TL16C550C, TL16C550CI

ASYNCHRONOUS COMMUNICATIONS ELEMENT

WITH AUTOFLOW CONTROL

SLLS177F – MARCH 1994 – REVISED MARCH 2001

3

POST OFFICE BOX 655303

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.

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 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).