STM32F103RB中的串口和端口

从《STM8S和STM32选型手册》中，可以得到如上图的STM32F103RB的一些硬件参数。
串口（USART）有3个，IO端口有51个。

从如下的原理图中看到，3个串口的引脚分别是（接收数据的引脚Rx，发送数据的引脚Tx）：
PA9/U1_TX/TIM1_CH2
PA10/U1_RX/TIM1_CH3
PA2/U2_TX/ADC2/TIM2_CH3
PA3/U2_RX/ADC3/TIM2_CH4
PB10/I2C2_SCL/U3_TX
PB11/I2C2_SDA/U3_RX

51个IO端口的引脚分别是：PA(0~15)16个，PB(0~15)16个，PC(0~15)16个，PD(0~2)3个，共51个。
PA0/WKUP/ADC0/TIM2_CH1_ETR
PA1/ADC1/TIM2_CH2
PA2/U2_TX/ADC2/TIM2_CH3
PA3/U2_RX/ADC3/TIM2_CH4
PA4/SPI1_NSS/ADC4
PA5/SPI1_SCK/ADC5
PA6/SPI1_MISO/ADC6/TIM3_CH1
PA7/SPI1_MOSI/ADC7/TIM3_CH2
PA8/TIM1_CH1/MCO
PA9/U1_TX/TIM1_CH2
PA10/U1_RX/TIM1_CH3
PA11/CAN_RX/USBDM/TIM1_CH4
PA12/CAN_TX/USBDP/TIM1_ETR
PA13/JTMS/SWDIO
PA14/JTCK/SWCLK
PA15/JTDI
PB0/ADC8/TIM3_CH3 PC8
PB1/ADC9/TIM3_CH4
PB2/BOOT1
PB3/JTDO/TRACESWO
PB4/JNTRST
PB5/I2C1_SMBAI
PB6/I2C1_SCL/TIM4_CH1
PB7/I2C1_SDA/TIM4_CH2
PB8/TIM4_CH3
PB9/TIM4_CH4
PB10/I2C2_SCL/U3_TX
PB11/I2C2_SDA/U3_RX
PB12/SPI2_NSS/I2C2_SMBAI/TIM1_BKIN
PB13/SPI2_SCK/TIM1_CH1N
PB14/SPI2_MISO/TIM1_CH2N
PB15/SPI2_MOSI/TIM1_CH3N
PC0/ADC10
PC1/ADC11
PC2/ADC12
PC3/ADC13
PC4/ADC14
PC5/ADC15
PC6
PC7
PC8
PC9
PC10
PC11
PC12
PC13/ANT1
PC14/OSC32
PC15/OSC32
PD0/OSCIN
PD1/OSCOUT
PD2/TIM3_ETR