CD2231 Intel Corporation, CD2231 Datasheet - Page 76

• Manufacturer Part Number: CD2231
• Description: CD2231 Intelligent Two-channel Lan And Wan Communications Controller
• Manufacturer: Intel Corporation

CD2231 — Intelligent Two-Channel LAN and WAN Communications Controller
6.5.1.1
6.5.1.2
76
Note: SLIP, MNP4, and Automatic In-Band Flow Control modes are only available on Revision B and
TxFloff = 0 is normal. TxFloff = 1 indicates that the channel has been requested by the remote to
stop transmission. This bit is reset to ‘0’ when the channel receives restart, as previously described.
This bit is reset to ‘0’ when the transmitter is enabled or disabled, or the channel is reset.
TxFlon = 0 is normal. TxFlon = 1 indicates 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 STCR[3] to a ‘1’.
The CSR contains status bits RxFloff (Receive Flow Off) and RxFlon (Receive Flow On), which
are used for receiver in-band flow control.
RxFloff = 0 is normal. RxFloff = 1 indicates the channel has requested that the remote stop
transmission. This bit is reset to ‘0’ when the channel requests that the remote restart 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 indicates that the channel has requested that the remote restart
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
later devices.
The CD2231 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 (Channel Option Register 5), the automatic feature is activated and
the CD2231 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 CD2231 keeps track of XON/XOFF 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 to the threshold being exceeded. For this reason, the user should not use the Send
Special Character command in the STCR (Special Transmit Command register) to send XON/
XOFF characters because the CD2231 does not keep track of flow control characters that it did not
send automatically. The result could cause confusion on the other end of the connection due to
conflicting flow control commands.
Automatic in-band flow control is functional only in standard Async and Async-HDLC/PPP
modes; SLIP and MNP4 expressly forbid in-band flow control. See the COR5 description on
page 99
for programming details.
Datasheet