Atmel SAM9X25, Manual

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Figure 39-27. Connection with a Remote Device for Hardware Handshaking
Setting the USART to operate with hardware handshaking is performed by writing the
USART_MODE field in the Mode Register (US_MR) to the value 0x2.
The USART behavior when hardware handshaking is enabled is the same as the behavior in
standard synchronous or asynchronous mode, except that the receiver drives the RTS pin as
described below and the level on the CTS pin modifies the behavior of the transmitter as
described below. Using this mode requires using the DMAC channel for reception. The transmit-
ter can handle hardware handshaking in any case.
Figure 39-28 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.
Figure 39-28. Transmitter Behavior when Operating with Hardware Handshaking
39.7.4 ISO7816 Mode
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.
39.7.4.1 ISO7816 Mode Overview
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
“Baud Rate Generator”
on page 802 ).
The USART connects to a smart card as shown in Figure 39-29 . The TXD line becomes bidirec-
tional and the Baud Rate 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 con-
sidered as the master of the communication as it generates the clock.
USART
TXD
CTS
Remote
Device
RXD
TXD RXD
RTS
RTS
CTS
CTS
TXD