Registers for serial communication

Abstract

Registers for serial communication.

UDRn

The UDRn is a buffer for transmit and receive data. Shares same address.

When a UDREn flag of UCSRnA is set, we can write data to transmit buffer.

The UCSRnA(USART Control and Status Register n A) is a register to control serial communication and status.

UCSRnA
Bit	7	6	5	4	3	2	1	0
Name	RXCn	TXCn	UDREn	FEn	DORn	UPEn	U2Xn	MPCMn

RXCn(USART Receive Complete): Set when unread data exists in receive buffer.
TXCn(USART Transmit Complete): Set when untransmitted data dose not exist in transmit buffer.
UDREn(USART Data Register Empty): Set when transmit buffer is ready to get data.
FEn(Frame Error): Set when next character in the receive buffer has a frame error.
DORn(Data OverRun): Set when detects data overrun. If the receive buffer is full and detect new start bit.
UPEn(USART Parity Error): Set when next character in the receive buffer has a parity error.。
U2Xn(Double the USART Transmission Speed): Effective for asynchronous operation mode. If write 1, set baud rate divider from 16 to 8, so transfer rate is doubling.
MPCMn(Multi-processor Communication Mode): Enables the multi-processor communication mode.

UCSRnB
Bit	7	6	5	4	3	2	1	0
Name	RXCIEn	TXCIEn	UDRIEn	RXENn	TXENn	UCSZn2	RXB8n	TXB8n

RXCIEn(RX Complete Interrupt Enable n): Enables interrupt on the RXCn Flag.
TXCIEn(TX Complete Interrupt Enable n): Enables interrupt on the TXCn Flag.
UDRIEn(USART Data Register Empty Interrupt Enable n): Enables interrupt on the UDREn Flag.
RXENn(Receiver Enable n): Enables the USART Receiver.
TXENn(Transmitter Enable n): Enables the USART Transmitter.
UCSZn2(Character Size n), UCSZn1 and UCSZn0: Sets the number of data bits
RXB8n(Receive Data Bit 8 n): The ninth data bit of the received character when 9 bit mode.
TXB8n(Transmit Data Bit 8 n): The ninth data bit of the transmitting character when 9 bit mode.

The UCSRnC(USART Control and Status Register n C) is a register for parity, stop bit and so on.

UCSRnC
Bit	7	6	5	4	3	2	1	0
Name	UMSELn1	UMSELn0	UPMn1	UPMn0	USBSn	UCSZn1	UCSZn0	UCPOLn

UMSELn: Sets the operation mode as below. The default is asynchronous USART.

UPMn: Enables and sets type of parity generation and check. The default is disabled.

USBSn: Selects the number of stop bits to be inserted by the transmitter. 1 bit when 0, 2 bit when 1. The default is 1 bit.
UCSZn2, UCSZn1, UCSZn0: Sets the length of data bit. The default is 8 bit.

UCSZn2	UCSXn1	UCSZn0	length
0	0	0	5 bit
0	0	1	6 bit
0	1	0	7 bit
0	1	1	8 bit
1	0	0	Reserved
1	0	1	Reserved
1	1	0	Reserved
1	1	1	9 bit

UCPOLn: Sets the relationship between data output change and data input sample, and the synchronous clock (XCKn).

Sets the baud rate.

12 bit of the 16 bit is valid. The communication speed is calculated as below. The F_CPU is a system clock.

U2Xn = 0: BAUD = F_CPU / (16 (UBRRn + 1)); UBRRn = (F_CPU / (16 * BAUD)) - 1
U2Xn = 1: BAUD = F_CPU / 8 (UBRRn + 1); UBRRn = (F_CPU / (8 * BAUD)) - 1

Arduino AVR Boards 1.8.6

March 21, 2023