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2014-07-02 - Gecko Family - d0001_Rev1.30
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Figure 18.3. LEUART Transmitter Overview
LEUn_TX
Transm it shift regist er
TXENS
d0- d8
cont rol
d0
d2
d4
d6
d8
d7
d5
d3
d1
cont rol
TXDATA
TXDATAX
BIT8DV
Transm it buffer
0
18.3.4.2 Frame Transmission Control
The transmission control bits, which can be written using LEUARTn_TXDATAX, affect the transmission
of the written frame. The following options are available:
• Generate break: By setting WBREAK, the output will be held low during the first stop-bit period to
generate a framing error. A receiver that supports break detection detects this state, allowing it to be
used e.g. for framing of larger data packets. The line is driven high for one baud period before the next
frame is transmitted so the next start condition can be identified correctly by the recipient. Continuous
breaks lasting longer than an UART frame are thus not supported by the LEUART. GPIO can be used
for this. Note that when AUTOTRI in LEUARTn_CTRL is used, the transmitter is not tristated before
the high-bit after the break has been transmitted.
• Disable transmitter after transmission: If TXDISAT is set, the transmitter is disabled after the frame
has been fully transmitted.
• Enable receiver after transmission: If RXENAT is set, the receiver is enabled after the frame has
been fully transmitted. It is enabled in time to detect a start-bit directly after the last stop-bit has been
transmitted.
The transmission control bits in the LEUART cannot tristate the transmitter. This is performed
automatically by hardware however, if AUTOTRI in LEUARTn_CTRL is set. See Section 18.3.7 (p. 257)
for more information on half duplex operation.
18.3.4.3 Jitter in Transmitted Data
Internally the LEUART module uses only the positive edges of the 32.768 kHz clock (LFBCLK) for
transmission and reception. Transmitted data will thus have jitter equal to the difference between the
optimal data set-up location and the closest positive edge on the 32.768 kHz clock. The jitter in on the
location data is set up by the transmitter will thus be no more than half a clock period according to the
optimal set-up location. The jitter in the period of a single baud output by the transmitter will never be
more than one clock period.
18.3.5 Data Reception
Data reception is enabled by setting RXEN in LEUARTn_CMD. When the receiver is enabled, it actively
samples the input looking for a transition from high to low indicating the start baud of a new frame. When
a start baud is found, reception of the new frame begins if the receive shift register is empty and ready
for new data. When the frame has been received, it is pushed into the receive buffer, making the shift
register ready for another frame of data, and the receiver starts looking for another start baud. If the
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