12.4.8 Interrupt Identification Register (IIR)

Table 12-14. IIR
Bit Number	Name	R/W	Reset Value	Description
[7:6]	Mode	R	0b11	Always 0b11. Enables FIFO mode.
[5:4]	Reserved	R	0	Software must not rely on the value of a reserved bit. To provide compatibility with future products, the value of a reserved bit must be preserved across a read-modify-write operation.
[3:0]	IIR	R	0b0001	Interrupt identification bits. 0b0110: Highest priority. Receiver line status interrupts due to overrun error, parity error, framing error, or break interrupt. Reading the line status register resets this interrupt. 0b0100: Second priority. Receive data available interrupt modem status interrupt. Reading the receiver buffer register (Table 12-5) or FIFO drops below the trigger level resets this interrupt. 0b1100: Second priority. Character timeout indication interrupt occurs when no characters have been read from the Rx FIFO during the last four character times and there was at least one character in it during this time. Reading the RBR resets this interrupt. 0b0010: Third priority. Transmit holding register empty interrupt. Reading the IIR or writing to the transmit holding register (Table 12-6) resets the interrupt. 0b0000: Fourth priority. Modem status interrupt due to clear to send, data set ready, ring indicator, or data carrier detect being asserted. Reading the modem status register resets this interrupt. 0b0011: Fifth priority. Multi-mode interrupts can occur due to any of the interrupts mentioned in IIM. For more details, see Table 12-15.

Table 12-15. Interrupt Identification Bit Values
IIR Value[3:0]	Priority Level	Interrupt Type	Interrupt Source	Interrupt Reset Control
0110	Highest	Receiver line status	Overrun error, parity error, framing error, or break interrupt	Reading the line status register.
0100	Second	Received data available	Receiver data available	Reading the receiver buffer register or FIFO drops below the trigger level.
1100	Second	Character timeout indication	No characters have been read from the Rx FIFO during the last four character times and there was at least one character in it during this time.	Reading the receiver buffer register.
0010	Third	Transmitter holding register empty	Transmitter holding register empty	Reading the IIR or writing into the transmitter holding register.
0000	Fourth	Modem status	Clear to send, data set ready, ring indicator, or data carrier detect	Reading the modem status register.
0011	Fifth	Multi mode interrupt	Any of the multi-mode interrupts in the Table 12-16 register.	See Table 12-16 (Table 12-16).