CD1865 datasheet(75/150 Pages) INTEL

75 / 150 page

Intelligent Eight-Channel Communications Controller — CD1865

Datasheet

7.3.4

Transmitter Flow Control

The CD1865 provides both automatic In-Band (Xon/Xoff) and Out-of-Band Flow Control

functions. In-Band Flow Control recognizes special characters or character sequences for Xon and

Xoff control embedded in the data stream. Out-of-Band Flow Control uses the modem handshake

signals, RTS/CTS, to control the flow of data. Both types of flow control are implemented between

the Transmit FIFO and the Transmit Holding register, not between the Transmit Holding register

and the Transmit Shift register. Figure 26 on page 76 diagrams the transmitter flow-control logic.

All automatic flow-control functions are controlled by bits in Channel Option register 2 (COR2),

except DTR threshold, which is controlled by Modem Change Option register 1 (MCOR1).

Channel enable and flow-control status is stored in the Channel Control Status register (CCSR). A

TxEn bit shows the enabled status of the channel’s transmitter. Two bits, TxFloff and TxFlon, are

used to indicate the current state of the channels’ flow control.

Once the Automatic Flow-Control Modes are invoked by the host, all actions are transparent to the

host. If receipt of flow-control characters by the host is not required, the Flow-Control

Transparency bit of COR3 may be set to not pass received flow-control characters onto the host. If

TxIBE is set, the CD1865 implements the flow-control function on the transmitter regardless of the

FCT mode. The host can review the status of the channel by reading the Channel Control Status

If flow-control status is needed by the host, the SCDE and RxSC Control bits must be set and the

FCT bit must not be set. A special character detect status and the special character is presented to

the host by a Receive Exception Service Request. If the host wishes to manually flow-control the

transmitter, it can do so by using the TxEn bit, which stops transmission after the current character

completes.

RxFloff

RxFlon

Meaning

Illegal mode.

Xoff is last flow-control character sent

(flow off).

Xon is last flow-control character sent

(flow on).

Flow is on, data has been received.

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