Datasheet

Table Of Contents
Figure 34-15. RS485 Bus Connection
TXD
TE
USART
RXD
Differential
Bus
The TE pin will remain high for the complete frame including stop bit(s). If a Guard Time is programmed in
the Control C register (CTRLC.GTIME), the line will remain driven after the last character completion. The
following figure shows a transfer with one stop bit and CTRLC.GTIME=3.
Figure 34-16. Example of TE Drive with Guard Time
TXD
Start StopData GTIME=3
TE
The Transmit Complete interrupt flag (INTFLAG.TXC) will be raised after the guard time is complete and
TE goes low.
34.6.3.7 ISO 7816 for Smart Card Interfacing
The SERCOM USART features an ISO/IEC 7816-compatible operating mode. This mode permits
interfacing with smart cards and Security Access Modules (SAM) communicating through an ISO 7816
link. Both T=0 and T=1 protocols defined by the ISO 7816 specification are supported.
ISO 7816 is available with the following configuration:
ISO 7816 format (CTRLA.FORM = 0x07)
Inverse transmission and reception (CTRLA.RXINV=1 and CTRLA.TXINV=1)
Single bidirectional data line (CTRLA.TXPO and CTRLA.RXPO configured to use the same data pin)
Even parity (CTRLB.PMODE=0)
8-bit character size (CTRLB.CHSIZE=0)
T=0 (CTRLA.CMODE=1) or T=1 (CTRLA.CMODE=0)
ISO 7816 is a half duplex communication on a single bidirectional line. The USART connects to a smart
card as shown below. The output is only driven when the USART is transmitting. The USART is
considered as the master of the communication as it generates the clock.
Figure 34-17. Connection of a Smart Card to the SERCOM USART
TXD/RXD
SERCOM
USART
SCK
I/O
Smart
Card
CLK
ISO 7816 characters are specified as 8 bits with even parity. The USART must be configured accordingly.
The USART cannot operate concurrently in both receiver and transmitter modes as the communication is
unidirectional. 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
ISO 7816 mode may lead to unpredictable results.
The ISO 7816 specification defines an inverse transmission format. Data bits of the character must be
transmitted on the I/O line at their negative value (CTRLA.RXINV=1 and CTRLA.TXINV=1).
SAM D5x/E5x Family Data Sheet
SERCOM USART - SERCOM Synchronous and Asyn...
© 2019 Microchip Technology Inc.
Datasheet
DS60001507E-page 935