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2016-04-28 - Giant Gecko Family - d0053_Rev1.20
465
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Figure 17.13. USART ISO 7816 Data Frame With Error
S
0
1
2
3
4
5
6
7
P
St op
St art or idle
St op or idle
ISO 7816 Fram e wit h error
St op
NAK
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. Because 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 17.14 (p.
465) . 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 configured 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.
Figure 17.14. USART SmartCard Stop Bit Sampling
13 14 15 16 1
2
3
4
5
6
7
8
9
10 11
1/ 2 st op bit
7
8
1
2
3
4
5
6
13
7
12
O
V
S
=
0
O
V
S
=
1
14 15 16
8
P
NAK or st op
17 18 X
9
10
X
X
X
X
X
X
X
St op
1
2
3
4
5
6
7
O
V
S
=
2
1
2
3
4
5
x
O
V
S
=
3
8
x
6
4
For communication with a SmartCard, a clock signal needs to be generated for the card. This clock
output can be generated using one of the timers. See the ISO 7816 specification for more info on this
clock signal.
SmartCard T1 mode is also supported. The T1 frame format used is the same as the asynchronous
frame format with parity bit enabled and one stop bit. The USART must then be configured to operate
in asynchronous half duplex mode.
17.3.3 Synchronous Operation
Most of the features in asynchronous mode are available in synchronous mode. Multi-processor mode
can be enabled for 9-bit frames, loopback is available and collision detection can be performed.
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