Speed up start up after leaving system off

[Marelcom]

Hi

We use the nRF52832 for an NFC device. The device is 99.9 %  in the system off state to save power as much as possible. because the device is power by a button cell battery. The device has an e-paper and the display data are transfered over NFC. The user has the possible the change the displayed data with a external button.

uP = nRF52832
SDK = nRF Connect v2.6.1

In two case the device must wake up.
- User pushes the button
- Device receives new Data over NFC

At the moment the device is running well. But our customer wants the have the possibility to do a DFU over BLE. After adding the follwing modules to the "prj.conf" file the start up takes about 1 second, after leaving the system off mode. This is way to long. The usere wants to have a reaction from device after around 100ms when pushing the external button.

# Enable Bluetooth for FOTA (DFU)

CONFIG_BT=y

CONFIG_BT_PERIPHERAL=y

CONFIG_BT_DEVICE_NAME="TestDevice"

# Enable MCUboot and FOTA

CONFIG_BOOTLOADER_MCUBOOT=y

CONFIG_NCS_SAMPLE_MCUMGR_BT_OTA_DFU=y

Is there a way to shorten the start up time from system off mode?
Like enabling the moduls for FOTA (DFU) only when the useres wants to start the DFU mode while pushing a button and save the maximum power in deep sleep mode.

thanks and best regards
Markus

[Marelcom]

Finaly this case could be solved. Here is the summary:

Nordic: I described the approach for sysbuild, but you are using the old child-parent image (which I should have assumed when you wrote you use SDK 2.6. So there is a slight difference. You should add the same config (CONFIG_BOOT_VALIDATE_SLOT0=n), but the folder name where you place mcuboot.conf must be "child_image" and not "sysbuild".

Markus: After adding mcuboot.conf file to the child_image folder (see screenshot) and CONFIG_BOOT_VALIDATE_SLOT0=n then it works.
In the file "mcuboot/zephyr/.config" CONFIG_BOOT_VALIDATE_SLOT0 is not added any more (see screenshot). Start up time is now decreased by 400ms around.

Nordic:  A second factor that could add 200-400 us to the startup time is if you have an external 32.768 kHz crystall, as this takes a significant amount of time to start (depending on the crystal). If so, you can disable the LFXO and use LFRC intead by setting CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y (remembed to change in the board files, or alternatively both in the bootloader and application configuration, so that there is no conflict).

Markus: while changing CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y then the start time is shortened again by almost 400ms. Current start up time is about 50ms. That's fast enough and I can move on.

thanks a lot for your very helpful support
bes regards
Markus