Datasheet
SLLS177H − MARCH 1994 − REVISED JANUARY 2006
26
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
PRINCIPLES OF OPERATION
FIFO interrupt mode operation (continued)
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 interrupt 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. Since 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:
D LSR0 is set as long as there is one byte in the receiver FIFO.
D LSR1 − 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.
D LSR5 indicates when the THR is empty.
D LSR6 indicates that both the THR and TSR are empty.
D LSR7 indicates whether there are any errors in the receiver FIFO.
There is no trigger level reached or time-out condition indicated in the FIFO polled mode. However, the receiver
and transmitter FIFOs are still fully capable of holding characters.