Reducing ROM size for the nRF52833

[cal.rubbo]

I think I've followed practically every "minimal configuration" example I could find, and implemented as many Kconfig options as possible, but our firmware image is still too large for DFU. I also created a static partition scheme and reduced MCU boot partition to 32 kilobytes and NVS partition to 8 kilobytes.

Despite this, our ROM size comes out to be around 13 kilobytes too large for MCU boot.

Here's my full project configuration file, with the Kconfig options I've found that actually reduce ROM size at the top. Please let me know if there's any Kconfig options I'm missing or anything else I can do to reduce the ROM size.

# not yet sure if this is helpful, harmful, or just unnecessary... but it actually decreases the project size!
CONFIG_NCS_SAMPLES_DEFAULTS=y

# to reduce image size
CONFIG_CBPRINTF_NANO=y
CONFIG_CBPRINTF_LIBC_SUBSTS=n
CONFIG_MINIMAL_LIBC=y
CONFIG_SIZE_OPTIMIZATIONS=y
CONFIG_BT_ASSERT=n
CONFIG_BT_DATA_LEN_UPDATE=n
#CONFIG_BT_GATT_SERVICE_CHANGED=n # unfortunately this is required by DFU
CONFIG_BT_GATT_READ_MULTIPLE=n
CONFIG_BT_GATT_READ_MULT_VAR_LEN=n
CONFIG_BT_GAP_PERIPHERAL_PREF_PARAMS=n
CONFIG_BT_PHY_UPDATE=n
CONFIG_BT_CTLR_LE_PING=n
CONFIG_BT_CTLR_PRIVACY=n
CONFIG_BT_CTLR_PHY_2M=n
CONFIG_TIMEOUT_64BIT=n
CONFIG_TIMESLICING=n
CONFIG_LOG=n
CONFIG_FPU=y # despite guidance from "Minimal" examples, enabling this actually decreases ROM size
CONFIG_SPI=n
CONFIG_ASSERT=n # should be enabled while developing, disabled for production
CONFIG_CONSOLE=n # should be enabled while developing, disabled for production
CONFIG_SERIAL=n # should be enabled while developing, disabled for production
CONFIG_USE_SEGGER_RTT=n # is this required for development? what does this really do?

# for DFU (remote firmware upgrades)
CONFIG_BOOTLOADER_MCUBOOT=y
CONFIG_NCS_SAMPLE_MCUMGR_BT_OTA_DFU=y

# for reading / writing to flash
CONFIG_FLASH=y
CONFIG_FLASH_PAGE_LAYOUT=y
CONFIG_FLASH_MAP=y
CONFIG_NVS=y
CONFIG_MPU_ALLOW_FLASH_WRITE=y

# software resets
CONFIG_REBOOT=y

# for button, LED, accelerometer
CONFIG_GPIO=y
CONFIG_I2C=y

# for printing heap usage stats
CONFIG_SYS_HEAP_RUNTIME_STATS=y

# for bluetooth (general)
CONFIG_BT=y

# for bluetooth (advertising)
CONFIG_BT_PERIPHERAL=y # technically our application only requires "broadcaster" but "peripheral" is required for DFU
CONFIG_BT_DEVICE_APPEARANCE=512
CONFIG_BT_DEVICE_NAME="AirA7v100"
CONFIG_BT_EXT_ADV=y
CONFIG_BT_EXT_ADV_MAX_ADV_SET=3

# for bluetooth (tx power control)
CONFIG_BT_HCI_VS_EXT=y
CONFIG_BT_CTLR_TX_PWR_DYNAMIC_CONTROL=y

# for bluetooth (scanning)
CONFIG_BT_CENTRAL=y
CONFIG_BT_SCAN=y
CONFIG_BT_ID_MAX=1
CONFIG_BT_SCAN_FILTER_ENABLE=y
CONFIG_BT_SCAN_NAME_CNT=2

# for bluetooth (connecting)
CONFIG_BT_SMP=y
CONFIG_BT_SMP_SC_ONLY=n
CONFIG_BT_SMP_APP_PAIRING_ACCEPT=n
CONFIG_BT_SMP_ENFORCE_MITM=y
CONFIG_BT_BONDABLE=n

# for blueooth (GATT)
CONFIG_BT_GATT_CLIENT=y
CONFIG_BT_GATT_DM=y
CONFIG_BT_GATT_DM_DATA_PRINT=y

# for reading voltage
CONFIG_NRFX_SAADC=y
CONFIG_NRFX_TIMER2=y
CONFIG_NRFX_PPI=y

# for power management
CONFIG_PM_DEVICE=y

# for power-off state
CONFIG_POWEROFF=y

[dejans]

Hi,

There are some resources which might help you in reducing ROM usage. For example, there are some general recommendations how to optimize memory footprint, how to optimize Zephyr for footprint, and minimal footprint sample. On the Bluetooth side, you can look at configuration options which deal with stack sizes and buffers. Finally, you can look at minimal configuration given in minimal.conf file of the Peripheral LBS and Peripheral UART samples.

Best regards,
Dejan

[cal.rubbo]

Thanks Dejan, however I believe I've gone through everything you've linked.

My question is, outside of what you've posted, are there any additional Kconfig parameters or guidance to reduce ROM size? Because I've found some on my own, and I imagine there are more, but I'm not an expert in Zephyr so I'm sort-of shooting in the dark.

We need to reduce the ROM size by at least 10 more kilobytes for DFU to be functional, so mainly I'm wondering: does that seem feasible based on the information I've given you?

Thanks for your help,
Cal

---

"For example, there are some general recommendations how to optimize memory footprint, how to optimize Zephyr for footprint, and minimal footprint sample."

Yes, I've looked through those and I believe I've implemented everything possible (i.e. didn't break our application, actually reduced ROM size, and wasn't already disabled by default). Unfortunately most of the guidance combines ROM and RAM reduction tips, however I spent a bunch of time going through each one to see which Kconfigs actually reduced ROM size.

Also, I've even found some more Kconfigs that weren't mentioned the documentation you listed. For instance, today I just found a couple more that reduced ROM size in this Github issue

"On the Bluetooth side, you can look at configuration options which deal with stack sizes and buffers."

I've found that none of those configuration options reduce the ROM size. They reduce the RAM size. 

"Finally, you can look at minimal configuration given in minimal.conf file of the Peripheral LBS and Peripheral UART samples."

Yes, this is where I found many of the Kconfig options that reduced ROM size.

[cal.rubbo]

Here you go! This is my mcuboot.conf placed in the child_image folder inside my application directory:

CONFIG_BOOT_SIGNATURE_TYPE_RSA=y
#CONFIG_BOOT_SIGNATURE_KEY_FILE="..."

CONFIG_BOOT_SERIAL_UART=n
CONFIG_UART_CONSOLE=n
CONFIG_CONSOLE=n
CONFIG_CONSOLE_HANDLER=n
CONFIG_MCUBOOT_SERIAL=n


CONFIG_SINGLE_APPLICATION_SLOT=y

According to the build output, this is how much space mcuboot is taking currently:

Memory region         Used Size  Region Size  %age Used
           FLASH:       26322 B        32 KB     80.33%
             RAM:       20672 B       128 KB     15.77%
        IDT_LIST:          0 GB         2 KB      0.00%

UPDATE: I didn't realize it was possible to make MCU boot smaller. I've got it down to 24 kilobytes, which still isn't quite enough. What's the smallest MCU boot can be? and what configuration options are required to make it that size? I think this is all the possible options: github.com/.../Kconfig

[dejans]

Hi,

Which board do you use?

Best regards,
Dejan

[cal.rubbo]

nRF52833

[dejans]

Hi,

I understand that you use nrf52833 SoC, but do you use nrf52833-dk or a custom board?

Best regards,
Dejan

[cal.rubbo]

Ah sorry about that. Right now, I'm testing my application on the nrf52833-dk, however it is going to eventually be for a custom board.

The custom board is very similar to the nrf52833-dk but with far less peripherals. 

[cal.rubbo]

Update: I was able to reduce the mcuboot size to 19 kilobytes using zephyr's "prj_minimal.conf" that I'm pasting below.

I also resized my partitions accordingly. I'm still about 4 kilobytes over the image limit!

child_image/mcuboot.conf

CONFIG_MAIN_STACK_SIZE=10240
CONFIG_MBEDTLS_CFG_FILE="mcuboot-mbedtls-cfg.h"

CONFIG_FLASH=y
CONFIG_FPROTECT=y
CONFIG_PM=n

CONFIG_BOOT_SWAP_SAVE_ENCTLV=n
CONFIG_BOOT_ENCRYPT_IMAGE=n

CONFIG_BOOT_BOOTSTRAP=n
CONFIG_BOOT_UPGRADE_ONLY=n

### Minimal Configurations ###
CONFIG_BOOT_USE_MIN_PARTITION_SIZE=y
CONFIG_ASSERT=n
CONFIG_BOOT_BANNER=n
CONFIG_CLOCK_CONTROL=n
CONFIG_CONSOLE=n
CONFIG_CONSOLE_HANDLER=n
CONFIG_GPIO=n
CONFIG_KERNEL_MEM_POOL=n
CONFIG_LOG=n
CONFIG_MINIMAL_LIBC_CALLOC=n
CONFIG_MINIMAL_LIBC_MALLOC=n
CONFIG_MINIMAL_LIBC_REALLOCARRAY=n
CONFIG_NCS_SAMPLES_DEFAULTS=n
CONFIG_NO_RUNTIME_CHECKS=y
CONFIG_NRF_RTC_TIMER=n
CONFIG_PRINTK=n
CONFIG_SECURE_BOOT_DEBUG=n
CONFIG_SERIAL=n
CONFIG_SIZE_OPTIMIZATIONS=y
CONFIG_SYS_CLOCK_EXISTS=n
CONFIG_UART_CONSOLE=n

[dejans]

Hi,

Which application do you use for testing? Do you use any of the NCS samples?

Best regards,
Dejan

[cal.rubbo]

I'm sorry but I'm a bit confused by the question.

I've used various NCS samples, yes. Using those samples, I've developed an application that does everything we need it to do. The application works well, but the issue is there isn't enough space for the application when I enable everything required by DFU.

[dejans]

Hi,

There are two more discussions that you may want to look at - build file size optimization and how to resize mcuboot partition in Zephyr.

In addition, you could try to use this config option which could decrease footprint by around 4 KB. 

Best regards,
Dejan

[cal.rubbo]

"There are two more discussions that you may want to look at - build file size optimization and how to resize mcuboot partition in Zephyr."

The guidance in the first link discusses NEWLIB_LIBC optimizations which don't apply to me, because I'm already using MINIMAL_LIBC (which is far leaner than NEWLIB_LIBC). It also references a few other Kconfig options that I've already disabled if you look at my original post. 

The guidance in the second link I've already utilized to shrink my MCU Boot partition. I've been able to shrink the MCU boot image to a little less than 19 kilobytes, which allowed me to resize the MCU boot partition 20 kilobytes.

"In addition, you could try to use this config option which could decrease footprint by around 4 KB."

If you look at my original post, you can see I already have that config option in my prj.conf file.

---

Just so we're clear, here is my latest project configuration environment. Note that this morning I found a few more Bluetooth options I could disable, so I'm now even closer: only 1.5 Kilobytes above the ROM size limitation!

app/prj.conf (application configuration)

# not yet sure if this is helpful, harmful, or just unnecessary... but it actually decreases the project size!
CONFIG_NCS_SAMPLES_DEFAULTS=y

# to reduce image size
CONFIG_CBPRINTF_NANO=y
CONFIG_CBPRINTF_LIBC_SUBSTS=n # untested
CONFIG_MINIMAL_LIBC=y
CONFIG_MINIMAL_LIBC_OPTIMIZE_STRING_FOR_SIZE=y
CONFIG_SIZE_OPTIMIZATIONS=y
CONFIG_BT_ASSERT=n
CONFIG_BT_DATA_LEN_UPDATE=n
#CONFIG_BT_GATT_SERVICE_CHANGED=n # unfortunately this is required by DFU
CONFIG_BT_GATT_READ_MULTIPLE=n
CONFIG_BT_GATT_READ_MULT_VAR_LEN=n
CONFIG_BT_GAP_PERIPHERAL_PREF_PARAMS=n
CONFIG_BT_PHY_UPDATE=n
CONFIG_BT_CTLR_LE_PING=n
CONFIG_BT_CTLR_PRIVACY=n
CONFIG_BT_CTLR_PHY_2M=n
CONFIG_TIMEOUT_64BIT=n
CONFIG_TIMESLICING=n
CONFIG_LOG=n
CONFIG_SPI=n
CONFIG_ASSERT=n # should be enabled while developing, disabled for production
CONFIG_CONSOLE=n # should be enabled while developing, disabled for production
CONFIG_SERIAL=n # should be enabled while developing, disabled for production

# to reduce image size even more (everything below doesn't break the application, but unknown how it affects DFU!)
CONFIG_BT_ATT_RETRY_ON_SEC_ERR=n
CONFIG_BT_GATT_AUTO_RESUBSCRIBE=n
CONFIG_BT_GATT_CACHING=n
CONFIG_BT_GATT_ENFORCE_SUBSCRIPTION=n

# for DFU (remote firmware upgrades)
CONFIG_BOOTLOADER_MCUBOOT=y # should be disabled while developing, enabled for production
CONFIG_NCS_SAMPLE_MCUMGR_BT_OTA_DFU=y # should be disabled while developing, enabled for production

# for reading / writing to flash
CONFIG_FLASH=y
CONFIG_FLASH_PAGE_LAYOUT=y
CONFIG_FLASH_MAP=y
CONFIG_NVS=y
CONFIG_MPU_ALLOW_FLASH_WRITE=y

# software resets
CONFIG_REBOOT=y

# for button, LED, accelerometer
CONFIG_GPIO=y
CONFIG_I2C=y

# for printing heap usage stats
CONFIG_SYS_HEAP_RUNTIME_STATS=y

# for bluetooth (general)
CONFIG_BT=y

# for bluetooth (advertising)
CONFIG_BT_PERIPHERAL=y # used to be CONFIG_BT_BROADCASTER=y but DFU BLE implementation requires peripheral
CONFIG_BT_DEVICE_APPEARANCE=512
CONFIG_BT_DEVICE_NAME="AirA7v100"
CONFIG_BT_EXT_ADV=y
CONFIG_BT_EXT_ADV_MAX_ADV_SET=3

# for bluetooth (tx power control)
CONFIG_BT_HCI_VS_EXT=y
CONFIG_BT_CTLR_TX_PWR_DYNAMIC_CONTROL=y

# for bluetooth (scanning)
CONFIG_BT_CENTRAL=y
CONFIG_BT_SCAN=y
CONFIG_BT_ID_MAX=1
CONFIG_BT_SCAN_FILTER_ENABLE=y
CONFIG_BT_SCAN_NAME_CNT=2

# for bluetooth (connecting)
CONFIG_BT_SMP=y
CONFIG_BT_SMP_SC_ONLY=n
CONFIG_BT_SMP_APP_PAIRING_ACCEPT=n
CONFIG_BT_SMP_ENFORCE_MITM=y
CONFIG_BT_BONDABLE=n

# for blueooth (GATT)
CONFIG_BT_GATT_CLIENT=y
CONFIG_BT_GATT_DM=y
CONFIG_BT_GATT_DM_DATA_PRINT=y

# for reading voltage
CONFIG_NRFX_SAADC=y
CONFIG_NRFX_TIMER2=y
CONFIG_NRFX_PPI=y

# for power management
CONFIG_PM_DEVICE=y
#CONFIG_PM_DEVICE_RUNTIME=y

# for power-off state
CONFIG_POWEROFF=y

app/child_image/mcuboot.conf (bootloader configuration)

CONFIG_MAIN_STACK_SIZE=10240
CONFIG_MBEDTLS_CFG_FILE="mcuboot-mbedtls-cfg.h"

CONFIG_FLASH=y
CONFIG_FPROTECT=y
CONFIG_PM=n

CONFIG_BOOT_SWAP_SAVE_ENCTLV=n
CONFIG_BOOT_ENCRYPT_IMAGE=n

CONFIG_BOOT_BOOTSTRAP=n
CONFIG_BOOT_UPGRADE_ONLY=n

### Minimal Configurations ###
CONFIG_BOOT_USE_MIN_PARTITION_SIZE=y
CONFIG_ASSERT=n
CONFIG_ASSERT_VERBOSE=n
CONFIG_BOOT_BANNER=n
CONFIG_CLOCK_CONTROL=n
CONFIG_CONSOLE=n
CONFIG_CONSOLE_HANDLER=n
CONFIG_GPIO=n
CONFIG_KERNEL_MEM_POOL=n
CONFIG_LOG=n
CONFIG_MINIMAL_LIBC_CALLOC=n
CONFIG_MINIMAL_LIBC_MALLOC=n
CONFIG_MINIMAL_LIBC_REALLOCARRAY=n
CONFIG_NCS_SAMPLES_DEFAULTS=n
CONFIG_NO_RUNTIME_CHECKS=y
CONFIG_NRF_RTC_TIMER=n
CONFIG_PRINTK=n
CONFIG_SECURE_BOOT_DEBUG=n
CONFIG_SERIAL=n
CONFIG_SIZE_OPTIMIZATIONS=y
CONFIG_SYS_CLOCK_EXISTS=n
CONFIG_UART_CONSOLE=n

# trying out some more (doesn't do much)
CONFIG_MPU=n
CONFIG_ARM_MPU=n
CONFIG_CBPRINTF_NANO=y
CONFIG_CBPRINTF_LIBC_SUBSTS=n
CONFIG_MINIMAL_LIBC=y

# trying out some more (doesn't do anything)
CONFIG_SPI=n
CONFIG_TIMEOUT_64BIT=n
CONFIG_TIMESLICING=n
CONFIG_MINIMAL_LIBC_OPTIMIZE_STRING_FOR_SIZE=y

app/pm_static.yml (static partition configuration, mcu == 20 kilobytes, nvs == 8 kilobytes)

app:
  address: 0x5200
  end_address: 0x41800
  region: flash_primary
  size: 0x3C600
mcuboot:
  address: 0x0
  end_address: 0x5000
  placement:
    before:
    - mcuboot_primary
  region: flash_primary
  size: 0x5000
mcuboot_pad:
  address: 0x5000
  end_address: 0x5200
  placement:
    align:
      start: 0x1000
    before:
    - mcuboot_primary_app
  region: flash_primary
  size: 0x200
mcuboot_primary:
  address: 0x5000
  end_address: 0x41800
  orig_span: &id001
  - mcuboot_pad
  - app
  region: flash_primary
  sharers: 0x1
  size: 0x3C800
  span: *id001
mcuboot_primary_app:
  address: 0x5200
  end_address: 0x41800
  orig_span: &id002
  - app
  region: flash_primary
  size: 0x3C600
  span: *id002
mcuboot_secondary:
  address: 0x41800
  end_address: 0x7E000
  placement:
    after:
    - mcuboot_primary
    align:
      start: 0x1000
  region: flash_primary
  share_size:
  - mcuboot_primary
  size: 0x3C800
nvs_storage:
  address: 0x7E000
  end_address: 0x80000
  placement:
    align:
      start: 0x1000
    before:
    - end
  region: flash_primary
  size: 0x2000
sram_primary:
  address: 0x20000000
  end_address: 0x20020000
  region: sram_primary
  size: 0x20000