USART_CR1 and USART_CR2 Registers in Microcontrollers

USART Registers

• USART Control 1 (USART_CR1) register: a 32-bit register used to set the number of bits per character (data length) in a frame, among other things.
• USART Control 2 (USART_CR2) register: a 32-bit register used for configuration.

USART_CR1 Register Bits

• Bit 15: OVER8: Oversampling mode (0: oversampling by 16, 1: oversampling by 8)
• Bit 13: UE: USART enable (0: USART prescaler and outputs disabled, 1: USART enabled)
• Bit 12: M: Word length (0: 1 Start bit, 8 Data bits, n Stop bit, 1: 1 Start bit, 9 Data bits, n Stop bit)
• Bit 7: TXEIE: TXE interrupt enable (0: interrupt is inhibited, 1: an USART interrupt is generated whenever TXE=1)
• Bit 6: TCIE: Transmission complete interrupt enable (0: interrupt is inhibited, 1: an USART interrupt is generated whenever TC=1)
• Bit 5: RXNEIE: RXNE interrupt enable (0: interrupt is inhibited, 1: an USART interrupt is generated whenever ORE=1 or RXNE=1)
• Bit 4: IDLEIE: IDLE interrupt enable (0: interrupt is inhibited, 1: an USART interrupt is generated whenever IDLE=1)
• Bit 3: TE: Transmitter enable (0: transmitter is disabled, 1: transmitter is enabled)
• Bit 2: RE: Receiver enable (0: receiver is disabled, 1: receiver is enabled and begins searching for a start bit)
• Bits 13:12: STOP: STOP bits (00: 1 Stop bit, 01: 0.5 Stop bit, 10: 2 Stop bits, 11: 1.5 Stop bit)
• Bit 11: CLKEN: Clock enable (0: SCLK pin disabled, 1: SCLK pin enabled)

USART Data Register (USART_DR)

• Used to transmit a byte of data
• A write to this register initiates a transmission from the USART
• The same register is used for both transmit and receive data in STM Arm microcontrollers

Simplified Block Diagram of USART

• Transmit Data register (USART_TDR): used to send data
• Receive Data register (USART_RDR): used to store received data
• USART peripheral sends out the content of the data transmit register through the serial transmit pin (TXD)
• Received data is stored in the Receive Data register (USART_RDR)

Enabling the USART

• Before enabling the USART, the bus clock to the USART should be enabled
• The USART module should be configured properly before it is enabled
• The USART module can be disabled by writing a 0 to the UE bit (UE, USART Enable) of the CR1 register

GPIO Pins Used for USART TxD and RxD

• In addition to the USART registers setup, the I/O pins for USART (TxD and RxD) must be configured to use their alternate functions
• This is done using the AFRx (Alternative Function Register) registers

Steps to Configure USART for Receiving Data

• Enable the Clock to GPIOA
• Enable the Clock to USART2
• Select the peripheral function AF7 for PA3 (USART2_RxD) pin using the GPIO_MODER and GPIO_AFRL registers
• Set the baud rate for USART2 using USART2_BRR register
• Configure the CR1 (Control 1) register for oversampling rate, character size (8bit or 9-bit) and enabling receive (RE)
• Configure the CR2 (Control 2) register for number of stop bit(s) and so on
• Configure the CR3 (Control 3) register for no hardware flow control and so on
• Enable USART2 after configuration complete
• Wait until the RXNE (Receive Not Empty) bit of the USART_SR register is set
• Read a byte from USART2_DR register that was received

Importance of the RXNE Flag Bit

• The RXNE flag bit is used to indicate when a character is received
• The receiver's shift register receives the start bit indicating that the next bit is the first bit of the character it is about to receive
• The character is received one bit at a time