Atmel SAM4S-XPLD Atmel ATSAM4S-XPLD ATSAM4S-XPLD Hoja De Datos

 712

SAM4S [DATASHEET]

11100E–ATARM–24-Jul-13

 shows how the receiver operates if hardware handshaking is enabled. The RTS pin is driven high if the

receiver is disabled and if the status RXBUFF (Receive Buffer Full) coming from the PDC channel is high. Normally, the
remote device does not start transmitting while its CTS pin (driven by RTS) is high. As soon as the Receiver is enabled,
the RTS falls, indicating to the remote device that it can start transmitting. Defining a new buffer to the PDC clears the
status bit RXBUFF and, as a result, asserts the pin RTS low.

 shows how the transmitter operates if hardware handshaking is enabled. The CTS pin disables the

transmitter. If a character is being processing, the transmitter is disabled only after the completion of the current
character and transmission of the next character happens as soon as the pin CTS falls.

The USART features an ISO7816-compatible operating mode. This mode permits interfacing with smart cards and
Security Access Modules (SAM) communicating through an ISO7816 link. Both T = 0 and T = 1 protocols defined by the
ISO7816 specification are supported.

Setting the USART in ISO7816 mode is performed by writing the USART_MODE field in the Mode Register (US_MR) to
the value 0x4 for protocol T = 0 and to the value 0x5 for protocol T = 1.

The ISO7816 is a half duplex communication on only one bidirectional line. The baud rate is determined by a division of
the clock provided to the remote device (see 

Generator feeds the ISO7816 clock on the SCK pin. As the TXD pin becomes bidirectional, its output remains driven by
the output of the transmitter but only when the transmitter is active while its input is directed to the input of the receiver.
The USART is considered as the master of the communication as it generates the clock. 

RTS

RXBUFF

Write

US_CR

RXEN = 1

RXD

RXDIS = 1

CTS

TXD

Smart

Card

SCK

CLK

TXD

I/O

USART