RAM retention / shutdown

[lee-marc]

Module:	ilumi H52 BLE module (nRF52832)
SDK:	nRF5_SDK_15.2.0_9412b96
Softdevice:	132_nrf52_6.1.0_softdevice.hex
Compiler:	gcc version 7.3.1 20180622 (release) [ARM/embedded-7-branch revision 261907] (15:7-2018-q2-4)

Hello,

I'm working with ble_central/ble_app_uart_c example and want to shutdown the device to save power.

nrf_ble_scan_stop();
nrf_gpio_cfg_sense_input(9, NRF_GPIO_PIN_PULLUP, NRF_GPIO_PIN_SENSE_LOW);
err_code = sd_power_system_off();
APP_ERROR_CHECK(err_code);

This code seems to work fine. After inserting the command "nrf_ble_scan_stop()" current consumtion dropped down to 0.5 µA. That's pretty good for our application.

But after waking up the program should continue at the point where the shutdown occured instead of starting from reset state. So I tried to insert the following line:

NRF_POWER->RAMON |= (POWER_RAMON_OFFRAM0_RAM0On << POWER_RAMON_OFFRAM0_Pos) | \
                    (POWER_RAMON_OFFRAM1_RAM1On << POWER_RAMON_OFFRAM1_Pos);

nrf_ble_scan_stop();
nrf_gpio_cfg_sense_input(9, NRF_GPIO_PIN_PULLUP, NRF_GPIO_PIN_SENSE_LOW);

NRF_POWER->RAMON |= (POWER_RAMON_OFFRAM0_RAM0On << POWER_RAMON_OFFRAM0_Pos) | \
                    (POWER_RAMON_OFFRAM1_RAM1On << POWER_RAMON_OFFRAM1_Pos);

err_code = sd_power_system_off();
APP_ERROR_CHECK(err_code);

Unfortunately the result was a fatal error (RTT: <error> app: Fatal error). The device resets itself instantly after shutting down.

What could be wrong? Thank you in advance for your assistance.

[haakonsh]

You need to use SystemON because SystemOFF resets the device on wake-up. 

[lee-marc]

Hello haakonsh,

Thanks four your reply. What do you mean with SystemON and SystemOFF?
Commands SystemON and SystemOFF don't exist. Also sd_power_system_on() doesn't exit.

Is there any documentation about shutdown and RAM retention?

[haakonsh]

See POWER — Power supply.

You can retain information in RAM in System OFF, but the device will still reset on wakeup from System OFF. 

See nrf_pwr_mgmt_run API docs.

[lee-marc]

haakonsh said:

You can retain information in RAM in System OFF, but the device will still reset on wakeup from System OFF. 

If System OFF can't retain RAM, which state does?

haakonsh said:

See nrf_pwr_mgmt_run API docs.

I can't find any documentaion about RAM retention there.

[haakonsh]

SystemOFF can retain RAM, but the device WILL reset upon wakeup from SystemON. After a reset the CPU will start execution from the reset_vector and the CPU registers are gone. 

RAM, CPU registers, and Call Stack are retained in SystemON. 



 

[lee-marc]

What is the benefit of retaining RAM in system off mode if it will be reset at wakeup?

Which ultra low power state can retain RAM and return back into normal state without reset?

In our application we want to save power if the device is not used. But RAM must be retained.

[haakonsh]

"What is the benefit of retaining RAM in system off mode if it will be reset at wakeup?"
The RAM is not reset at wakeup, but the CPU and its registers are. 

"Which ultra low power state can retain RAM and return back into normal state without reset?

In our application we want to save power if the device is not used. But RAM must be retained."

System ON, IDLE.

[lee-marc]

Ok, I see. But I don't understand the benefit of RAM retention if it's not possible to resume program code.

I was thinking that nRF52832 could go into ultra low power mode and resume program code after going back to normal mode.

[haakonsh]

If you're using a basic state machine for program flow then you can save its state and continue its execution after you've reset the CPU and initialized whatever you need to initialize. You can also use retained RAM as storage.

[lee-marc]

But how to do this?
Where can I find a documentation about implementing RAM retention correctly?

[haakonsh]

See chapter 8 Memory, and 18 POWER — Power supply of the Prod Spec. 

We do not have any example or library that covers this topic in our SDKs. 

I suggest you start by storing a single variable in a section of RAM, turn on RAM retention on that section, go to SystemOFF and wake up again, then read back the variable to verify that your data has been retained. 
Do note that the lower part of the RAM is used by the SoftDevice and application and it should not be used to retain data.