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SAM9G15 [DATASHEET]
11052E–ATARM–06-Feb-13
39.7.7.4 Receiver and Transmitter Control
See “Receiver and Transmitter Control” on page 777.
39.7.7.5 Character Transmission
The characters are sent by writing in the Transmit Holding Register (US_THR). The transmitter reports two status bits in
the Channel Status Register (US_CSR): TXRDY (Transmitter Ready), which indicates that US_THR is empty and
TXEMPTY, which indicates that all the characters written in US_THR have been processed. When the current character
processing is completed, the last character written in US_THR is transferred into the Shift Register of the transmitter and
US_THR becomes empty, thus TXRDY rises.
Both TXRDY and TXEMPTY bits are low when the transmitter is disabled. Writing a character in US_THR while TXRDY
is low has no effect and the written character is lost.
If the USART is in SPI Slave Mode and if a character must be sent while the Transmit Holding Register (US_THR) is
empty, the UNRE (Underrun Error) bit is set. The TXD transmission line stays at high level during all this time. The UNRE
bit is cleared by writing the Control Register (US_CR) with the RSTSTA (Reset Status) bit to 1.
In SPI Master Mode, the slave select line (NSS) is asserted at low level 1 Tbit (Time bit) before the transmission of the
MSB bit and released at high level 1 Tbit after the transmission of the LSB bit. So, the slave select line (NSS) is always
released between each character transmission and a minimum delay of 3 Tbits always inserted. However, in order to
address slave devices supporting the CSAAT mode (Chip Select Active After Transfer), the slave select line (NSS) can
be forced at low level by writing the Control Register (US_CR) with the RTSEN bit to 1. The slave select line (NSS) can
be released at high level only by writing the Control Register (US_CR) with the RTSDIS bit to 1 (for example, when all
data have been transferred to the slave device).
In SPI Slave Mode, the transmitter does not require a falling edge of the slave select line (NSS) to initiate a character
transmission but only a low level. However, this low level must be present on the slave select line (NSS) at least 1 Tbit
before the first serial clock cycle corresponding to the MSB bit.
39.7.7.6 Character Reception
When a character reception is completed, it is transferred to the Receive Holding Register (US_RHR) and the RXRDY bit
in the Status Register (US_CSR) rises. If a character is completed while RXRDY is set, the OVRE (Overrun Error) bit is
set. The last character is transferred into US_RHR and overwrites the previous one. The OVRE bit is cleared by writing
the Control Register (US_CR) with the RSTSTA (Reset Status) bit to 1.
To ensure correct behavior of the receiver in SPI Slave Mode, the master device sending the frame must ensure a
minimum delay of 1 Tbit between each character transmission. The receiver does not require a falling edge of the slave
select line (NSS) to initiate a character reception but only a low level. However, this low level must be present on the
slave select line (NSS) at least 1 Tbit before the first serial clock cycle corresponding to the MSB bit.
39.7.7.7 Receiver Timeout
Because the receiver baudrate clock is active only during data transfers in SPI Mode, a receiver timeout is impossible in
this mode, whatever the Time-out value is (field TO) in the Time-out Register (US_RTOR).
39.7.8 LIN Mode
The LIN Mode provides Master node and Slave node connectivity on a LIN bus.
The LIN (Local Interconnect Network) is a serial communication protocol which efficiently supports the control of
mechatronic nodes in distributed automotive applications.
The main properties of the LIN bus are:
Single Master/Multiple Slaves concept
Low cost silicon implementation based on common UART/SCI interface hardware, an equivalent in software, or as
a pure state machine
Self synchronization without quartz or ceramic resonator in the slave nodes
Deterministic signal transmission
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