AT90S/LS4434 and AT90S/LS8535
56
If however, a valid start bit is detected, sampling of the data bits following the start bit is performed. These bits are also
sampled at samples 8, 9 and 10. The logical value found in at least two of the three samples is taken as the bit value. All
bits are shifted into the Transmitter Shift register as they are sampled. Sampling of an incoming character is shown in
Figure 43.
Figure 43.
Sampling Received Data
When the stop bit enters the receiver, the majority of the three samples must be one to accept the stop bit. If two or more
samples are logical “0”s, the Framing Error (FE) flag in the UART Status Register (USR) is set. Before reading the UDR
register, the user should always check the FE bit to detect framing errors.
Whether or not a valid stop bit is detected at the end of a character reception cycle, the data is transferred to UDR and the
RXC flag in USR is set. UDR is in fact two physically separate registers, one for transmitted data and one for received data.
When UDR is read, the Receive Data register is accessed and when UDR is written, the Transmit Data register is
accessed. If 9-bit data word is selected (the CHR9 bit in the UART Control Register, UCR is set), the RXB8 bit in UCR is
loaded with bit 9 in the Transmit Shift register when data is transferred to UDR.
If, after having received a character, the UDR register has not been read since the last receive, the OverRun (OR) flag in
UCR is set. This means that the last data byte shifted into the shift register could not be transferred to UDR and has been
lost. The OR bit is buffered and is updated when the valid data byte in UDR is read. Thus, the user should always check the
OR bit after reading the UDR register in order to detect any overruns if the baud rate is high or CPU load is high.
When the RXEN bit in the UCR register is cleared (zero), the receiver is disabled. This means that the PD0 pin can be used
as a general I/O pin. When RXEN is set, the UART Receiver will be connected to PD0, which is forced to be an input pin
regardless of the setting of the DDD0 bit in DDRD. When PD0 is forced to input by the UART, the PORTD0 bit can still be
used to control the pull-up resistor on the pin.
When the CHR9 bit in the UCR register is set, transmitted and received characters are 9 bits long, plus start and stop bits.
The ninth data bit to be transmitted is the TXB8 bit in UCR register. This bit must be set to the wanted value before a trans-
mission is initiated by writing to the UDR register. The ninth data bit received is the RXB8 bit in the UCR register.
