SC68C752B_4

© NXP B.V. 2010. All rights reserved.

Product data sheet

Rev. 04 — 20 January 2010

9 of 48

NXP Semiconductors

SC68C752B

5 V, 3.3 V and 2.5 V dual UART, 5 Mbit/s (max.), with 64-byte FIFOs

6.2.2 Auto-CTS

The transmitter circuitry checks CTSn before sending the next data byte. When CTSn is

active, the transmitter sends the next byte. To stop the transmitter from sending the

following byte, CTSn must be de-asserted before the middle of the last stop bit that is

currently being sent. The auto-CTS function reduces interrupts to the host system. When

flow control is enabled, CTSn 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.

6.3 Software flow control

Software flow control is enabled through the Enhanced Feature Register and the Modem

Control Register. Different combinations of software flow control can be enabled by setting

different combinations of EFR[3:0]. Table 4 shows software flow control options.

When CTSn is LOW, the transmitter keeps sending serial data out.

When CTSn goes HIGH before the middle of the last stop bit of the current byte, the transmitter finishes sending the current

byte, but is does not send the next byte.

When CTSn goes from HIGH to LOW, the transmitter begins sending data again.

Fig 6.

CTSn functional timing

Start

byte 0 to 7

Stop

TXn

CTSn

002aaa227

Start

byte 0 to 7

Stop

Table 4.

Software flow control options (EFR[3:0])

EFR[3]

EFR[2]

EFR[1]

EFR[0]

TX, RX software flow controls

0

0

X

X

no transmit flow control

1

0

X

X

transmit Xon1, Xoff1

0

1

X

X

transmit Xon2, Xoff2

1

1

X

X

transmit Xon1, Xon2, Xoff1, Xoff2

X

X

0

0

no receive flow control

X

X

1

0

receiver compared Xon1, Xoff1

X

X

0

1

receiver compares Xon2, Xoff2

1

0

1

1

transmit Xon1, Xoff1

receiver compares Xon1 and Xon2, Xoff1 and Xoff2

0

1

1

1

transmit Xon2, Xoff2

receiver compares Xon1 and Xon2, Xoff1 and Xoff2

1

1

1

1

transmit Xon1, Xon2, Xoff1, Xoff2

receiver compares Xon1 and Xon2, Xoff1 and Xoff2