• STM32L433 microcontroller selection and integration
• 3.3 V rail generation for the MCU and support circuitry
• Reset, clock, decoupling, boot configuration, and SWD debug interface for the MCU
• Wiring the required host UART and control signals between the MCU and the Blues modem connector
• Integration of an external SPI Arducam Mega 3MP NoIR camera and an external Dorhea 850 nm IR illuminator through one board-mounted 8-pin JST XH connector
• Integration of an MCU-controlled switched 3.3 V path for the IR illuminator on that combined connector
• Integration of a dedicated 4-wire external illuminated pushbutton connector with one STM32 GPIO for button input and one STM32 GPIO for LED control
• Board-level metadata and architecture updates for the MCU plus modem control subsystem

• Battery charging, fuel gauging, or chemistry-specific charge management
• Full breakout of every unused modem connector signal not required for host control

• External 12 V battery input through barrel connector
• 5 V regulated modem supply delivered to the Blues M.2 connector
• 3.3 V regulated rail for the STM32L433 domain and the combined external camera / IR connector
• Switched 3.3 V rail for the IR power pin on the combined external connector
• STM32L433 Blues modem host interface over UART plus required control/status lines
• STM32L433 Arducam Mega camera interface over SPI plus dedicated chip select
• STM32L433 GPIO-controlled enable for the external IR illuminator high-side switch
• STM32L433 active-low external pushbutton input on PC13 and low-side LED control output on PB13 through a dedicated 4-wire connector
• SWD debug header for programming and debug access

Diagram

• The board shall accept external power from a 12 V LiPo or NiCad battery pack using a barrel connector.
• The board shall generate a regulated 5 V rail dedicated to the Blues modem input.
• The board shall generate a regulated 3.3 V rail for the STM32L433 domain.
• The board shall route the 5 V rail to the existing M.2 connector used by the Blues modem card.
• The board shall provide a reliable host MCU interface between the STM32L433 and the Blues modem connector.
• The board shall provide a combined 8-pin external board connector for the Arducam Mega 3MP NoIR SPI camera module and the Dorhea 850 nm IR illuminator module.
• The board shall switch the IR illuminator 3.3 V supply from an MCU GPIO so the illuminator can be disabled during sleep.

• Input power: nominal 12 V battery source
• Key rails: protected battery input rail, regulated 5 V modem rail, regulated 3.3 V MCU rail, and a switched 3.3 V IR illuminator rail
• Modem rail must support cellular burst current with design margin; target regulator capability is 3 A
• The MCU domain shall run at 3.3 V and only connect to modem signals with compatible logic levels
• The camera interface shall use SPI-compatible 3.3 V logic levels and a power rail capable of supporting up to 317 mA camera working current
• The combined external connector shall expose this pinout: pin 1 = GND, pin 2 = 3V3_MCU, pin 3 = CAM_SCLK, pin 4 = CAM_MISO, pin 5 = CAM_MOSI, pin 6 = CAM_CS, pin 7 = IR_ILLUM_3V3_SW, pin 8 = GND
• The IR illuminator switched path shall default OFF when the MCU GPIO is high-impedance during reset or sleep
• External power input shall include reverse-polarity protection and transient suppression
• The board shall provide a dedicated 4-wire connector for the external illuminated pushbutton with pin 1 = GND, pin 2 = VIN_12V LED supply, pin 3 = button sense, and pin 4 = LED switched return
• The external pushbutton input shall connect to STM32L433 PC13 through a pull-up, RC filtering, and connector-side ESD protection
• The external pushbutton LED shall be controlled from STM32L433 PB13 through a board-side low-side MOSFET with a default-OFF pull-down at reset

• Board size is fixed at 127 mm x 127 mm
• Stackup is fixed at standard 4 layer
• CN1 is already placed near the lower board edge
• Power input connector must support board-edge access and not interfere with corner mounting holes
• The SWD/debug connector should be reachable from a board edge for programming access

• Prefer readily sourceable library parts and mechanically robust connector choices
• Use a switching regulator, not an LDO, because 12 V to 5 V conversion at modem current would dissipate excessive heat linearly
• Keep power loop area small around the buck regulator and reserve short, wide paths for modem power delivery
• Keep the camera connector in the 3.3 V logic domain to avoid level shifting between the STM32L433 and the external SPI camera module
• Use a high-side switched 3.3 V path for the IR illuminator so the module is not back-powered through its ground return and can be fully disabled in sleep

• Final verification of Blues modem host UART and control pin mapping from modem documentation
• Availability of a suitable STM32L433 library part and a compatible SWD header in the library
• Physical cable / mating harness definition for the combined external camera + IR module assembly

• Cellular burst current can cause brownout if the 5 V regulator and output bulk capacitance are undersized
• Incorrect modem host pin mapping could prevent modem bring-up or firmware control
• Logic-level mismatch between 3.3 V MCU I/O and any modem-side pins could damage the MCU or cause unreliable operation
• External battery input can be reverse-connected or expose the board to cable transients
• External camera cable attachment increases risk of ESD coupling on the SPI lines unless protected in a future refinement
• External IR cable attachment can inject transients onto the switched 3.3 V rail unless the connector-side TVS and local decoupling are kept in the design

• Schematic includes the STM32L433 MCU and required support circuitry
• Schematic includes a complete 3.3 V MCU supply path and debug access
• Required host interface signals are wired between the MCU and the modem connector
• Combined external camera + IR connector is wired so the camera retains its dedicated SPI interface and 3.3 V power while the IR module receives a separate switched 3.3 V feed with default-off behavior
• Design remains consistent with the existing 127 mm x 127 mm 4-layer board and connector placement intent

• Verify M.2 modem host signal mapping against Blues documentation
• Review schematic connectivity and floating pins after modifications
• Review MCU decoupling, reset, clock, and SWD connectivity against STM32 documentation
• Review 3.3 V and 5 V power-path integrity and PCB placement constraints
• Verify combined connector pinout, available 3.3 V rail capacity, switched-rail labeling, and default-off gate bias before production release

• Updated schematic
• Updated layout constraints and placement intent notes
• BOM entries for the MCU support subsystem and any new debug/power parts

• If a true locking barrel jack is unavailable in the library, use the closest mechanically robust board-edge barrel jack and document the intended locking family for later substitution
• External camera + IR connector is currently assigned as a JST XH 8-pin board header compatible with an 8-position JST XH cable assembly

• Added requirement for STM32L433 host MCU integration
• Added dedicated 3.3 V MCU power architecture
• Added modem host-interface requirement between MCU and CN1
• Added dedicated USB Type-C field firmware-update interface for STM32 USB DFU
• Added explicit requirement that USB VBUS remain isolated from VIN_12V and 5V_MODEM so the board cannot be unintentionally bench-powered from the update connector
• Added combined external camera + IR connector requirement with preserved camera SPI nets, dedicated camera 3.3 V power, and MCU-controlled switched IR 3.3 V power path
• Added dedicated external illuminated pushbutton interface requirement using PC13 for button input and PB13 for LED control on a new 4-wire connector

• Confirm exact Blues modem host UART and control pin mapping from the modem documentation
• Confirm whether external HSE crystal is required by firmware, or if the internal oscillator is acceptable
• Confirm whether additional ESD or series damping is desired on the external camera SPI harness
• Confirm the final external cable assembly used for the combined camera + IR harness
• Confirm whether the VIN_12V-driven illumination level is acceptable for the selected 24 V pushbutton LED variant, or if a later dedicated 24 V LED supply is needed

• USB connector is for firmware update only, not normal board power input.
• USB data lines shall connect only to STM32L433 PA11 and PA12 through ESD protection.
• USB VBUS shall remain local to the connector and protection network and shall not feed VIN_12V, 5V_MODEM, or 3V3_MCU.
• USB ground shall connect to board ground.
• Existing SWD header remains available and unmodified.

• USB Type-C receptacle with CC1 and CC2 pull-down resistors for USB device role.
• ESD protection for USB D+ and D-.
• Small local VBUS capacitor only for connector-side transient filtering.