SC16C554DB datasheet(16/51 Pages) PHILIPS

16 / 51 page

9397 750 13133

Product data sheet

Rev. 01 — 9 February 2005

16 of 51

Philips Semiconductors

SC16C554B/554DB

5 V, 3.3 V and 2.5 V quad UART, 5 Mbit/s (max.) with 16-byte FIFOs

6.4 Autoﬂow control (see Figure 7)

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 UART 1 and UART 2 from a

SC16C554B/554DB with the autoﬂow control enabled. If not, overrun errors occur when

the transmit data rate exceeds the receiver FIFO read latency.

6.4.1 Auto-RTS (see Figure 7)

Auto-RTS data ﬂow control originates in the receiver timing and control block (see block

diagrams in Figure 1 and Figure 2) 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 9), RTS

is de-asserted. With trigger levels of 1, 4, and 8, the sending UART may send an

additional byte after the trigger level is reached (assuming the sending UART has another

byte to send) because it may not recognize the de-assertion of RTS until after it has

begun sending the additional byte. RTS is automatically reasserted once the RX FIFO is

emptied by reading the receiver buffer register. When the trigger level is 14 (see

Figure 10), RTS is de-asserted after the ﬁrst data bit of the 16th character is present on

the RX line. RTS is reasserted when the RX FIFO has at least one available byte space.

6.4.2 Auto-CTS (see Figure 7)

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 8). 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.

Fig 7.

Autoﬂow control (auto-RTS and auto-CTS) example

RCV

FIFO

SERIAL TO

PARALLEL

FLOW

CONTROL

XMT

FIFO

PARALLEL

TO SERIAL

FLOW

CONTROL

PARALLEL

TO SERIAL

FLOW

CONTROL

SERIAL TO

PARALLEL

FLOW

XMT

FIFO

RCV

FIFO

ACE1

ACE2

D7 to D0

RTS

CTS

D7 to D0

CONTROL

002aaa048

CTS

RTS

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