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Chapter 4 – SocketModem Parallel Interface – A Programmer's Description
Multi-Tech Systems, Inc. SocketModem MT5600SMI Developer’s Guide 22
IIR – Interrupt Identifier Register (Addr = 2)
The Interrupt Identifier Register (IIR) identifies the existence and type of up to five prioritized pending
interrupts. Four priority levels are set to assist interrupt processing in the host. The four levels, in
order of decreasing priority, are Highest: Receiver Line Status, 2: Receiver Data Available or
Receiver Character Timeout. 3: TX Buffer Empty, and 4: Modem Status.
When the IIR is accessed, the modem freezes all interrupts and indicates the highest priority interrupt
pending to the host. Any change occurring in interrupt conditions are not indicated until this access is
complete.
Bits 7-6 FIFO Mode
These two bits copy FCR0.
Bits 5-4 Not Used
Always 0.
Bits 3-1 Highest Priority Pending Interrupt
These three bits identify the highest priority pending interrupt (Table 4-2). Bit 3 is applicable
only when FIFO mode is selected; otherwise, bit 3 is a 0.
Bit 0 Interrupt Pending
When this bit is a 0, an interrupt is pending; IIR bits 1-3 can be used to determine the source
of the interrupt. When this bit is a 1, an interrupt is not pending.
Table 4–2. Interrupt Sources and Reset Control
Interrupt Identification Register Interrupt Set and Reset Functions
Bit 3
(Note 1)
Bit 2 Bit 1 Bit 0 Priority
Level
Interrupt Type Interrupt Source Interrupt Reset
Control
0 0 0 1 — None None —
0 1 1 0 Highest Receiver Line
Status
Overrun Error (OE) (LSR1),
Parity Error (PE) (LSR2),
Framing Error (FE) (LSR3),
or Break Interrupt (BI) (LSR4)
Reading the LSR
0 1 0 0 2 Received Data
Available
Received Data Available
(LSR0)
or RX FIFO Trigger Level
(FCR6-FCR7)
Reached
1
Reading the RX
Buffer or the RX
FIFO drops
below the
Trigger Level
1 1 0 0 2 Character Timeout
Indication
1
The RX FIFO contains at
least 1 character and no
characters have been
removed from or input to the
RX FIFO during the last 4
character times.
Reading the RX
Buffer
0 0 1 0 3 TX Buffer Empty TX Buffer Empty Reading the IIR
or writing to the
TX Buffer
0 0 0 0 4 Modem Status Delta CTS (DCTS) (MSR0),
Delta DSR (DDST) (MSR1),
Trailing Edge Ring Indicator
(TERI) (MSR3), or Delta
DCD (DCD) (MSR4)
Reading the
MSR
Notes:
1. FIFO Mode only.