Slow AMBA Peripherals
13-16
GMS30C7201 Data Sheet
When the CPU accesses the IIR, the UART freezes all interrupts and indicates the
highest priority pending interrupt to the CPU. While this CPU access is occurring, the
UART records new interrupts, but does not change its current indication until the access
is complete.
Table 13-6: Summary of registers
on page 13-10 shows the contents of
the IIR.
Details on each bit are outlined below.
Bit 0:
This bit can be used in a prioritized interrupt environment to indicate
whether an interrupt is pending. When bit 0 is a logic 0, an interrupt is
pending and the IIR contents may be used as a pointer to the
appropriate interrupt service routine. When bit 0 is a logic 1, no
interrupt is pending.
Bit 1 and 2: These two bits of the IIR are used to identify the highest priority
interrupt pending as indicated in
Table 13-10: Interrupt control
functions
on page 13-16.
Bit 3:
In the 16450 mode this bit is 0. In the FIFO mode, this bit is set along
with bit 2 when a time-out interrupt is pending.
Bit 4 and 5: These two bits of the IIR are always logic 0.
Bit 6 and 7: These two bits are set when FCR0 = 1.
FIFO
Mode
Only
Interrupt Identification
Register
Interrupt Set and Reset Functions
Bit 3
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 or Parity
Error or Framing Error or
Break Interrupt
Reading the Line Status
Register
0
1
0
0
Second
Receiver Data
Available
Receiver Data Available or
Trigger Level Reached
Reading the Receiver
Buffer Register or the
FIFO drops below the
trigger level
1
1
0
0
Second
Character Time-
out Indication
No Characters have been
removed from or input to
the RCVR FIFO during the
last 4 Character times and
there is at least 1 Character
in it during this time
Reading the Receiver
Buffer Register
0
0
1
0
Third
Transmitter
Holding Register
Empty
Transmitter Holding
Register Empty
Reading the IIR Register
(if source of interrupt) or
writing into the
Transmitter Holding
Register
0
0
0
0
Fourth
MODEM Status
Clear to Send or Data Set
Ready or Ring Indicator or
Data Carrier Detect
Reading the MODEM
Status Register
Table 13-10: Interrupt control functions