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Table 16.7. USART Baud Rates @ 4MHz Peripheral Clock
Desired
USARTn_OVS =00
USARTn_OVS =01
baud rate
[baud/s]
600
1200
2400
4800
9600
14400
19200
28800
38400
57600
76800
115200
230400
USARTn_CLKDIV/256
415,75
207,25
103,25
51
25
16,25
12
7,75
5,5
3,25
2,25
1,25
0
Actual baud
rate [baud/s]
599,88
1200,48
2398,082
4807,692
9615,385
14492,75
19230,77
28571,43
38461,54
58823,53
76923,08
111111,1
250000
Error %
-0,02
0,04
-0,08
0,16
0,16
0,64
0,16
-0,79
0,16
2,12
0,16
-3,55
8,51
USARTn_CLKDIV/256
832,25
415,75
207,25
103,25
51
33,75
25
16,25
12
7,75
5,5
3,25
1,25
Actual baud
rate [baud/s]
600,06
1199,76
2400,96
4796,163
9615,385
14388,49
19230,77
28985,51
38461,54
57142,86
76923,08
117647,1
222222,2
Error %
0,01
-0,02
0,04
-0,08
0,16
-0,08
0,16
0,64
0,16
-0,79
0,16
2,12
-3,55
16.3.2.3 Data Transmission
Asynchronous data transmission is initiated by writing data to the transmit buffer using one of the
methods described in Section 16.3.2.3.1 (p. 182) . When the transmission shift register is empty and
ready for new data, a frame from the transmit buffer is loaded into the shift register, and if the transmitter
is enabled, transmission begins. When the frame has been transmitted, a new frame is loaded into the
shift register if available, and transmission continues. If the transmit buffer is empty, the transmitter goes
to an idle state, waiting for a new frame to become available.
Transmission is enabled through the command register USARTn_CMD by setting TXEN, and disabled
by setting TXDIS in the same command register. When the transmitter is disabled using TXDIS, any
ongoing transmission is aborted, and any frame currently being transmitted is discarded. When disabled,
the TX output goes to an idle state, which by default is a high value. Whether or not the transmitter is
enabled at a given time can be read from TXENS in USARTn_STATUS.
When the USART transmitter is enabled and there is no data in the transmit shift register or transmit
buffer, the TXC flag in USARTn_STATUS and the TXC interrupt flag in USARTn_IF are set, signaling
that the transmitter is idle. The TXC status flag is cleared when a new frame becomes available for
transmission, but the TXC interrupt flag must be cleared by software.
16.3.2.3.1 Transmit Buffer Operation
The transmit-buffer is a 2-level FIFO buffer. A frame can be loaded into the buffer by writing
to USARTn_TXDATA, USARTn_TXDATAX, USARTn_TXDOUBLE or USARTn_TXDOUBLEX. Using
USARTn_TXDATA allows 8 bits to be written to the buffer, while using USARTn_TXDOUBLE will write
2 frames of 8 bits to the buffer. If 9-bit frames are used, the 9th bit of the frames will in these cases be
set to the value of BIT8DV in USARTn_CTRL.
To set the 9th bit directly and/or use transmission control, USARTn_TXDATAX and
USARTn_TXDOUBLEX must be used. USARTn_TXDATAX allows 9 data bits to be written, as well
as a set of control bits regarding the transmission of the written frame. Every frame in the buffer is
stored with 9 data bits and additional transmission control bits. USARTn_TXDOUBLEX allows two
2011-04-12 - d0001_Rev1.10
182
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