TL16C550D_09 Datasheet, PDF (34/54 Pages) Texas Instruments – ASYNCHRONOUS COMMUNICATIONS ELEMENT WITH AUTOFLOW CONTROL

TL16C550D, TL16C550DI
SLLS597E – APRIL 2004 – REVISED DECEMBER 2008 .................................................................................................................................................. www.ti.com
Table 4. Receiver FIFO Trigger Level
BIT 7
0
0
1
1
BIT 6
0
1
0
1
RECEIVER FIFO
TRIGGER LEVEL (BYTES)
01
04
08
14
FIFO Interrupt Mode Operation
When the receiver FIFO and receiver interrupts are enabled (FCR0 = 1, IER0 = 1, IER2 = 1), a receiver interrupt
occurs as follows:
1. The received data available interrupt is issued to the microprocessor when the FIFO has reached its
programmed trigger level. It is cleared when the FIFO drops below its programmed trigger level.
2. The IIR receive data available indication also occurs when the FIFO trigger level is reached, and like the
interrupt, it is cleared when the FIFO drops below the trigger level.
3. The receiver line status interrupt (IIR = 06) has higher priority than the received data available (IIR = 04)
interrupt.
4. The data ready bit (LSR0) is set when a character is transferred from the shift register to the receiver FIFO. It
is cleared when the FIFO is empty.
When the receiver FIFO and receiver interrupts are enabled:
1. FIFO time-out interrupt occurs if the following conditions exist:
a. At least one character is in the FIFO.
b. The most recent serial character was received more than four continuous character times ago (if two
stop bits are programmed, the second one is included in this time delay).
c. The most recent microprocessor read of the FIFO has occurred more than four continuous character
times before. This causes a maximum character received command to interrupt an issued delay of 160
ms at a 300-baud rate with a 12-bit character.
2. Character times are calculated by using the RCLK input for a clock signal (makes the delay proportional to
the baud rate).
3. When a time-out interrupt has occurred, it is cleared and the timer is cleared when the microprocessor reads
one character from the receiver FIFO.
4. When a time-out interrupt has not occurred, the time-out timer is cleared after a new character is received or
after the microprocessor reads the receiver FIFO.
When the transmitter FIFO and THRE interrupts are enabled (FCR0 = 1, IER1 = 1), transmit interrupts occur as
follows:
1. The transmitter-holding-register-empty interrupt [IIR (3−0) = 2] occurs when the transmit FIFO is empty. It is
cleared [IIR (3−0) = 1] when the THR is written to (1 to 16 characters may be written to the transmit FIFO
while servicing this interrupt) or the IIR is read.
2. The transmitter-holding-register-empty interrupt is delayed one character time minus the last stop bit time
when there have not been at least two bytes in the transmitter FIFO at the same time since the last time that
the FIFO was empty. The first transmitter interrupt after changing FCR0 is immediate if it is enabled.
FIFO-Polled Mode Operation
With FCR0 = 1 (transmitter and receiver FIFOs enabled), clearing IER0, IER1, IER2, IER3, or all four to 0 puts
the ACE in the FIFO-polled mode of operation. Because the receiver and transmitter are controlled separately,
either one or both can be in the polled mode of operation.
In this mode, the user program checks receiver and transmitter status using the LSR. As stated previously:
• LSR0 is set as long as one byte is in the receiver FIFO.
• LSR1 through LSR4 specify which error(s) have occurred. Character error status is handled the same way as
when in the interrupt mode; the IIR is not affected since IER2 = 0.
• LSR5 indicates when the THR is empty.
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