Atmel Xplained Evaluation Board AT32UC3A3-XPLD AT32UC3A3-XPLD Data Sheet

32072H–AVR32–10/2012

Figure 25-21. Elementary Time Unit (ETU) 

In T=0 protocol, a character is made up of one start bit, eight data bits, one parity bit, and a two
bit period guard time. During the guard time, the line will be high if the receiver does not signal a
parity error, as shown in 

. The receiver signals a parity error, aka non-acknowledge

(NACK), by pulling the line low for a bit period within the guard time, resulting in the total charac-
ter length being incremented by one, see 

. The USART will not load data to RHR if

it detects a parity error, and will set PARE if it receives a NACK.

Figure 25-22. T=0 Protocol without Parity Error 

Figure 25-23. T=0 Protocol with Parity Error 

In T=1 protocol, the character resembles an asynchronous format with only one stop bit. The
parity is generated when transmitting and checked when receiving. Parity errors set PARE.

The USART receiver keeps count of up to 255 errors in the Number Of Errors field in the Num-
ber of Error Register (NER.NB_ERRORS). Reading NER automatically clears NB_ERRORS.

The USART can be configured to ignore parity errors by writing a one to the Inhibit Non
Acknowledge bit (MR.INACK). Erroneous characters will be treated as if they were ok, not gen-
erating a NACK, loaded to RHR, and raising RXRDY.

1 ETU

FI_DI_RATIO

ISO7816 Clock Cycles

ISO7816 Clock 

on CLK

ISO7816 I/O Line 

on TXD

D0

D1

D2

D3

D4

D5

D6

D7

RXD

Parity

Bit

Baud Rate

Clock

Start 

Bit

Guard

Time 1

Next 
Start 

Bit

Guard

Time 2

D0

D1

D2

D3

D4

D5

D6

D7

I/O

Parity

Bit

Baud Rate

Clock

Start

Bit

Guard

Time 1

Start 

Bit

Guard

Time 2

D0

D1

Error

Repetition