SAM4S_14 Datasheet, PDF (779/1231 Pages) ATMEL Corporation – ARM-based Flash MCU

Figure 36-28. Receiver Behavior when Operating with Hardware Handshaking
RXD
RXEN = 1
Write
US_CR
RXDIS = 1
RTS
RXBUFF
Figure 36-29 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 36-29. Transmitter Behavior when Operating with Hardware Handshaking
CTS
TXD
36.7.4 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 US_MR to the value 0x4
for protocol T = 0 and to the value 0x5 for protocol T = 1.
36.7.4.1 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 761).
The USART connects to a smart card as shown in Figure 36-30. The TXD line becomes bidirectional 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 considered as the master of the communication as it generates the clock.
Figure 36-30. Connection of a Smart Card to the USART
USART
SCK
TXD
CLK
Smart
Card
I/O
When operating in ISO7816, either in T = 0 or T = 1 modes, the character format is fixed. The configuration is 8
data bits, even parity and 1 or 2 stop bits, regardless of the values programmed in the CHRL, MODE9, PAR and
CHMODE fields. MSBF can be used to transmit LSB or MSB first. Parity Bit (PAR) can be used to transmit in
normal or inverse mode. Refer to “USART Mode Register” on page 798 and “PAR: Parity Type” on page 799.
The USART cannot operate concurrently in both receiver and transmitter modes as the communication is
unidirectional at a time. It has to be configured according to the required mode by enabling or disabling either the
SAM4S Series [DATASHEET]
Atmel-11100G-ATARM-SAM4S-Datasheet_27-May-14
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