SAM7S64_14 Datasheet, PDF (380/775 Pages) ATMEL Corporation – Internal High-speed Flash

CHMODE fields. MSBF can be used to transmit LSB or MSB first. Parity Bit (PAR) can be used to transmit in nor-
mal or inverse mode. Refer to “USART Mode Register” on page 390 and “PAR: Parity Type” on page 392.
The USART cannot operate concurrently in both receiver and transmitter modes as the communication is unidirec-
tional at a time. It has to be configured according to the required mode by enabling or disabling either the receiver
or the transmitter as desired. Enabling both the receiver and the transmitter at the same time in ISO7816 mode
may lead to unpredictable results.
The ISO7816 specification defines an inverse transmission format. Data bits of the character must be transmitted
on the I/O line at their negative value. The USART does not support this format and the user has to perform an
exclusive OR on the data before writing it in the Transmit Holding Register (US_THR) or after reading it in the
Receive Holding Register (US_RHR).
31.6.4.2 Protocol T = 0
In T = 0 protocol, a character is made up of one start bit, eight data bits, one parity bit and one guard time, which
lasts two bit times. The transmitter shifts out the bits and does not drive the I/O line during the guard time.
If no parity error is detected, the I/O line remains at 1 during the guard time and the transmitter can continue with
the transmission of the next character, as shown in Figure 31-23.
If a parity error is detected by the receiver, it drives the I/O line at 0 during the guard time, as shown in Figure 31-
24. This error bit is also named NACK, for Non Acknowledge. In this case, the character lasts 1 bit time more, as
the guard time length is the same and is added to the error bit time which lasts 1 bit time.
When the USART is the receiver and it detects an error, it does not load the erroneous character in the Receive
Holding Register (US_RHR). It appropriately sets the PARE bit in the Status Register (US_SR) so that the software
can handle the error.
Figure 31-23. T = 0 Protocol without Parity Error
Baud Rate
Clock
RXD
Start D0 D1 D2 D3 D4 D5 D6 D7 Parity Guard Guard Next
Bit
Bit Time 1 Time 2 Start
Bit
Figure 31-24. T = 0 Protocol with Parity Error
Baud Rate
Clock
I/O
Error
Start D0 D1 D2 D3 D4 D5 D6 D7 Parity Guard
Bit
Bit Time 1
Guard Start D0 D1
Time 2 Bit
Repetition
31.6.4.3 Receive Error Counter
The USART receiver also records the total number of errors. This can be read in the Number of Error (US_NER)
register. The NB_ERRORS field can record up to 255 errors. Reading US_NER automatically clears the
NB_ERRORS field.
SAM7S Series [DATASHEET]
6175M–ATARM–26-Oct-12
380