RM0377 Datasheet, PDF (664/862 Pages) STMicroelectronics – This reference manual targets application developers

RM0377
Universal synchronous asynchronous receiver transmitter (USART)
Figure 219. ISO 7816-3 asynchronous protocol
:LWKRXW3DULW\HUURU
6       S
6WDUWELW
:LWK3DULW\HUURU
6       S
6WDUWELW
*XDUGWLPH
*XDUGWLPH
/LQHSXOOHGORZE\UHFHLYHU
GXULQJVWRSLQFDVHRISDULW\HUURU
06Y9
When connected to a smartcard, the TX output of the USART drives a bidirectional line that
is also driven by the smartcard. The TX pin must be configured as open drain.
Smartcard mode implements a single wire half duplex communication protocol.
• Transmission of data from the transmit shift register is guaranteed to be delayed by a
minimum of 1/2 baud clock. In normal operation a full transmit shift register starts
shifting on the next baud clock edge. In Smartcard mode this transmission is further
delayed by a guaranteed 1/2 baud clock.
• In transmission, if the smartcard detects a parity error, it signals this condition to the
USART by driving the line low (NACK). This NACK signal (pulling transmit line low for 1
baud clock) causes a framing error on the transmitter side (configured with 1.5 stop
bits). The USART can handle automatic re-sending of data according to the protocol.
The number of retries is programmed in the SCARCNT bit field. If the USART
continues receiving the NACK after the programmed number of retries, it stops
transmitting and signals the error as a framing error. The TXE bit can be set using the
TXFRQ bit in the USARTx_RQR register.
• Smartcard auto-retry in transmission: a delay of 2.5 baud periods is inserted between
the NACK detection by the USART and the start bit of the repeated character. The TC
bit is set immediately at the end of reception of the last repeated character (no guard-
time). If the software wants to repeat it again, it must insure the minimum 2 baud
periods required by the standard.
• If a parity error is detected during reception of a frame programmed with a 1.5 stop bit
period, the transmit line is pulled low for a baud clock period after the completion of the
receive frame. This is to indicate to the smartcard that the data transmitted to the
USART has not been correctly received. A parity error is NACKed by the receiver if the
NACK control bit is set, otherwise a NACK is not transmitted (to be used in T=1 mode).
If the received character is erroneous, the RXNE/receive DMA request is not activated.
According to the protocol specification, the smartcard must resend the same character.
If the received character is still erroneous after the maximum number of retries
specified in the SCARCNT bit field, the USART stops transmitting the NACK and
signals the error as a parity error.
• Smartcard auto-retry in reception: the BUSY flag remains set if the USART NACKs the
card but the card doesn’t repeat the character.
• In transmission, the USART inserts the Guard Time (as programmed in the Guard Time
register) between two successive characters. As the Guard Time is measured after the
stop bit of the previous character, the GT[7:0] register must be programmed to the
DocID025942 Rev 2
664/862
738