Silicon Laboratories EFR32xG14 Wireless Gecko Reference Manual page 545

Reference Manual
USART - Universal Synchronous Asynchronous Receiver/Transmitter
18.3.2.22 SmartCard Mode
In SmartCard mode, the USART supports the ISO 7816 I/O line T0 mode. With exception of the stop-bits (guard time), the 7816 data
frame is equal to the regular asynchronous frame. In this mode, the receiver pulls the line low for one baud, half a baud into the guard
time to indicate a parity error. This NAK can for instance be used by the transmitter to re-transmit the frame. SmartCard mode is a half
duplex asynchronous mode, so the transmitter must be tristated whenever not transmitting data.
To enable SmartCard mode, set SCMODE in USARTn_CTRL, set the number of databits in a frame to 8, and configure the number of
stopbits to 1.5 by writing to STOPBITS in USARTn_FRAME.
The SmartCard mode relies on half duplex communication on a single line, so for it to work, both the receiver and transmitter must work
on the same line. This can be achieved by setting LOOPBK in USARTn_CTRL or through an external connection. The TX output
should be configured as open-drain in the GPIO module.
When no parity error is identified by the receiver, the data frame is as shown in Figure 18.15 USART ISO 7816 Data Frame Without
Error on page 545. The frame consists of 8 data bits, a parity bit, and 2 stop bits. The transmitter does not drive the output line during
the guard time.
ISO 7816 Frame without error
Stop or idle Start or idle
3 Stop
S 0 6 7 P
1 2 4 5
Figure 18.15. USART ISO 7816 Data Frame Without Error
If a parity error is detected by the receiver, it pulls the line I/O line low after half a stop bit, see Figure 18.16 USART ISO 7816 Data
Frame With Error on page 545. It holds the line low for one bit-period before it releases the line. In this case, the guard time is exten-
ded by one bit period before a new transmission can start, resulting in a total of 3 stop bits.
ISO 7816 Frame with error
Start or idle
Stop or idle
Stop
3 NAK
S 0 6 7 P
1 2 4 5
Stop
Figure 18.16. USART ISO 7816 Data Frame With Error
On a parity error, the NAK is generated by hardware. The NAK generated by the receiver is sampled as the stop-bit of the frame. Be-
cause of this, parity errors when in SmartCard mode are reported with both a parity error and a framing error.
When transmitting a T0 frame, the USART receiver on the transmitting side samples position 16, 17 and 18 in the stop-bit to detect the
error signal when in 16x oversampling mode as shown in Figure 18.17 USART SmartCard Stop Bit Sampling on page 546. Sampling
at this location places the stop-bit sample in the middle of the bit-period used for the error signal (NAK).
If a NAK is transmitted by the receiver, it will thus appear as a framing error at the transmitter, and the FERR interrupt flag in
USARTn_IF will be set. If SCRETRANS USARTn_CTRL is set, the transmitter will automatically retransmit a NACK'ed frame. The
transmitter will retransmit the frame until it is ACK'ed by the receiver. This only works when the number of databits in a frame is config-
ured to 8.
Set SKIPPERRF in USARTn_CTRL to make the receiver discard frames with parity errors. The PERR interrupt flag in USARTn_IF is
set when a frame is discarded because of a parity error.
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