Datasheet

TL16C554A, TL16C554AI
ASYNCHRONOUS-COMMUNICATIONS ELEMENT
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SLLS509E − AUGUST 2001 − REVISED JUNE 2010
33
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
PRINCIPLES OF OPERATION
autoflow control (see Figure 20)
Autoflow control is comprised of auto-CTS and auto-RTS. With auto-CTS, the CTS input must be active before
the transmitter FIFO can send data. With auto-RTS
, RTS becomes active when the receiver can handle more
data and notifies the sending serial device. When RTS
is connected to CTS, data transmission does not occur
unless the receiver FIFO has space for the data; thus, overrun errors are eliminated using ACE1 and ACE2 from
a TL16C554A with the autoflow control enabled. Otherwise, overrun errors may occur when the transmit-data
rate exceeds the receiver FIFO read latency.
RCV
FIFO
Serial to
Parallel
Flow
Control
XMT
FIFO
Parallel
to Serial
Flow
Control
Parallel
to Serial
Flow
Control
Serial to
Parallel
Flow
Control
XMT
FIFO
RCV
FIFO
ACE1 ACE2
D7 −D0
SIN SOUT
RTS
CTS
SOUT SIN
CTS
RTS
D7 −D0
Figure 20. Autoflow Control (Auto-RTS and Auto-CTS) Example
auto-RTS (see Figure 20)
Auto-RTS data flow control originates in the receiver timing and control block (see functional block diagram)
and is linked to the programmed receiver FIFO trigger level. When the receiver FIFO level reaches a trigger level
of 1, 4, or 8 (see Figure 22) RTS
is deasserted. With trigger levels of 1, 4, and 8, the sending ACE may send
an additional byte after the trigger level is reached (assuming the sending ACE has another byte to send)
because it may not recognize the deassertion of RTS
until after it has begun sending the additional byte. RTS
is automatically reasserted once the RCV FIFO is emptied by reading the receiver-buffer register.
When the trigger level is 14 (see Figure 23), RTS
is deasserted after the first data bit of the 16th character is
present on the SIN line. RTS
is reasserted when the RCV FIFO has at least one available byte space.
auto-CTS (see Figure 20)
The transmitter circuitry checks CTS before sending the next data byte. When CTS is active, it sends the next
byte. To stop the transmitter from sending the following byte, CTS
must be released before the middle of the
last stop bit currently being sent (see Figure 21). The auto-CTS
function reduces interrupts to the host system.
When flow control is enabled, CTS
level changes do not trigger host interrupts because the device automatically
controls its own transmitter. Without auto-CTS
the transmitter sends any data present in the transmit FIFO and
a receiver overrun error may result.
enabling autoflow control and auto-CTS
Autoflow control is enabled by setting modem-control register bits 5 (autoflow enable or AFE) and 1 (RTS) to
a 1. Autoflow incorporates both auto-RTS
and auto-CTS. When only auto-CTS is desired, bit 1 in the modem-
control register should be cleared (this assumes that an external control signal is driving CTS
).