CD1284 Datasheet, PDF (57/176 Pages) Intel Corporation – IEEE 1284-Compatible Parallel Interface Controller with Two High-Speed Asynchronous Serial Ports

IEEE 1284-Compatible Parallel Interface Controller — CD1284
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.
Table 16. CCSR[2:1] Encoding
TxFloff
0
0
1
1
TxFlon
0
1
0
1
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.
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.
Datasheet
57