TL16C552A, TL16C552AM
DUAL ASYNCHRONOUS COMMUNICATIONS ELEMENT
WITH FIFO
SLLS189D – NOVEMBER 1994 – REVISED JANUARY 1999
PRINCIPLES OF OPERATION
interrupt enable register (IER) (continued)
Bit 1: When IER1 is set, the transmitter holding register empty interrupt is enabled.
Bit 2: When IER2 is set, the receiver line status interrupt is enabled.
Bit 3: When IER3 is set, the modem status interrupt is enabled.
Bits 4 – 7: IER4 through IER7 are cleared.
In order to minimize software overhead during data character transfers, the serial channel prioritizes interrupts
into four levels. The four levels of interrupt conditions are as follows:
Priority 1 – Receiver line status (highest priority)
Priority 2 – Receiver data ready or receiver character time out
Priority 3 –Transmitter holding register empty
Priority 4–Modem status (lowest priority)
Information indicating that a prioritized interrupt is pending and the type of interrupt is stored in the IIR. The IIR
indicates the highest priority interrupt pending. The contents of the IIR are indicated in Table 5.
Table 5. Interrupt Control Functions
INTERRUPT
IDENTIFICATION
REGISTER
INTERRUPT SET AND RESET FUNCTIONS
PRIORITY
LEVEL
INTERRUPT RESET
CONTROL
BIT 3 BIT 2 BIT 1 BIT 0
INTERRUPT TYPE
INTERRUPT SOURCE
0
0
0
0
1
1
0
1
0
1
0
0
None
First
None
None
OE, PE, FE, or BI
None
Receiver line status
LSR read
Second
Received data available Receiver data available or trigger level RBR read until FIFO
reached
drops below the
trigger level
1
1
0
0
Second
Character time-out
indicator
No characters have been removed from or RBR read
input to the receiver FIFO during the last
four character times and there is at least
one character in it during this time.
0
0
0
0
1
0
0
0
Third
THRE
THRE
IIR read if THRE is
the interrupt source
or THR write
Fourth
Modem status
CTS, DSR, RI, or DCD
MSR read
Bit 0: IIR0 indicates whether an interrupt is pending. When IIR0 is cleared, an interrupt is pending.
Bits 1 and 2: IIR1 and IIR2 identify the highest priority interrupt pending, as indicated in Table 5.
Bit 3: IIR3 is always cleared in TL16C450 mode. This bit is set along with bit 2 in FIFO mode and when a
trigger change level interrupt is pending.
Bits 4 and 5: IIR4 and IIR5 are always cleared.
Bits 6 and 7: IIR6 and IIR7 are set when FCR0 = 1.
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