TL16C550C, TL16C550CI
ASYNCHRONOUS COMMUNICATIONS ELEMENT
WITH AUTOFLOW CONTROL
SLLS177E – MARCH 1994 – REVISED APRIL1998
detailed description
autoflow control (see Figure 1)
Autoflow control is comprised of auto-CTS and auto-RTS. With auto-CTS, the CTS input must be active before
the transmitter FIFO can emit data. With auto-RTS, RTS becomes active when the receiver needs 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
TLC16C550C with the autoflow control enabled. If not, overrun errors occur when the transmit data rate exceeds
the receiver FIFO read latency.
ACE1
Serial to
Parallel
SIN
SOUT
ACE2
Parallel
to Serial
XMT
FIFO
Flow
Control
D7 – D0
Parallel
to Serial
SOUT
SIN
Serial to
Parallel
RCV
FIFO
Flow
Control
RCV
FIFO
Flow
Control
D7 – D0
RTS
CTS
XMT
FIFO
Flow
Control
CTS
RTS
Figure 1. Autoflow Control (Auto-RTS and Auto-CTS) Example
auto-RTS (see Figure 1)
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 3), 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 4), 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 1)
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 that is currently being sent (see Figure 2). 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 a control signal is driving CTS).
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