SCD1284 datasheet(57/176 Pages) INTEL

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IEEE 1284-Compatible Parallel Interface Controller — CD1284

Datasheet

If enabled in SCD12 and a character matching the contents of SCHR2 is received (the XOFF

character), the CD1284 checks that automatic transmit in-band flow control is enabled in COR2[6].

If this function is enabled, the CD1284 stops transmission after the current transmitting character

and the character in the Transmitter Holding register, if any, are sent. If enabled, the CD1284 also

attempts to match against errored characters. This function is enabled by the CMOE bit (COR5[5]).

COR2[7] enables IXM (Implied XON mode), which determines the character that restarts

transmission after a stop by automatic flow control. If IXM (COR2[7]) is ‘0’, only a programmed

XON character (SCHR1) can restart the transmitter; all other characters are received and placed in

the FIFO. If IXM is reset, any character received restarts data transmission. TxIBE (COR2[6])

must be set to active automatic flow control, otherwise IXM (COR2[7]) has no effect.

As with receiver flow control, the CPU can determine the current state of the transmitter through

TXFloff and TxFlon (CCSR[2:1]). When automatic in-band flow control is enabled and the

CD1284 receives an XOFF character, TxFloff is set. When an XON character is received, TxFlon

is set. Once transmission resumes, TxFlon is cleared. The encoding for TxFloff and TxFlon is

shown in Table 16.

TxFloff and TxFlon are cleared whenever the transmitter is disabled or enabled, regardless of the

state of flow control when the disable/enable occurred. This feature can force transmission to

resume regardless of remote-initiated flow control.

One final aspect of automatic in-band flow control is FCT (Flow Control Transparency). FCT is

enabled/disabled in COR3[5] and determines if remote-initiated flow control is transparent to the

CPU. If FCT is not set, in addition to stopping transmission when an XOFF character is received,

the CD1284 places the received XOFF character in the receive FIFO and informs the CPU with a

receive exception service request. When the XON character is received, it is also sent to the CPU

by an exception service request, then restarts data transmission.

If FCT is enabled, received flow control characters control transmission, but are discarded instead

of being placed in the FIFO. If the CPU does not require to know when its transmit data has been

stopped, this bit can be set to reduce the number of service requests that must be handled.

Table 17 summarizes the control bits in the CORs that enable the various modes of in-band flow

control.

Table 16. CCSR[2:1] Encoding

TxFloff

TxFlon

Encoded Status

Transmission resumes, transmitter is

enabled/disabled, or the transmitter is

in the default reset state.

XON was received, but transmission

has not restarted.

XOFF was received, transmission has

stopped.

Not used.

SCD1284 Datasheet(PDF) 57 Page - Intel Corporation

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