STM32 Pin System Deep Dive — AF, GPIO Ports, Pin Mapping

STM32H743VITx LQFP-100 pinout diagram

2.1 Physical Pins vs. GPIO Ports

To understand the STM32 pin system, you need to distinguish between two different "pin numbers":

1. Physical pin number: The leg number on the IC package (LQFP-100: pins 1–100)
2. GPIO port name: The software name like PA0, PB3, PC13

GPIO port naming convention:

P + [Port letter] + [Pin number]
│    │              │
│    A~K            0~15
│    (port group)   (pin number within group)
GPIO

Examples:

• PA0 = Port A, Pin 0
• PB7 = Port B, Pin 7
• PD1 = Port D, Pin 1

Available GPIO ports on the STM32H743VITx (LQFP-100):

Port	Available Pins	Notes
GPIOA	PA0–PA15	All 16 pins available
GPIOB	PB0–PB15	All 16 pins available
GPIOC	PC0–PC13	14 pins (PC14/15 are OSC32)
GPIOD	PD0–PD15	All 16 pins available
GPIOE	PE0–PE15	All 16 pins available

⚠️ Note: GPIOF–GPIOK are not available on the LQFP-100 package (no physical pins). These ports are only accessible on packages with 144 pins or more.

2.2 LQFP-100 Physical Pin to GPIO Mapping

Mapping between physical pin numbers and GPIO names for the STM32H743VITx (LQFP-100):

                      Pins 76–100 (top)
                ┌──────────────────────┐
                │  76 77 78 ... 99 100 │
        Pins    │                      │  Pins
       51–75    │                      │  1–25
       (right)  │    STM32H743VITx     │  (left)
                │      LQFP-100        │
                │                      │
                │  51 50 49 ... 27 26  │
                └──────────────────────┘
                      Pins 26–50 (bottom)

Key pin mappings (commonly used):

Physical Pin	GPIO	Main Functions	AF Examples
14	PA0	ADC1_IN0, TIM2_CH1	AF1(TIM2), AF-(Analog)
15	PA1	ADC1_IN1, TIM2_CH2	AF1(TIM2), AF-(Analog)
16	PA2	ADC1_IN2, USART2_TX	AF7(USART2), AF-(Analog)
17	PA3	ADC1_IN3, USART2_RX	AF7(USART2), AF-(Analog)
20	PA4	ADC1_IN4, SPI1_NSS, DAC1_OUT1	AF5(SPI1)
21	PA5	ADC1_IN5, SPI1_SCK, DAC1_OUT2	AF5(SPI1)
22	PA6	ADC1_IN6, SPI1_MISO, TIM3_CH1	AF5(SPI1)
23	PA7	ADC1_IN7, SPI1_MOSI, TIM3_CH2	AF5(SPI1)
68	PA8	TIM1_CH1, MCO1	AF1(TIM1), AF0(MCO1)
69	PA9	USART1_TX, TIM1_CH2	AF7(USART1)
70	PA10	USART1_RX, TIM1_CH3	AF7(USART1)
71	PA11	USB_OTG_FS_DM, FDCAN1_RX	AF9(FDCAN1)
72	PA12	USB_OTG_FS_DP, FDCAN1_TX	AF9(FDCAN1)
76	PA13	SWDIO (debugger)	Debug only — do not reassign!
77	PA14	SWCLK (debugger)	Debug only — do not reassign!
78	PA15	SPI3_NSS, TIM2_CH1	AF6(SPI3)
35	PB0	ADC1_IN8, TIM3_CH3	AF2(TIM3)
36	PB1	ADC1_IN9, TIM3_CH4	AF2(TIM3)
37	PB2	QUADSPI_CLK	AF9(QUADSPI)
89	PB3	SPI3_SCK, TIM2_CH2	AF6(SPI3)
90	PB4	SPI3_MISO, TIM3_CH1	AF6(SPI3)
91	PB5	SPI3_MOSI, FDCAN2_RX	AF6(SPI3), AF9(FDCAN2)
92	PB6	USART1_TX, FDCAN2_TX, I2C1_SCL	AF7(USART1), AF9(FDCAN2)
93	PB7	USART1_RX, I2C1_SDA	AF7(USART1), AF4(I2C1)
95	PB8	FDCAN1_RX, I2C1_SCL, TIM4_CH3	AF9(FDCAN1), AF4(I2C1)
96	PB9	FDCAN1_TX, I2C1_SDA, TIM4_CH4	AF9(FDCAN1), AF4(I2C1)
47	PB10	USART3_TX, I2C2_SCL, TIM2_CH3	AF7(USART3)
48	PB11	USART3_RX, I2C2_SDA, TIM2_CH4	AF7(USART3)
51	PB12	SPI2_NSS, USART3_CK	AF5(SPI2)
52	PB13	SPI2_SCK, FDCAN2_TX	AF5(SPI2), AF9(FDCAN2)
53	PB14	SPI2_MISO, USART3_RTS	AF5(SPI2)
54	PB15	SPI2_MOSI, TIM12_CH2	AF5(SPI2)
8	PC0	ADC1_IN10	AF-(Analog)
9	PC1	ADC1_IN11	AF-(Analog)
10	PC2	ADC1_IN12, SPI2_MISO	AF5(SPI2)
11	PC3	ADC1_IN13, SPI2_MOSI	AF5(SPI2)
33	PC4	ADC1_IN14	AF-(Analog)
34	PC5	ADC1_IN15	AF-(Analog)
63	PC6	USART6_TX, TIM8_CH1	AF7(USART6)
64	PC7	USART6_RX, TIM8_CH2	AF7(USART6)
65	PC8	TIM8_CH3, SDMMC1_D0	AF3(TIM8)
66	PC9	TIM8_CH4, SDMMC1_D1	AF3(TIM8)
67	PC10	USART3_TX, SPI3_SCK	AF7(USART3)
79	PC11	USART3_RX, SPI3_MISO	AF7(USART3)
80	PC12	SPI3_MOSI, UART5_TX	AF6(SPI3)
7	PC13	RTC_TAMP, WKUP	General-purpose GPIO
82	PD0	FDCAN1_RX, FMC_D2	AF9(FDCAN1)
83	PD1	FDCAN1_TX, FMC_D3	AF9(FDCAN1)
84	PD2	UART5_RX, TIM3_ETR	AF8(UART5)
85	PD3	USART2_CTS, FMC_CLK	AF7(USART2)
86	PD4	USART2_RTS, FMC_NOE	AF7(USART2)
87	PD5	USART2_TX, FMC_NWE	AF7(USART2)
88	PD6	USART2_RX, FMC_NWAIT	AF7(USART2)
55	PD8	USART3_TX	AF7(USART3)
56	PD9	USART3_RX	AF7(USART3)
100	PE0	UART8_RX, TIM4_ETR	AF8(UART8)
1	PE1	UART8_TX	AF8(UART8)
2	PE2	SPI4_SCK, UART8_TX	AF5(SPI4)
98	PE9	TIM1_CH1, FMC_D6	AF1(TIM1) — for PWM
99	PE11	TIM1_CH2, FMC_D8	AF1(TIM1) — for PWM

2.3 Pin Function Categories

The 100 pins on the LQFP-100 fall into four broad categories:

Category	Pins	Count	Description
Power	VDD, VSS, VDDA, VSSA, VREF+, VCAP	~18	Power supply (3.3V, GND)
Clock	OSC_IN, OSC_OUT (PH0/PH1)	2	External crystal connection
Reset	NRST	1	System reset (active low)
Debug	PA13 (SWDIO), PA14 (SWCLK)	2	SWD debugger only
Boot	BOOT0	1	Boot mode selection
GPIO	PA0–PE15	~76	General-purpose I/O + AF

🔴 Hard rule: PA13/PA14 are the SWD debugger pins. Reassigning them to any other function will make debugging and programming impossible. Always reserve them for SWD.

2.4 Alternate Function (AF) System

One of the most important concepts in STM32. A single GPIO pin can be configured for up to 16 different functions.

AF numbering (AF0–AF15):

AF Number	Main Functions
AF0	SYS (MCO, SWD, TRACE)
AF1	TIM1, TIM2
AF2	TIM3, TIM4, TIM5
AF3	TIM8, TIM15-17
AF4	I2C1-4
AF5	SPI1-6
AF6	SPI2/3, SAI1/2
AF7	USART1-3, USART6
AF8	UART4/5, UART7/8, LPUART1
AF9	FDCAN1/2, QUADSPI, TIM12-14
AF10	USB OTG, SAI2
AF11	ETH, UART7
AF12	FMC, SDMMC, MDIOS
AF13	DCMI, COMP
AF14	LTDC, UART5
AF15	EVENTOUT

AF0–AF15 multiplexer — select one of 16 functions per pin

AF example — PA7:

PA7 can be assigned any of the following:

• AF0: MCO (microcontroller clock output)
• AF2: TIM3_CH2 (Timer 3 channel 2 = PWM output)
• AF5: SPI1_MOSI (SPI1 master output)
• Analog: ADC1_IN7 (analog input)

Only one AF can be active at a time. If PA7 is configured as SPI1_MOSI, ADC and TIM3 cannot be used on that pin simultaneously.

How to avoid AF conflicts:

1. List all the peripherals you need (CAN, UART, SPI, ADC, PWM, etc.)
2. Check the pin options for each peripheral (Datasheet Tables 9–12)
3. Arrange pins so AFs don't overlap → CubeMX checks for conflicts automatically

2.5 Pin Configuration Registers

Each GPIO pin is controlled through the following registers:

Register          Bits/pin    Function
──────────────────────────────────────────────
GPIOx_MODER      2 bits/pin  Mode selection
                              00 = Input
                              01 = Output
                              10 = Alternate Function
                              11 = Analog

GPIOx_OTYPER     1 bit/pin   Output type
                              0 = Push-Pull
                              1 = Open-Drain

GPIOx_OSPEEDR    2 bits/pin  Output speed
                              00 = Low (up to 12 MHz)
                              01 = Medium (up to 60 MHz)
                              10 = High (up to 85 MHz)
                              11 = Very High (up to 100 MHz)

GPIOx_PUPDR      2 bits/pin  Pull-up/pull-down
                              00 = No Pull
                              01 = Pull-Up
                              10 = Pull-Down
                              11 = Reserved

GPIOx_AFRL       4 bits/pin  AF select (pins 0–7)
GPIOx_AFRH       4 bits/pin  AF select (pins 8–15)
                              0000 = AF0
                              0001 = AF1
                              ...
                              1111 = AF15

Direct register example (configure PA7 as SPI1_MOSI with AF5):

// 1. Enable GPIOA clock
RCC->AHB4ENR |= RCC_AHB4ENR_GPIOAEN;

// 2. Set PA7 to Alternate Function mode
GPIOA->MODER &= ~(0x3 << (7 * 2));     // clear bits
GPIOA->MODER |=  (0x2 << (7 * 2));     // 10 = AF mode

// 3. Select AF5 (SPI1) — PA7 is pin 0–7, so use AFRL
GPIOA->AFR[0] &= ~(0xF << (7 * 4));    // clear bits
GPIOA->AFR[0] |=  (0x5 << (7 * 4));    // AF5 = SPI1

// 4. Push-Pull, Very High Speed
GPIOA->OTYPER &= ~(1 << 7);            // Push-Pull
GPIOA->OSPEEDR |= (0x3 << (7 * 2));    // Very High Speed

In practice you'll use the HAL library, so direct register manipulation is rare. That said, understanding the register layout lets you read register values during debugging and diagnose problems quickly.

2.6 Finding Pin Information in the Datasheet

You need two documents to look up pin information for the STM32H743VITx:

Document	Contents	What to look for
Datasheet (DS12110)	Pinout, electrical characteristics, package	Physical pin numbers, AF tables, voltage/current specs
Reference Manual (RM0433)	Registers, peripheral operation	GPIO registers, timer config, CAN protocol, etc.

Key sections to read in the datasheet:

• Tables 9–12: "Alternate function mapping" — full AF0–AF15 mapping for every pin
• Figure 1: LQFP-100 pinout diagram
• Table 7: "Pin definitions" — power/IO type and 5V-tolerance for each pin

💡 Tip: CubeMX lets you browse these tables in a GUI, but getting comfortable reading the datasheet directly pays off significantly when doing hardware design.

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