CD2481 Datasheet, PDF (84/222 Pages) Intel Corporation – Programmable Four-Channel Communications Controller

CD2481 — Programmable Four-Channel Communications Controller
7.5.2
7.5.2.1
When in-band flow control is enabled (TxIBE = 1), and an XOFF character is received, the channel
will stop transmission after the current character in the Transmit Shift register and the current
character in the Transmit Holding register are transmitted. When IXM = 0, transmission will restart
after an XON character is received. When IXM = 1, transmission will restart after any character is
received.
The Flow Control Transparency (FCT) Mode bit (COR3[5]) is used to determine if the received
flow control characters are passed to the host. If FCT = 1, the characters are not passed to the host.
If FCT = 0, they are passed to the host as exception characters. This bit does not affect non-flow
control special characters.
Additional status information about the state of transmitter in-band flow control is available in the
Channel Status register (CSR). The Transmit Flow Off (TxFloff) and the Transmit Flow On
(TxFlon) (CSR[2:1]) bits indicate the current state.
TxFloff = 0 is normal. TxFloff = 1 means that the channel has been requested by the remote to stop
transmission. This bit is reset to 0 when the channel restarts transmission, as described above. This
bit is reset to 0 when the transmitter is enabled or disabled, or the channel is reset.
TxFlon = 0 is normal. TxFlon = 1 means that the channel has been requested by the remote to
restart transmission. This bit is reset to 0 once the channel has restarted transmission. This bit is
reset to 0 when the transmitter is enabled or disabled, or the channel is reset.
Receiver In-Band Flow Control
The channel can request the remote to stop transmission by sending an XOFF character. Likewise,
the channel can request the remote to restart transmission by sending an XON characters. The
XON/XOFF characters is transmitted by setting the SndSpc bit in STCR to a 1.
The CSR contains status bits Receive Flow Off (RxFloff) and Receive Flow On (RxFlon) that are
used for receiver in-band flow control.
RxFloff = 0 is normal. RxFloff = 1 means the channel has requested that the remote stop its
transmission. This bit is reset to 0 when the channel requests the that the remote restart its
transmission. This bit is reset to 0 when the receiver is enabled or disabled, or the channel is reset.
RxFlon = 0 is normal. RxFlon = 1 means that the channel has requested that the remote restart its
transmission. This bit is reset to 0 when the next non-flow control character is received. This bit is
reset to 0 when the receiver is enabled or disabled, or the channel is reset.
Automatic Receive In-Band Flow Control
The CD2481 can perform automatic in-band flow control, if desired. Automatic in-band flow
control means that the device sends XON and XOFF characters based on the level of characters in
the receive FIFO. This function is identical to the automatic out-of-band (hardware) flow control
that uses the DTR bin.
As with automatic hardware flow control, when the number of characters in the FIFO exceeds the
number programmed in COR5[3:0], the automatic feature is activated and the CD2481 transmits an
XOFF character (as defined by SCHR2). When the number of characters falls back to equal or
below the programmed value, an XON character (as defined by SCHR1) is transmitted. The
CD2481 keeps track of X-ON/X-OFF characters that it has sent so that erroneous flow control
characters are not transmitted. For example, it does not transmit an XON simply because the
number of characters is below the threshold; it only does so if it had previously sent an XOFF due
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Datasheet