at91rm3400 ATMEL Corporation, at91rm3400 Datasheet - Page 276

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at91rm3400

Manufacturer Part Number
at91rm3400
Description
Atmel Advanced At91 Arm Microcontroller
Manufacturer
ATMEL Corporation
Datasheet

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Asynchronous
Receiver
276
AT91RM3400
Figure 94. Character Transmit
The characters are sent by writing in the Transmit Holding Register (US_THR). The transmit-
ter 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 raises.
Both TXRDY and TXEMPTY bits are low since the transmitter is disabled. Writing a character
in US_THR while TXRDY is active has no effect and the written character is lost.
Figure 95. Transmitter Status
If the USART is programmed in asynchronous operating mode (SYNC = 0), the receiver over-
samples the RXD input line. The oversampling is either 16 or 8 times the Baud Rate clock,
depending on the OVER bit in the Mode Register (US_MR).
The receiver samples the RXD line. If the line is sampled during one half of a bit time at 0, a
start bit is detected and data, parity and stop bits are successively sampled on the bit rate
clock.
If the oversampling is 16, (OVER at 0), a start is detected at the eighth sample at 0. Then, data
bits, parity bit and stop bit are sampled on each 16 sampling clock cycle. If the oversampling is
8 (OVER at 1), a start bit is detected at the fourth sample at 0. Then, data bits, parity bit and
stop bit are sampled on each 8 sampling clock cycle.
The number of data bits, first bit sent and parity mode are selected by the same fields and bits
as the transmitter, i.e. respectively CHRL, MODE9, MSBF and PAR. The number of stop bits
has no effect on the receiver as it considers only one stop bit, regardless of the field NBSTOP,
so that resynchronization between the receiver and the transmitter can occur. Moreover, as
soon as the stop bit is sampled, the receiver starts looking for a new start bit so that resynchro-
nization can also be accomplished when the transmitter is operating with one stop bit.
Baud Rate
TXEMPTY
Baud Rate
Example: 8-bit, Parity Enabled One Stop
US_THR
TXRDY
Clock
Write
Clock
TXD
TXD
Start
Bit
Start
Bit
D0
D1
D0
D2
D3
D1
D4
D5
D2
D6
D7
D3
Parity
Bit
Stop
Bit
D4
Start
Bit
D0
D5
D1
D2
D6
D3
D4
D7
D5
D6
Parity
Bit
1790A–ATARM–11/03
D7
Parity
Bit
Stop
Bit
Stop
Bit

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