SCD1284 datasheet(56/176 Pages) INTEL

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CD1284 — IEEE 1284-Compatible Parallel Interface Controller

Datasheet

As previously discussed, the send special character command is preemptive to data currently in the

transmit FIFO. The XOFF character is transmitted immediately after the current character and the

character in the Transmitter Holding register are sent (a maximum delay of two character times).

When the CPU is again ready to start receiving characters, the XON character is sent by another

send special character command. At this time, the CD1284 is issued the command to send the

character programmed in SCHR1.

Send special character commands override any flow-control by a remote of the CD1284. For

example, even if the CD1284 transmitter is shut off by the remote, it can still send flow control

characters.

The current state of the flow-control condition is always made available to the CPU through the

CCSR. In addition to the enabled/disabled status of the receiver and transmitter, the CCSR displays

the flow-control status.

Two bits in the CCSR pertain to receiver flow control, RxFloff and RxFlon. Whenever the CPU

issues the send special character 2 (send XOFF) command, the CD1284 sets the RxFloff bit,

indicating a request for the remote to stop transmission.

When the CPU issues the send special character 1 (send XON) command, RxFlon is set and

RxFloff reset. RxFlon remains set until the first character is received after XON is transmitted.

Table 15 shows the bit encoding for RxFloff and RxFlon.

RxFloff and RxFlon are cleared whenever the receiver is disabled or enabled, regardless of the

state of flow control when the disable/enable occurred.

Note:

Regardless of the current state of RxFloff, the CD1284 continues to receive characters. If the

remote ignores or is slow to respond to the XOFF character, there an overrun condition can occur.

5.6.2.1

Transmitter In-Band Flow Control

The CD1284 can automatically flow control its own transmitter when it receives the XON and

XOFF characters, as programmed in SCHR1 and SCHR2. There are control bits in COR2 and

COR3 to enable or disable various aspects of automatic flow control.

Special-character detection must be enabled through the SCD12 bit (COR3[4]) for flow-control

characters to be acted upon. When SCD12 is set, the CD1284 scans received characters for a match

with one of the special characters programmed in SCHR1–SCHR2.

Table 15. CCSR[6:5] Encoding

RxFloff

RxFlon

Encoded Status

Transmission resumes, the receiver is

enabled/disabled, or receiver is in the

default reset state.

XON is sent, but transmission has not

restarted.

XOFF was sent.

Not used.

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