Atmel ARM-Based Evaluation Kit AT91SAM9N12-EK AT91SAM9N12-EK Data Sheet

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SAM9N12/SAM9CN11/SAM9CN12 [DATASHEET]

11063K–ATARM–05-Nov-13

The characters are sent by writing in the Transmit Holding Register (US_THR). An additional condition for transmitting a
character can be added when the USART is configured in SPI master mode. In the USART_MR, the value configured on
INACK field can prevent any character transmission (even if US_THR has been written) while the receiver side is not
ready (character not read). When WRDBT equals 0, the character is transmitted whatever the receiver status. If WRDBT
is set to 1, the transmitter waits for the receiver holding register to be read before transmitting the character (RXRDY flag
cleared), thus preventing any overflow (character loss) on the receiver side.

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 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 a
one to the RSTSTA (Reset Status) bit in the Control Register (US_CR).

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 a one to the RTSEN bit in the US_CR. The slave select line (NSS) can be released at
high level only by writing a one to the RTSDIS bit in the US_CR (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.

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 a
one to the RSTSTA (Reset Status) bit the US_CR. 

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.

Because the receiver baud rate 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).