AP03B STM32 MCU

Documentation: Design reference available; AP03B upload instructions in preparation

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Read this official Leafony page and explain its key points in English. For hardware help, first ask for a photo or the installed leaf models, assembly and power sources, then the goal and tools. Clarify which LED or display changed and whether a cable or leaf was replaced. Do not equate an unseen reset button with an absent one, or I2C acknowledgement with a working display. Verify specifications against sources; distinguish observations from hypotheses. Follow a redirect to its destination; if unreadable, ask for the relevant text instead of guessing.

Page: AP03B STM32 MCU

URL: https://docs.leafony.com/en/leaf/processor/ap03b/

Description / headings (excerpt): AP03B STM32L452REI6 specifications, Leafony Bus connections, BOOT0 and reset operation, and design documents. / Features / Specifications / Power and external connections / Pin connections

These reference images were generated from the KiCad board data. Component 3D models are omitted, and pads for unpopulated components are included.

Features

This MCU (microcontroller) leaf runs programs to read sensors and control communication leaves and other modules. It is a redesigned version of AP03 STM32 MCU, using the STMicroelectronics STM32L452REI6.

It connects to other leaves through the Leafony Bus and operates from 3.3V. The board includes a PG switch for BOOT0, a reset switch, and an LED indicating the BOOT0 state. It is the MCU leaf in the Cellular LPWA Cat-M1 Kit.

Specifications

Item	Description
MCU	STMicroelectronics STM32L452REI6
CPU	Arm Cortex-M4 with FPU
Maximum clock	80 MHz (MCU specification)
Flash memory	512 KB
SRAM	160 KB
Power	Leafony Bus 3.3V / GND
Low-speed external clock	32.768 kHz crystal (X1)
Main bus signals	D0–D13, A0–A5, SDA / SCL, SWDIO / SWCLK, RESET
Bus contacts	29 contacts on each side (CN1)
Board dimensions	20 × 20 mm (PCB design dimensions)
Board thickness / layers	0.8 mm, 6 layers (excluding component height)
Design revision	v1.0, April 27, 2026

See STMicroelectronics product information for CPU and memory specifications. The actual clock depends on the firmware configuration.

Power and external connections

Supply the Leafony Bus 3.3V terminal from a power leaf or other suitable source. The VBUS terminal is not connected to the AP03B circuitry. There is no USB connector on the board; provide a separate interface for a computer or debugger.

SDA and SCL each have a 10 kΩ pull-up resistor to 3.3V. Before connecting external devices, check the Leafony Bus specification and the other device’s power and signal voltages.

AREF connection resistor R5 is DNP (not populated), so the bus AREF terminal is not connected to the MCU’s analog supply/reference circuit in the standard configuration.

Pin connections

The following table describes the standard Leafony Bus to MCU connections. Front and back contacts with the same number (such as F1 / B1) carry the same signal. MCU port names are schematic names, not Arduino sketch pin numbers.

Contact (front / back)	Leafony Bus signal	MCU port / connection
F1 / B1	3.3V	3.3V power
F2 / B2	SWDIO	PA13
F3 / B3	VBUS	Not connected to the MCU or power circuit
F4 / B4	AREF	Not connected to MCU because R5 is not fitted
F5 / B5	RESET	NRST
F6 / B6	A0	PA4
F7 / B7	D6	PA8
F8 / B8	A1	PA0
F9 / B9	D7	PB12
F10 / B10	A2	PA1
F11 / B11	D8	PA9
F12 / B12	A3	PB0
F13 / B13	D9	PA10 (through U2; connected when BOOT0 is low)
F14 / B14	A4	PC1
F15 / B15	D10	PB6
F16 / B16	A5	PC0
F17 / B17	D11	PA7
F18 / B18	D0	PA3
F19 / B19	D12	PA6
F20 / B20	D1	PA2
F21 / B21	D13	PA5
F22 / B22	D2	PC7
F23 / B23	SDA	PB9 (R2: 10 kΩ pull-up to 3.3V)
F24 / B24	D3	PB3
F25 / B25	SCL	PB8 (R1: 10 kΩ pull-up to 3.3V)
F26 / B26	D4	PB5
F27 / B27	GND	GND
F28 / B28	D5	PB4
F29 / B29	SWCLK	PA14

Connections are based on the Leafony Bus schematic and MCU schematic. R5, R7, R12 and R14 are not populated.

Switches and LED

Component	Board marking	Function
SW1	PG	Selects BOOT0 high / low.
SW2	RST	Connects RESET to GND when pressed.
D1	D1	Lights when BOOT0 is high.

The LED indicates the BOOT0 state, not program execution or a successful upload. For boot-mode details, check the MCU configuration and the reset / program selection circuit.

D9 switching by BOOT0

Analog switch U2 is between D9 and PA10. They connect when BOOT0 is low and disconnect when it is high. Check the PG switch when combining the board with a communication leaf that uses D9. D0, D1, D6 and D7 connect directly to the MCU ports listed above.

Development environment

The following STM32 MCU documentation provides background:

These guides include AP03-specific instructions. For AP03B, check the pin mapping, upload interface and firmware board settings. The AP03B design repository does not yet include build settings, upload scripts or runnable example code.

Design documents

The product is named AP03B, but its design files are named leafony-ap03-stm32. Open them in an environment that reads KiCad 9.0 files, preserving the directory structure and dedicated libraries. Some component 3D models are not included in the repository.

Revision history

September 19, 2026: Added the AP03B STM32 MCU page.