Atmel Xplained Pro Evaluation Kit ATSAM4E-XPRO ATSAM4E-XPRO Data Sheet
Product codes
ATSAM4E-XPRO
SAM4E [DATASHEET]
Atmel-11157D-ATARM-SAM4E16-SAM4E8-Datasheet_12-Jun-14
868
Figure 37-2.
Baud Rate Generator
37.5.2 Receiver
37.5.2.1 Receiver Reset, Enable and Disable
After device reset, the UART receiver is disabled and must be enabled before being used. The receiver can be
enabled by writing the Control Register (UART_CR) with the bit RXEN at 1. At this command, the receiver starts
looking for a start bit.
enabled by writing the Control Register (UART_CR) with the bit RXEN at 1. At this command, the receiver starts
looking for a start bit.
The programmer can disable the receiver by writing UART_CR with the bit RXDIS at 1. If the receiver is waiting for
a start bit, it is immediately stopped. However, if the receiver has already detected a start bit and is receiving the
data, it waits for the stop bit before actually stopping its operation.
a start bit, it is immediately stopped. However, if the receiver has already detected a start bit and is receiving the
data, it waits for the stop bit before actually stopping its operation.
The receiver can be put in reset state by writing UART_CR with the bit RSTRX at 1. In this case, the receiver
immediately stops its current operations and is disabled, whatever its current state. If RSTRX is applied when data
is being processed, this data is lost.
immediately stops its current operations and is disabled, whatever its current state. If RSTRX is applied when data
is being processed, this data is lost.
37.5.2.2 Start Detection and Data Sampling
The UART only supports asynchronous operations, and this affects only its receiver. The UART receiver detects
the start of a received character by sampling the URXD signal until it detects a valid start bit. A low level (space) on
URXD is interpreted as a valid start bit if it is detected for more than seven cycles of the sampling clock, which is
16 times the baud rate. Hence, a space that is longer than 7/16 of the bit period is detected as a valid start bit. A
space which is 7/16 of a bit period or shorter is ignored and the receiver continues to wait for a valid start bit.
the start of a received character by sampling the URXD signal until it detects a valid start bit. A low level (space) on
URXD is interpreted as a valid start bit if it is detected for more than seven cycles of the sampling clock, which is
16 times the baud rate. Hence, a space that is longer than 7/16 of the bit period is detected as a valid start bit. A
space which is 7/16 of a bit period or shorter is ignored and the receiver continues to wait for a valid start bit.
When a valid start bit has been detected, the receiver samples the URXD at the theoretical midpoint of each bit. It
is assumed that each bit lasts 16 cycles of the sampling clock (1-bit period) so the bit sampling point is eight cycles
(0.5-bit period) after the start of the bit. The first sampling point is therefore 24 cycles (1.5-bit periods) after
detecting the falling edge of the start bit.
is assumed that each bit lasts 16 cycles of the sampling clock (1-bit period) so the bit sampling point is eight cycles
(0.5-bit period) after the start of the bit. The first sampling point is therefore 24 cycles (1.5-bit periods) after
detecting the falling edge of the start bit.
Each subsequent bit is sampled 16 cycles (1-bit period) after the previous one.
Figure 37-3.
Start Bit Detection
peripheral clock
16-bit Counter
0
Baud Rate
Clock
CD
CD
OUT
Divide
by 16
0
1
>1
Receiver
Sampling Clock
Sampling Clock
D0
D1 D2
D3
D4 D5 D6
D7
P
S
S
D0
D1 D2
D3 D4
D5
D6
D7
P
URXD
RSTSTA
RXRDY
OVRE
stop
stop