STMicroelectronics STM32F05 series Reference Manual page 598

Universal synchronous asynchronous receiver transmitter (USART)
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 may be cleared using
the TXFRQ bit in the USART_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
guardtime). 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 desired CGT (Character Guard Time, as defined by the 7816-3 specification) minus
12 (the duration of one character).
● The assertion of the TC flag can be delayed by programming the Guard Time register.
In normal operation, TC is asserted when the transmit shift register is empty and no
further transmit requests are outstanding. In Smartcard mode an empty transmit shift
register triggers the Guard Time counter to count up to the programmed value in the
Guard Time register. TC is forced low during this time. When the Guard Time counter
reaches the programmed value TC is asserted high.
●
The de-assertion of TC flag is unaffected by Smartcard mode.
● If a framing error is detected on the transmitter end (due to a NACK from the receiver),
the NACK is not detected as a start bit by the receive block of the transmitter. According
to the ISO protocol, the duration of the received NACK can be 1 or 2 baud clock
periods.
● On the receiver side, if a parity error is detected and a NACK is transmitted the receiver
does not detect the NACK as a start bit.
Note: A break character is not significant in Smartcard mode. A 0x00 data with a framing error is
treated as data and not as a break.
No Idle frame is transmitted when toggling the TE bit. The Idle frame (as defined for the
other configurations) is not defined by the ISO protocol.
598/742 Doc ID 018940 Rev 1
RM0091