UART: (MEGA163)
The minimum requirements to use the serial port are as follows:
1. Load the values for the Baud Rate into UBRR and UBRRHI.
2. Enable the Transmitter and / or Receiver in the UCSRB register.
These two steps will allow the programmer to include the Standard I/O header and to use the standard library functions such as printf, putchar, getchar, etc. <stdio.h>
Alternatively, the programmer can monitor the RCX and TCX bits in the UCSRA register to determine when a character has been received or transmitted. The UDR register contains the data to be transmitted or the data that has been received.
UART Baud Rate Registers - UBRR and UBRRHI
This is a 12-bit register which contains the UART Baud Rate.
|
Bit |
15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | |
| MSB | LSB | UBRRHI | |||||||
| MSB | LSB |
UBRR |
|||||||
|
Bit |
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
| Read/Write | R | R | R | R | R/W | R/W | R/W | R/W | |
| R/W | R/W | R/W | R/W | R/W | R/W | R/W | R/W | ||
| Initial Value | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
The value for UBRR and UBRRHI can be selected from the tables or calculated with the equations located below. Both the equations and the tables use the value of UBR. UBR is equal to UBRRHI and UBRR where UBRRHI is the upper four-bits and UBRR is the lower eight-bits of the 12-bit number represented by UBR.
Equations to Calculate the BAUD Rate.
BAUD = Fclk / ( 16 * (UBR + 1) )
or re-written
UBR = (Fclk / (BAUD * 16)) - 1
UART Control and Status Register B - UCSRB
|
Bit |
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
|
|
RXCIE | TXCIE | UDRIE | RXEN | TXEN | CHR9 | RXB8 | TXB8 |
UCSRB |
| Read/Write | R/W | R/W | R/W | R/W | R/W | R/W | R | W | |
| Initial Value | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 |
Bit 4 - RXEN: Receiver Enable
This bit enables the UART receiver when set (one).
Bit 3 - TXEN: Transmitter Enable
This bit enables the UART transmitter when set (one).
UART Control and Status Register A - UCSRA
|
Bit |
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
|
|
RXC | TXC | UDRE | FE | OR | - | U2X | MPCM |
UCSRA |
| Read/Write | R | R/W | R | R | R | R | R/W | R/W | |
| Initial Value | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
Bit 7 - RXC: UART Receive Complete
This bit is set when a received character is transferred from the Receiver Shift register to UDR.
Bit 6 - TXC: UART Transmit Complete
This bit is set when the entire character in the Transmit Shift register has been shifted out.
|
Bit |
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
|
|
MSB | LSB |
UDR |
||||||
| Read/Write | R/W | R/W | R/W | R/W | R/W | R/W | R/W | R/W | |
| Initial Value | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
The UDR register is actually two physically separate registers sharing the same I/O address. When writing to the register, the UART Transmit Data register is written. When reading from the register, the UART Receive Data register is read.
Usage:
UBRR = 25; // set for 9600 BAUD with 4MHz Clock
UBRRHI = 0;
UCSRB = 0x18; // enable transmitter and receiver
if ( (UCSRA & 0x80) != 0 ); // Received a character...
ch = UDR; // retrieve character from serial register
Example:
// setup the UART for serial communications
// by having the compiler calculate the values
void init_UART(void)
{
#define XTAL 6000000L
#define BAUD 9600unsigned int UBR; // 16 bit variable to hold serial port calculations
UBR = XTAL / 16 / BAUD - 1; // calculate the load values
UBRR = (unsigned char)(UBR & 0xFF); // load the lower 8 bits
UBRRHI = (unsigned char)(UBR >> 8); // load the upper bits
UCSRB = 0x18; // enable transmit and receive
}