OXCB950
OXFORD SEMICONDUCTOR LTD.
7.9 Automatic Flow Control
Automatic in-band flow control, automatic out-of-band flow
control and special character detection features can be
used when in Enhanced mode (flow control is software
compatible with the 16C654). Alternatively, 750-compatible
automatic out-of-band flow control can be enabled when in
non-Enhanced mode. In 950 mode, in-band and out-of-
band flow controls are compatible with 16C654 with the
addition of fully programmable flow control thresholds.
EFR[3:2]: In-band transmit flow control mode
When in-band transmit flow control is enabled, XON/XOFF
character(s) are inserted into the data stream whenever the
RFL passes the upper trigger level and falls below the
lower trigger level respectively.
For automatic in-band flow control, bit 4 of EFR must be
set. The combinations of software transmit flow control can
then be selected by programming EFR[3:2] as follows:
7.9.1 Enhanced Features Register ‘EFR’
logic [00] ⇒ In-band transmit flow control is disabled.
logic [01] ⇒ Single character in-band transmit flow control
enabled, using XON2 as the XON character
and XOFF2 as the XOFF character.
Writing 0xBF to LCR enables access to the EFR and other
Enhanced mode registers. This value corresponds to an
unused data format. Writing 0xBF to LCR will set LCR[7]
but leaves LCR[6:0] unchanged. Therefore, the data format
of the transmitter and receiver data is not affected. Write
the desired LCR value to exit from this selection.
logic [10] ⇒ Single character in-band transmit flow control
enabled, using XON1 as the XON character
and XOFF1 as the XOFF character.
logic[11] ⇒ The value EFR[3:2] = “11” is reserved for
future use and should not be used
Note: In-band transmit and receive flow control is disabled
in 9-bit mode.
EFR[4]: Enhanced mode
EFR[1:0]: In-band receive flow control mode
logic 0 ⇒ Non-Enhanced mode. Disables IER bits 4-7,
ISR bits 4-5, FCR bits 4-5, MCR bits 5-7 and in-
band flow control. Whenever this bit is cleared,
the setting of other bits of EFR are ignored.
logic 1 ⇒ Enhanced mode. Enables the Enhanced Mode
functions. These functions include enabling
IER bits 4-7, FCR bits 4-5, MCR bits 5-7. For
in-band flow control the software driver must
set this bit first. If this bit is set, out-of-band flow
control is configured with EFR bits 6-7,
otherwise out-of-band flow control is compatible
with 16C750.
When in-band receive flow control is enabled, the UART
compares the received data with the programmed XOFF
character(s). When this occurs, the UART will disable
transmission as soon as any current character
transmission is complete. The UART then compares the
received data with the programmed XON character(s).
When
a match occurs, the UART will re-enable
transmission (see section 7.11.6).
For automatic in-band flow control, bit 4 of EFR must be
set. The combinations of software receive flow control can
be selected by programming EFR[1:0] as follows:
EFR[5]: Enable special character detection
logic 0 ⇒ Special character detection is disabled.
logic 1 ⇒ While in Enhanced mode (EFR[4]=1), the
UART compares the incoming receiver data
with the XOFF2 value. Upon a correct match,
the received data will be transferred to the RHR
and a level 5 interrupt (XOFF or special
character) will be asserted if level 5 interrupts
are enabled (IER[5] set to 1).
logic [00] ⇒ In-band receive flow control is disabled.
logic [01] ⇒ Single character in-band receive flow control
enabled, recognising XON2 as the XON
character and XOFF2 as the XOFF
character.
logic [10] ⇒ Single character in-band receive flow control
enabled, recognising XON1 as the XON
character and XOFF1 and the XOFF
character.
EFR[6]: Enable automatic RTS flow control.
logic 0 ⇒ RTS flow control is disabled (default).
logic 1 ⇒ RTS flow control is enabled in Enhanced mode
(i.e. EFR[4] = 1), where the RTS# pin will be
forced inactive high if the RFL reaches the
upper flow control threshold. This will be
released when the RFL drops below the lower
threshold. 650 and 950-mode drivers should
use this bit to enable RTS flow control.
logic [11] ⇒ The behaviour of the receive flow control is
dependent on the configuration of EFR[3:2].
Single character in-band receive flow control
is enabled, accepting XON1 or XON2 as valid
XON characters and XOFF1 or XOFF2 as
valid XOFF characters when EFR[3:2] = “01”
or “10”. EFR[1:0] should not be set to “11”
when EFR[3:2] is ‘00’.
DS-0033 Sep 05
External-Free Release
Page 42
OXFORD [ OXFORD SEMICONDUCTOR ]
