrf_delay_ms(1);

    // Check if the system woke up from System IDLE mode by reading the NRF_POWER->GPREGRET register which has
    // retained the value written before going to System IDLE. Below is the layout for usage for
    // NRF_POWER->GPREGRET register.
    //  BITS |   7   |   6   |   5   |   4   |   3   |   2   |   1   |   0
    //  --------------------------------------------------------------------
    //       |        SPECIAL_SEQUENCE       |          LOOP_COUNT
    //  --------------------------------------------------------------------
    ///

    if ((NRF_POWER->GPREGRET >> 4) == RESET_MEMORY_TEST_BYTE)
    {
        // Take the loop_count value.
        loop_count          = (uint8_t)(NRF_POWER->GPREGRET & 0xFUL);
        NRF_POWER->GPREGRET = 0;

        if (loop_count >= (uint8_t)MAX_TEST_ITERATIONS)
        {
            // clear GPREGRET register before exit.
            NRF_POWER->GPREGRET = 0;
            bsp_board_led_on(SUCCESS_PIN_NUMBER);

            while (true)
            {
                // Do nothing.
            }
        }

        if (*p_ram_test != RAM_MEMORY_TEST_WORD)
        {
            display_failure();
        }

        *p_ram_test = 0;
    }

    // Write the known sequence + loop_count to the GPREGRET register.
    loop_count++;
    NRF_POWER->GPREGRET = ( (RESET_MEMORY_TEST_BYTE << 4) | loop_count);

    // Write the known value to the known address in RAM, enable RAM retention, set System Sleep, and wait
    // for GPIO wakeup from external source.

    bsp_board_led_on(READY_PIN_NUMBER);
    nrf_delay_ms(1000);

    //Turn off everything but a single RAM bank for minimum current draw.
    NRF_POWER->RAM[0].POWERSET = (POWER_RAM_POWER_S0POWER_On << POWER_RAM_POWER_S0POWER_Pos)      |
                            (POWER_RAM_POWER_S1POWER_Off      << POWER_RAM_POWER_S1POWER_Pos)      |
                            (POWER_RAM_POWER_S2POWER_Off      << POWER_RAM_POWER_S2POWER_Pos)      |
                            (POWER_RAM_POWER_S3POWER_Off      << POWER_RAM_POWER_S3POWER_Pos)      |
                            (POWER_RAM_POWER_S4POWER_Off      << POWER_RAM_POWER_S4POWER_Pos)      |
                            (POWER_RAM_POWER_S5POWER_Off      << POWER_RAM_POWER_S5POWER_Pos)      |
                            (POWER_RAM_POWER_S6POWER_Off      << POWER_RAM_POWER_S6POWER_Pos)      |
                            (POWER_RAM_POWER_S7POWER_Off      << POWER_RAM_POWER_S7POWER_Pos)      |
                            (POWER_RAM_POWER_S8POWER_Off      << POWER_RAM_POWER_S8POWER_Pos)      |
                            (POWER_RAM_POWER_S9POWER_Off      << POWER_RAM_POWER_S9POWER_Pos)      |
                            (POWER_RAM_POWER_S10POWER_Off     << POWER_RAM_POWER_S10POWER_Pos)     |
                            (POWER_RAM_POWER_S11POWER_Off     << POWER_RAM_POWER_S11POWER_Pos)     |
                            (POWER_RAM_POWER_S12POWER_Off     << POWER_RAM_POWER_S12POWER_Pos)     |
                            (POWER_RAM_POWER_S13POWER_Off     << POWER_RAM_POWER_S13POWER_Pos)     |
                            (POWER_RAM_POWER_S14POWER_Off     << POWER_RAM_POWER_S14POWER_Pos)     |
                            (POWER_RAM_POWER_S15POWER_Off     << POWER_RAM_POWER_S15POWER_Pos)     |
                            (POWER_RAM_POWER_S0RETENTION_On  << POWER_RAM_POWER_S0RETENTION_Pos)  |
                            (POWER_RAM_POWER_S1RETENTION_Off  << POWER_RAM_POWER_S1RETENTION_Pos)  |
                            (POWER_RAM_POWER_S2RETENTION_Off  << POWER_RAM_POWER_S2RETENTION_Pos)  |
                            (POWER_RAM_POWER_S3RETENTION_Off  << POWER_RAM_POWER_S3RETENTION_Pos)  |
                            (POWER_RAM_POWER_S4RETENTION_Off  << POWER_RAM_POWER_S4RETENTION_Pos)  |
                            (POWER_RAM_POWER_S5RETENTION_Off  << POWER_RAM_POWER_S5RETENTION_Pos)  |
                            (POWER_RAM_POWER_S6RETENTION_Off  << POWER_RAM_POWER_S6RETENTION_Pos)  |
                            (POWER_RAM_POWER_S7RETENTION_Off  << POWER_RAM_POWER_S7RETENTION_Pos)  |
                            (POWER_RAM_POWER_S8RETENTION_Off  << POWER_RAM_POWER_S8RETENTION_Pos)  |
                            (POWER_RAM_POWER_S9RETENTION_Off  << POWER_RAM_POWER_S9RETENTION_Pos)  |
                            (POWER_RAM_POWER_S10RETENTION_Off << POWER_RAM_POWER_S10RETENTION_Pos) |
                            (POWER_RAM_POWER_S11RETENTION_Off << POWER_RAM_POWER_S11RETENTION_Pos) |
                            (POWER_RAM_POWER_S12RETENTION_Off << POWER_RAM_POWER_S12RETENTION_Pos) |
                            (POWER_RAM_POWER_S13RETENTION_Off << POWER_RAM_POWER_S13RETENTION_Pos) |
                            (POWER_RAM_POWER_S14RETENTION_Off << POWER_RAM_POWER_S14RETENTION_Pos) |
                            (POWER_RAM_POWER_S15RETENTION_Off << POWER_RAM_POWER_S15RETENTION_Pos);
                          
      // Write test word to RAM memory.
      *p_ram_test = RAM_MEMORY_TEST_WORD;
      bsp_board_leds_off();

      //Enter System Idle sleep with partial Ram Retention. 
      while (true)
      {
          __WFE();
      }
    


}

[Håkon Alseth]

Hi,

 

You should set the NRF_POWER->RAM[0].POWER when setting blocks directly with a mix of '0' and '1'.

SET / CLR registers has to be written '1' to in order to perform a specific set or clear task.

For more info, see this chapter:

https://infocenter.nordicsemi.com/topic/ps_nrf52840/power.html?cp=5_0_0_4_2_6_20#register.RAM-0-8.POWER

 

Kind regards,

Håkon

[Adam Cook]

I have tried that an not measured any noticeable difference. Could there be other components on the DK that could be consuming extra power?

[Hung Bui]

Hi Adam, 

Håkon is on vacation and I will take over the ticket. Due to reduced staff in the summer there will be some delay in our response. I'm sorry for that. 

[Hung Bui]

Hi again Adam, 

I'm sorry for the late response. We are a bit low on staff in the summer. 

I just want to share some update on my measurement. 

I edited the RTC example for testing.
Here is my code to turn off RAM retention: 

I have to retain RAM[8], most likely because the stack is located at the top of RAM. 

Here is with full ram Retention (RAM_RETENTION=1): 

Here is after turn off most blocks (set RAM_RETENTION=0)

You can find that the current consumption dropped almost 1.6uA. This matched with the calculation from our expert in the email thread with Gene. 

I found that nRF_POWER->RAM[8] plays an important role in the power consumption. If I don't put RAM[8] (part of it) into OFF mode, I will have this: 

So the current dropped only 0.60 uA when RAM[8] was in full retention. 

I was testing with PCA10056 DK v3.0 with PPK2 in Ampere mode. My suggestion is to try switch SW6 to nRFOnly if you don't see low current consumption in your measurement to avoid leakage current to other components on the board. As power reset between each measurement is also suggested. 

rtc - Ramretention.zip