Philips Semiconductors Product data sheet
SC28L202Dual UART
2005 Nov 01
16
The modes of control are described in MR3[3:2]
00 = Host mode
01 = Auto transmit
10 = Auto Receive
11 = Auto receive and transmit
Mode control
Xon/Xoff mode control is accomplished via the MR3[3:2].
00 Xon/Xoff processing disabled. The host will control Rx, Tx.
01 Auto Tx control. Tx is stopped/started when Xoff/Xon is
received.
10 Auto Rx control. Receiver commands Tx to send Xoff at
trigger level.
11 Auto Rx and Tx control. Receiver commands Tx to send
Xoff as the receiver fills and commands the Tx to send
Xon when Rx FIFO fill level is lowered. This results in total
automatic control. No processor interrupt is required.
Note that MR3[7] controls the stripping of Xoff/Xon characters.
0 Xon/Xoff characters are sent to the Rx FIFO
1 Xon/Xoff characters are discarded.
The MR3[7] functions regardless of the setting of MR3[3:2]. This
allows for general purpose character recognition and processing.
(See ‘Character Stripping’.)
Xon Xoff Characters
The programming of these characters is usually done individually.
The standard Xon/Xoff characters are . Xon is 0x11, Xoff 0x13. Any
enabling of the Xon/Xoff functions will use the contents of the Xon
and Xoff character registers as the basis on which recognition is
predicated.
Host mode
When neither the auto-receiver or auto-transmitter modes are set,
the Xon/Xoff logic is operating in the host mode. In host mode, all
activity of the Xon/Xoff logic is initiated by commands to the CRx.
The Xoff command forces the transmitter to disable exactly as
though a Xoff character had been received by the RxFIFO. The
transmitter will remain disabled until the chip is reset or the CR (7:3)
= 10110 (Xoff resume) command is given. In particular, reception of
a Xon or disabling or re-enabling the transmitter will NOT cause
resumption of transmission. Redundant CRTXxx commands, i.e.
CRTXon, CRTXon, are harmless, although they waste time. A
CRTXon may be used to cancel a CRTXoff (and vice versa) but both
may be transmitted depending on the command timing with respect
to that of the transmitter state machine.
Auto-transmitter mode
When a channel receiver loads a Xoff character into the RxFIFO, the
channel transmitter will finish transmission of the current character
and then stop transmitting. A transmitter so idled can be restarted by
the receipt of a Xon character by the receiver or by a hardware or
software reset. The last option results in the loss of the
untransmitted contents of the TxFIFO. When operating in this mode
the Command Register commands for the transmitter are not
effective.
While idle data may be written to the TxFIFO and it continue to
present its fill level to the interrupt arbiter and maintains the integrity
of its status registers.
Use of ’00’ as a Xon/Xoff character is complicated by the Receiver
break operation which loads a ’00’ character on the RxFIFO. The
Xon/Xoff character detectors do not discriminate in this case from a
Xon/Xoff character received through the RxD pin.
NOTE: To be recognized as a Xon or Xoff character, the receiver
must have room in the RxFIFO to accommodate the character. An
Xon/Xoff character that is received resulting in a receiver overrun
does not effect the transmitter nor is it loaded into the RxFIFO,
regardless of the state of the Xon/Xoff transparency bit, MR3[7].
Receiver Mode
Since the receiving FIFO resources in the Dual UART are limited,
some means of controlling a remote transmitter is desirable in order
to lessen the probability of receiver overrun. The Dual UART
provides two methods of controlling the data flow. There is a
hardware-assisted means of accomplishing control, the so-called
out-of-band flow control, and an in-band flow control method.
The out-of-band flow control is implemented through the
CTSN–RTSN signaling via the I/O ports. The operation of these
hardware handshake signals is described in the receiver and
transmitter discussions.
In-band flow control is a protocol for controlling a remote transmitter
by embedding special characters within the message stream, itself.
Two characters, Xon and Xoff, which do not represent normal
printable character take on flow control definitions when the
Xon/Xoff capability is enabled. Flow control characters received may
be used to gate the channel transmitter on and off. This activity is
referred to as Auto-transmitter mode. To protect the channel receiver
from overrun, fixed fill levels (hardware set at 240 characters) of the
RxFIFO may be employed to automatically insert Xon/Xoff
characters in the transmitter’s data stream. This mode of operation
is referred to as auto-receiver mode. Commands issued by the host
CPU via the CR can simulate all these conditions.
Auto Receive and Transmit
This is a combination of both modes.
NOTE: Xon /Xoff characters
The Xon/Xoff character with errors will be accepted as valid. The
user has the option sending or not sending these characters to the
FIFO. Error bits associated with Xon/Xoff will be stored normally to
the receiver FIFO.
The channel’s transmitter may be programmed to automatically
transmit a Xoff character without host CPU intervention when the
RxFIFO fill level exceeds a fixed limit (240). In this mode it will
transmit a Xon character when the RxFIFO level drops below a
second fixed limit (16). A character from the TxFIFO that has been
loaded into the TxD shift register will continue to transmit.
Character(s) in the TxFIFO that have not been loaded to the
transmitter shift register are unaffected by the Xon or Xoff
transmission. They will be transmitted after the Xon/Xoff activity
concludes.
If the fill level condition that initiates Xon activity negates before the
flow control character can begin transmission, the transmission of
the flow control character will not occur. That is, either of the
following sequences may be transmitted depending on the timing of
the FIFO level changes with respect to the normal character times:
Fix This
Character Xoff Xon Character
Character Character
Hardware keeps track of Xoff characters sent that are not rescinded
by a Xon. This logic is reset by writing MR3[3:2] to ‘00’. If the user
drops out of Auto-receiver mode while the XISR shows Xoff as the
