Datasheet

188
ATmega169A/PA/329A/PA/649A/P/3290A/PA/6490A/P [DATASHEET]
8284E–AVR–02/2013
20.11 Register description
20.11.1 UDRn – USART I/O Data Register n
The USART Transmit Data Buffer Register and USART Receive Data Buffer Registers share the same I/O address
referred to as USART Data Register or UDRn. The Transmit Data Buffer Register (TXB) will be the destination for
data written to the UDRn Register location. Reading the UDRn Register location will return the contents of the
Receive Data Buffer Register (RXB).
For 5-, 6- or 7-bit characters the upper unused bits will be ignored by the Transmitter and set to zero by the
Receiver.
The transmit buffer can only be written when the UDREn Flag in the UCSRnA Register is set. Data written to UDRn
when the UDREn Flag is not set, will be ignored by the USART Transmitter. When data is written to the transmit
buffer, and the Transmitter is enabled, the Transmitter will load the data into the Transmit Shift Register when the
Shift Register is empty. Then the data will be serially transmitted on the TxD pin.
The receive buffer consists of a two level FIFO. The FIFO will change its state whenever the receive buffer is
accessed. Due to this behavior of the receive buffer, do not use Read-Modify-Write instructions (SBI and CBI) on
Table 20-7. Examples of UBRRn settings for commonly used oscillator frequencies. (Continued)
Baud
Rate
(bps)
f
osc
= 16.0000MHz f
osc
= 18.4320MHz f
osc
= 20.0000MHz
U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1
UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 416 -0.1% 832 0.0% 479 0.0% 959 0.0% 520 0.0% 1041 0.0%
4800 207 0.2% 416 -0.1% 239 0.0% 479 0.0% 259 0.2% 520 0.0%
9600 103 0.2% 207 0.2% 119 0.0% 239 0.0% 129 0.2% 259 0.2%
14.4k 68 0.6% 138 -0.1% 79 0.0% 159 0.0% 86 -0.2% 173 -0.2%
19.2k 51 0.2% 103 0.2% 59 0.0% 119 0.0% 64 0.2% 129 0.2%
28.8k 34 -0.8% 68 0.6% 39 0.0% 79 0.0% 42 0.9% 86 -0.2%
38.4k 25 0.2% 51 0.2% 29 0.0% 59 0.0% 32 -1.4% 64 0.2%
57.6k 16 2.1% 34 -0.8% 19 0.0% 39 0.0% 21 -1.4% 42 0.9%
76.8k 12 0.2% 25 0.2% 14 0.0% 29 0.0% 15 1.7% 32 -1.4%
115.2k 8 -3.5% 16 2.1% 9 0.0% 19 0.0% 10 -1.4% 21 -1.4%
230.4k 3 8.5% 8 -3.5% 4 0.0% 9 0.0% 4 8.5% 10 -1.4%
250k 3 0.0% 7 0.0% 4 -7.8% 8 2.4% 4 0.0% 9 0.0%
0.5M 1 0.0% 3 0.0% – – 4 -7.8% – – 4 0.0%
1M00.0%10.0%––––––––
Max.
(1)
1 Mbps 2 Mbps 1.152 Mbps 2.304 Mbps 1.25 Mbps 2.5 Mbps
1. UBRR = 0, Error = 0.0%.
Bit 76543210
RXBn[7:0] UDRn (Read)
TXBn[7:0] UDRn (Write)
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000