MCS9845 MosChip Semiconductor, MCS9845 Datasheet - Page 15

• Manufacturer Part Number: MCS9845
• Description: PCI Dual UART
• Manufacturer: MosChip Semiconductor

Rev. 2.5
Interrupt Identifi cation Register (IIR)
An on-chip interrupt generation and prioritization
capability permits a fl exible interface with most popular
microprocessors.
IIR Bit-0:
0 = An interrupt is pending. Used either in a hardware
1 = No interrupt is pending.
IIR Bits 3-1:
There are fi ve prioritized levels of interrupts:
Priority 1 - Receiver Line Status (highest priority)
Priority 2 - Receiver Data Ready
Priority 3 - Receiver Character Time-Out
Priority 4 - Transmitter Holding Register Empty
Priority 5 - Modem Status (lowest priority)
When an interrupt is generated, the Interrupt
Identifi cation Register indicates that an interrupt is
pending and encodes the type of interrupt in its four
least signifi cant bits (bits 0, 1, 2, and 3).
Bits 3, 2, & 1 are used to identify the highest priority
interrupt pending.
Bit-0 will clear to “1” when no interrupt is pending.
Bit-3
prioritized or polled interrupt system.
0
0
1
0
0
Bit-2
1
1
1
0
0
Interrupt Priority Decode
Bit-1
1
0
0
1
0
Bit-0
0
0
0
0
0
Status Change
Data Ready
Transmitter
Data Error
Interrupt
Time-Out
Register
Receive
Receive
Receive
Source
Holding
Modem
Empty
To clear the interrupts, specifi c registers must be read
or written. The actions needed to clear each type of
interrupt are listed below:
Receive Data Error:
Reading the LSR will clear this interrupt. Software
should save the LSR value after reading the register if
it needs to remember the error condition.
Receive Data Ready:
Reading the RHR until the FIFO becomes empty will
clear this interrupt.
Receive Timeout:
Reading entire characters from RHR will clear this
interrupt.
Transmitter Holding Register Empty:
Writing a character into the THR or reading the IIR (if
source of interrupt) will clear this interrupt.
Modem Status Change:
Reading the MSR will clear this interrupt.
IIR Bit 4:
This bit is not used (always 0).
IIR Bit 5:
This bit is not used (always 0).
IIR Bits 7-6:
0 = In the 16C450 mode.
1 = When FCR-0 is equal to 1.
PCI Dual UART with ISA Bridge
MCS9845
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Page 15