AT90S/LS4434 and AT90S/LS8535
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UART
The AT90S4434/8535 features a full duplex (separate receive and transmit registers) Universal Asynchronous Receiver
and Transmitter (UART). The main features are:
• Baud rate generator that can generate a large number of baud rates (bps)
• High baud rates at low XTAL frequencies
• 8 or 9 bits data
• Noise filtering
• Overrun detection
• Framing Error detection
• False Start Bit detection
• Three separate interrupts on TX Complete, TX Data Register Empty and RX Complete
• Buffered Transmit and Receive
Data Transmission
A block schematic of the UART transmitter is shown in Figure 41.
Figure 41.
UART Transmitter
Data transmission is initiated by writing the data to be transmitted to the UART I/O Data Register, UDR. Data is transferred
from UDR to the Transmit shift register when:
• A new character is written to UDR after the stop bit from the previous character has been shifted out. The shift register is
loaded immediately.
• A new character is written to UDR before the stop bit from the previous character has been shifted out. The shift register
is loaded when the stop bit of the character currently being transmitted is shifted out.
If the 10(11)-bit Transmitter shift register is empty, data is transferred from UDR to the shift register. The UDRE (UART
Data Register Empty) bit in the UART Status Register, USR, is set. When this bit is set (one), the UART is ready to receive