AT90S2313
43
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Bit 5 - UDRE: UART Data Register Empty
This bit is set (one) when a character written to UDR is transferred to the Transmit shift register. Setting of this bit indicates
that the transmitter is ready to receive a new character for transmission.
When the UDRIE bit in UCR is set, the UART Transmit Complete interrupt to be executed as long as UDRE is set. UDRE is
cleared by writing UDR. When interrupt-driven data transmittal is used, the UART Data Register Empty Interrupt routine
must write UDR in order to clear UDRE, otherwise a new interrupt will occur once the interrupt routine terminates.
UDRE is set (one) during reset to indicate that the transmitter is ready.
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Bit 4 - FE: Framing Error
This bit is set if a Framing Error condition is detected, i.e. when the stop bit of an incoming character is zero.
The FE bit is cleared when the stop bit of received data is one.
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Bit 3 - OR: Overrun
This bit is set if an Overrun condition is detected, i.e. when a character already present in the UDR register is not read
before the next character has been shifted into the Receiver Shift register. The OR bit is buffered, which means that it will
be set once the valid data still in UDRE is read.
The OR bit is cleared (zero) when data is received and transferred to UDR.
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Bits 2..0 - Res: Reserved bits
These bits are reserved bits in the AT90S2313 and will always read as zero.
UART Control Register - UCR
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Bit 7 - RXCIE: RX Complete Interrupt Enable
When this bit is set (one), a setting of the RXC bit in USR will cause the Receive Complete interrupt routine to be executed
provided that global interrupts are enabled.
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Bit 6 - TXCIE: TX Complete Interrupt Enable
When this bit is set (one), a setting of the TXC bit in USR will cause the Transmit Complete interrupt routine to be executed
provided that global interrupts are enabled.
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Bit 5 - UDRIE: UART Data Register Empty Interrupt Enable
When this bit is set (one), a setting of the UDRE bit in USR will cause the UART Data Register Empty interrupt routine to be
executed provided that global interrupts are enabled.
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Bit 4 - RXEN: Receiver Enable
This bit enables the UART receiver when set (one). When the receiver is disabled, the TXC, OR and FE status flags cannot
become set. If these flags are set, turning off RXEN does not cause them to be cleared.
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Bit 3 - TXEN: Transmitter Enable
This bit enables the UART transmitter when set (one). When disabling the transmitter while transmitting a character, the
transmitter is not disabled before the character in the shift register plus any following character in UDR has been com-
pletely transmitted.
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Bit 2 - CHR9: 9 Bit Characters
When this bit is set (one) transmitted and received characters are 9 bit long plus start and stop bits. The 9th bit is read and
written by using the RXB8 and TXB8 bits in UCR, respectively. The 9th data bit can be used as an extra stop bit or a parity
bit.
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Bit 1 - RXB8: Receive Data Bit 8
When CHR9 is set (one), RXB8 is the 9th data bit of the received character.
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Bit 0 - TXB8: Transmit Data Bit 8
When CHR9 is set (one), TXB8 is the 9th data bit in the character to be transmitted.
Bit 76543210
$0A ($2A) RXCIE TXCIE UDRIE RXEN TXEN CHR9 RXB8 TXB8 UCR
Read/Write R/W R/W R/W R/W R/W R/W R W
Initial value 0 0 0 0 0 0 1 0
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