Datasheet

SAM9G20
DS60001516A-page 466  2017 Microchip Technology Inc.
31.6.4 ISO7816 Mode
The USART features an ISO7816-compatible operating mode. This mode permits interfacing with smart cards and Security Access Mod-
ules (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.
31.6.4.1 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 Section 31.6.1 Baud Rate Generator).
The USART connects to a smart card as shown in Figure 31-30. 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 31-30: 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 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 Section 31.7.2 USART Mode Register and
Section PAR: Parity Type.
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 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
Transmit Holding Register (US_THR) or after reading it in the Receive Holding Register (US_RHR).
31.6.4.2 Protocol T = 0
In T = 0 protocol, a character is made up of one start bit, eight data bits, one parity bit and one guard time, which lasts two bit times. The
transmitter shifts out the bits and does not drive the I/O line during the guard time.
If no parity error is detected, the I/O line remains at 1 during the guard time and the transmitter can continue with the transmission of the
next character, as shown in Figure 31-31.
If a parity error is detected by the receiver, it drives the I/O line at 0 during the guard time, as shown in Figure 31-32. This error bit is also
named NACK, for Non Acknowledge. In this case, the character lasts 1 bit time more, as the guard time length is the same and is added
to the error bit time which lasts 1 bit time.
When the USART is the receiver and it detects an error, it does not load the erroneous character in the Receive Holding Register
(US_RHR). It appropriately sets the PARE bit in the Status Register (US_SR) so that the software can handle the error.
Figure 31-31: T = 0 Protocol without Parity Error
Smart
Card
SCK
CLK
TXD
I/O
USART
D0
D1
D2
D3
D4
D5 D6 D7
RXD
Parity
Bit
Baud Rate
Clock
Start
Bit
Guard
Time 1
Next
Start
Bit
Guard
Time 2