Information indicating that a prioritized interrupt is
pending and the source of that interrupt is stored
in the Interrupt Identification Register (refer to
Interrupt Control Table). When the CPU accesses
the IIR, the Serial Port freezes all interrupts and
indicates the highest priority pending interrupt to
the CPU. During this CPU access, even if the
Serial Port records new interrupts, the current
indication does not change until access is
completed. The contents of the IIR are described
below.
Bit 2
Setting this bit to a logic "1" clears all bytes in the
XMIT FIFO and resets its counter logic to 0. The
shift register is not cleared. This bit is self-
clearing.
Bit 3
Writing to this bit has no effect on the operation of
the UART. The RXRDY and TXRDY pins are not
available on this chip.
Bit 4,5
Reserved
Bit 6,7
Bit 0
These bits are used to set the trigger level for the
RCVR FIFO interrupt.
This bit can be used in either a hardwired
prioritized or polled environment to indicate
whether an interrupt is pending. When bit 0 is a
logic "0", an interrupt is pending and the contents
of the IIR may be used as a pointer to the
appropriate internal service routine. When bit 0 is
a logic "1", no interrupt is pending.
RCVR FIFO
Bit 6 Trigger Level (BYTES)
Bit 7
0
0
1
0
1
1
0
1
1
4
8
Bits 1 and 2
14
These two bits of the IIR are used to identify the
highest priority interrupt pending as indicated by
the Interrupt Control Table.
INTERRUPT IDENTIFICATION REGISTER (IIR)
Address Offset = 2H, DLAB = X, READ
Bit 3
In non-FIFO mode, this bit is a logic "0". In FIFO
mode this bit is set along with bit 2 when a timeout
interrupt is pending.
By accessing this register, the host CPU can
determine the highest priority interrupt and its
source. Four levels of priority interrupt exist. They
are in descending order of priority:
Bits 4 and 5
These bits of the IIR are always logic "0".
1. Receiver Line Status (highest priority)
2. Received Data Ready
Bits 6 and 7
3. Transmitter Holding Register Empty
4. MODEM Status (lowest priority)
These two bits are set when the FIFO CONTROL
Register bit 0 equals 1.
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