SAM7X256_14 Datasheet, PDF (309/662 Pages) ATMEL Corporation – ARM-based Flash MCU

Figure 30-17. Connection with a Remote Device for Hardware Handshaking
USART
TXD
RXD
CTS
RTS
Remote
Device
RXD
TXD
RTS
CTS
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 PDC channel for
reception. The transmitter can handle hardware handshaking in any case.
Figure 30-18 shows how the receiver operates if hardware handshaking is enabled. The RTS pin is driven high if the
receiver is disabled and if the status RXBUFF (Receive Buffer Full) coming from the PDC channel is high. Normally, the
remote device does not start transmitting while its CTS pin (driven by RTS) is high. As soon as the Receiver is enabled,
the RTS falls, indicating to the remote device that it can start transmitting. Defining a new buffer to the PDC clears the
status bit RXBUFF and, as a result, asserts the pin RTS low.
Figure 30-18. Receiver Behavior when Operating with Hardware Handshaking
RXD
RXEN = 1
Write
US_CR
RXDIS = 1
RTS
RXBUFF
Figure 30-19 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 30-19. Transmitter Behavior when Operating with Hardware Handshaking
CTS
TXD
30.6.4 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.
SAM7X Series [DATASHEET]
6120K–ATARM–11-Feb-14
309