at91rm3400 ATMEL Corporation, at91rm3400 Datasheet - Page 285

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

ISO7816 Mode
ISO7816 Mode
overview
1790A–ATARM–11/03
Figure 107. Transmitter Behavior when Operating with Hardware Handshaking
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 “Baud Rate Genera-
tor” on page 271).
The USART connects to a smart card. as shown in Figure 108. 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
considered as the master of the communication as it generates the clock.
Figure 108. Connection of a Smart Card to the USART
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 pro-
grammed in the CHRL, MODE9, PAR and CHMODE fields. MSBF can be used to transmit
LSB or MSB first.
The USART cannot operate concurrently in both receiver and transmitter modes as the com-
munication is unidirectional at a time. It has to be configured according to the required mode
by enabling or disabling either the receiver or the transmitter as desired. Enabling both the
receiver and the transmitter at the same time in ISO7816 mode may lead to unpredictable
results.
The ISO7816 specification defines an inverse transmission format. Data bits of the character
must be transmitted on the I/O line at their negative value. The USART does not support this
format and the user has to perform an exclusive OR on the data before writing it in the Trans-
mit Holding Register (US_THR) or after reading it in the Receive Holding Register (US_RHR).
CTS
TXD
USART
SCK
TXD
CLK
I/O
Smart
Card
AT91RM3400
285