FDC37M60X SMSC Corporation, FDC37M60X Datasheet - Page 79

• Manufacturer Part Number: FDC37M60X
• Description: ENHANCED SUPER I/O CONTROLLER WITH INFRARED SUPPORT
• Manufacturer: SMSC Corporation

NOTES ON SERIAL PORT OPERATION
FIFO MODE OPERATION:
GENERAL
The RCVR FIFO will hold up to 16 bytes
regardless of which trigger level is selected.
TX AND RX FIFO OPERATION
The Tx portion of the UART transmits data
through TXD as soon as the CPU loads a byte
into the Tx FIFO.
loads to the Tx FIFO if it currently holds 16
characters. Loading to the Tx FIFO will again
be enabled as soon as the next character is
transferred to the Tx shift register.
capabilities account for the largely autonomous
operation of the Tx.
The UART starts the above operations typically
with a Tx interrupt.
interrupt whenever the Tx FIFO is empty and the
Tx interrupt is enabled, except in the following
instance. Assume that the Tx FIFO is empty
and the CPU starts to load it. When the first
byte enters the FIFO the Tx FIFO empty
interrupt will transition from active to inactive.
Depending on the execution speed of the service
routine software, the UART may be able to
transfer this byte from the FIFO to the shift
register before the CPU loads another byte. If
this happens, the Tx FIFO will be empty again
and typically the UART's interrupt line would
transition to the active state. This could cause a
system with an interrupt control unit to record a
Tx FIFO empty condition, even though the CPU
is currently servicing that interrupt. Therefore,
after the first byte has been loaded into the
FIFO the UART will wait one serial character
transmission time before issuing a new Tx
FIFO empty interrupt. This one character Tx
interrupt
delay
The UART will prevent
will
The chip issues a Tx
remain
active
These
79
until at least two bytes have the Tx FIFO
empties after this condition, the Tx been
loaded into the FIFO, concurrently.
interrupt will be activated without a one
character delay.
Rx support functions and operation are quite
different
transmitter. The Rx FIFO receives data until the
number of bytes in the FIFO equals the selected
interrupt trigger level.
interrupts are enabled, the UART will issue an
interrupt to the CPU. The Rx FIFO will continue
to store bytes until it holds 16 of them. It will
not accept any more data when it is full. Any
more data entering the Rx shift register will set
the Overrun Error flag. Normally, the FIFO
depth and the programmable trigger levels will
give the CPU ample time to empty the Rx FIFO
before an overrun occurs.
One side-effect of having a Rx FIFO is that the
selected interrupt trigger level may be above the
data level in the FIFO. This could occur when
data at the end of the block contains fewer bytes
than the trigger level.
issued to the CPU and the data would remain in
the UART.
having to check for this situation the chip
incorporates a timeout interrupt.
The timeout interrupt is activated when there is
a least one byte in the Rx FIFO, and neither the
CPU nor the Rx shift register has accessed the
Rx FIFO within 4 character times of the last
byte. The timeout interrupt is cleared or reset
when the CPU reads the Rx FIFO or another
character enters it.
These FIFO related features allow optimization
of CPU/UART transactions and are especially
useful given the higer baud rate capability (256
kbaud).
from
To prevent the software from
those
No interrupt would be
described
At that time if Rx
for
When
the