RC56D, RC336D, and RC144D Modem Device Sets Designer’s Guide
1154
Conexant
4-5
4.2.3 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 a1, 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
Time-out
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 (DDSR) (MSR1),
Trailing Edge Ring Indicator (TERI)
(MSR3), or Delta DCD (DCD) (MSR4)
Reading the MSR
Notes:
1. FIFO Mode only.
Summary of Contents for RC144D
Page 87: ...INSIDE BACK COVER NOTES ...