Rainbow Electronics ATmega103L - Manual (Page 62)

ATmega603/103
62
When the stop bit enters the receiver, the majority of the three samples must be one to accept the stop bit. If two or more
samples are logical zeros, the Framing Error (FE) flag in the UART Status Register (USR) is set when the received byte is
transferred to UDR. Before reading the UDR register, the user should always check the FE bit to detect Framing Errors. FE
is cleared when UDR is read.
Whether or not a valid stop bit is detected at the end of a character reception cycle, the data is transferred to UDR and the
RXC flag in USR is set. UDR is in fact two physically separate registers, one for transmitted data and one for received data.
When UDR is read, the Receive Data register is accessed, and when UDR is written, the Transmit Data register is
accessed. If 9 bit data word is selected (the CHR9 bit in the UART Control Register, UCR is set), the RXB8 bit in UCR is
loaded with bit 9 in the Transmit shift register when data is transferred to UDR.
If, after having received a character, the UDR register has not been accessed since the last receive, the OverRun (OR) flag
in USR is set. This means that the new data transferred to the shift register could not be transferred to UDR and is lost. The
OR bit is buffered, and is available when the valid data byte in UDR has been read. The user should always check the OR
after reading from the UDR register in order to detect any overruns if the baud rate is high or CPU load is high.
When the RXEN bit in the UCR register is cleared (zero), the receiver is disabled. This means that the PE0 pin can be used
as a general I/O pin. When RXEN is set, the UART Receiver will be connected to PE0, which is forced to be an input pin
regardless of the setting of the DDE0 bit in DDRE. When PE0 is forced to input by the UART, the PORTE0 bit can still be
used to control the pull-up resistor on the pin.
When the CHR9 bit in the UCR register is set, transmitted and received characters are 9-bit long plus start and stop bits.
The 9th data bit to be transmitted is the TXB8 bit in UCR register. This bit must be set to the wanted value before a trans-
mission is initated by writing to the UDR register. The 9th
UART Control
UART I/O Data Register - UDR
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 UDR, the UART Receive Data register is read.
UART Status Register - USR
The USR register is a read-only register providing information on the UART Status.
•
Bit 7 - RXC: UART Receive Complete
This bit is set (one) when a received character is transferred from the Receiver Shift register to UDR. The bit is set regard-
less of any detected framing errors. When the RXCIE bit in UCR is set, the UART Receive Complete interrupt will be
executed when RXC is set(one). RXC is cleared by reading UDR. When interrupt-driven data reception is used, the UART
Receive Complete Interrupt routine must read UDR in order to clear RXC, otherwise a new interrupt will occur once the
interrupt routine terminates.
•
Bit 6 - TXC: UART Transmit Complete
This bit is set (one) when the entire character (including the stop bit) in the Transmit Shift register has been shifted out and
no new data has been written to the UDR. This flag is especially useful in half-duplex communications interfaces, where a
transmitting application must enter receive mode and free the communications bus immediately after completing the
transmission.
Bit 7 6 5 4 3 2 1 0
$0C ($2C) 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
Bit 7 6 5 4 3 2 1 0
$0B ($2B) RXC TXC UDRE FE OR - - - USR
Read/Write R R/W R R R R R R
Initial value 0 0 1 0 0 0 0 0