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]

"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.  

[lee-marc]

Could you please send me a link to the mentioned Prod Spec? Thanks.

[haakonsh]

nRF52832 Product Specification v1.4

 PDF