Disabling the Receiver
Writing '0' to the Receiver Enable bit in the CTRLB register (CTRLB.RXEN) will disable the receiver, flush
the two-level receive buffer, and data from ongoing receptions will be lost.
Error Bits
The USART receiver has three error bits in the Status (STATUS) register: Frame Error (FERR), Buffer
Overflow (BUFOVF), and Parity Error (PERR). Once an error happens, the corresponding error bit will be
set until it is cleared by writing ‘1’ to it. These bits are also cleared automatically when the receiver is
disabled.
There are two methods for buffer overflow notification, selected by the Immediate Buffer Overflow
Notification bit in the Control A register (CTRLA.IBON):
When CTRLA.IBON=1, STATUS.BUFOVF is raised immediately upon buffer overflow. Software can then
empty the receive FIFO by reading RxDATA, until the receiver complete interrupt flag (INTFLAG.RXC) is
cleared.
When CTRLA.IBON=0, the buffer overflow condition is attending data through the receive FIFO. After the
received data is read, STATUS.BUFOVF will be set along with INTFLAG.RXC.
Asynchronous Data Reception
The USART includes a clock recovery and data recovery unit for handling asynchronous data reception.
The clock recovery logic can synchronize the incoming asynchronous serial frames at the RxD pin to the
internally generated baud-rate clock.
The data recovery logic samples and applies a low-pass filter to each incoming bit, thereby improving the
noise immunity of the receiver.
Asynchronous Operational Range
The operational range of the asynchronous reception depends on the accuracy of the internal baud-rate
clock, the rate of the incoming frames, and the frame size (in number of bits). In addition, the operational
range of the receiver is depending on the difference between the received bit rate and the internally
generated baud rate. If the baud rate of an external transmitter is too high or too low compared to the
internally generated baud rate, the receiver will not be able to synchronize the frames to the start bit.
There are two possible sources for a mismatch in baud rate: First, the reference clock will always have
some minor instability. Second, the baud-rate generator cannot always do an exact division of the
reference clock frequency to get the baud rate desired. In this case, the BAUD register value should be
set to give the lowest possible error. Refer to
Clock Generation – Baud-Rate Generator
for details.
Recommended maximum receiver baud-rate errors for various character sizes are shown in the table
below.
Table 32-3. Asynchronous Receiver Error for 16-fold Oversampling
D
(Data bits+Parity)
R
SLOW
[%] R
FAST
[%] Max. total error [%] Recommended max. Rx error [%]
5
94.12
107.69
+5.88/-7.69
±2.5
6
94.92
106.67
+5.08/-6.67
±2.0
7
95.52
105.88
+4.48/-5.88
±2.0
8
96.00
105.26
+4.00/-5.26
±2.0
9
96.39
104.76
+3.61/-4.76
±1.5
10
96.70
104.35
+3.30/-4.35
±1.5
Atmel SAM L22G / L22J / L22N [DATASHEET]
Atmel-42402E-SAM L22G / L22J / L22N_Datasheet_Complete-07/2016
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