SC28L202 datasheet(23/77 Pages) PHILIPS

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Philips Semiconductors

Objective specification

SC28L202

Dual UART

2000 Feb 10

Traditional methods of polling status registers may also be used.

Their lower efficiency may be greatly offset by use of the UCIR

command and the read of the CIR. They reduce the many reads and

tests of status registers to only one read and one write. This would

normally be accomplished by setting the interrupt threshold to zero.

Then the moment any system within the DUART needs service the

next poll of the CIR would return a non zero value and the type field

will inform the processor which of the possible 18 systems needs

service. In the case of the FIFOs the number of bytes to be written

or read is also available.

Character and Address Recognition

(Also used for Multi–drop, Xon/Xoff systems)

Character recognition is specific to each of the two UARTs. Three

programmable characters are provided for the character recognition

for each channel. The three are general purpose in nature and may

be set to only cause an interrupt or to initiate some rather complex

operations specific to ”Multi–drop” address recognition or in–band

Xon/Xoff flow control.

Character recognition system continually examines the incoming

data stream. Upon the recognition of a character bits appropriate for

the character recognized are set in the Xon/Xoff Interrupt Status

setting of these bit(s) will initiate any of the automatic sequences or

and/or an interrupt that may have enabled via the MR0 register.

NOTE: Reading the XISR Clears the status bits associated with the

recognition.

The characters of the recognition system are fully programmable.

The Xon/Xoff characters will be set to the standard characters if the

hardware or software reset is used.

The character recognition circuits are basically designed to provide

general–purpose character recognition. Additional control logic has

been added to allow for Xon/Xoff flow control and for recognition of

the address character in the multi–drop or ”wake–up” mode. This

logic also allows for the generation of interrupts in either the

general–purpose recognition mode or the specific conditions

mentioned above.

The generality of the above provides a modicum of compatibility to

BOP (Bit Oriented Protocol) where the generation and detection of

“flags” is required. Parts of usually synchronous BOP protocols

(HDLC in particular) are beginning to show up in asynchronous

formats.

Character Stripping

The MR0 register provides for stripping the characters used for

character recognition. Recall that the character recognition may be

conditioned to control several aspects of the communication.

However this system is first a character recognition system. The

status of the various states of this system is reported in the XISR

and ISR registers. The character stripping of this system allows for

the removal of the specified control characters from the data stream:

two for the Xon /Xoff and one for the wake up. Via control in the

MR0 register these characters may be discarded (stripped) from the

data stream when the recognition system “sees” them or they may

be sent on the RxFIFO. Whether they are stripped or not the

recognition system will process them according to the action

requested; flow control, wake up, interrupt generation, etc. Care

should be exercised in programming the stripping option if noisy

environments are encountered. If a normal character were corrupted

to a Xoff character the transmitter would be stopped. If that

character were now stripped from the FIFO stack, then that stripping

action would make it difficult to determine the cause of transmitter

stopping.

When character stripping is invoked and a recognition character is

received that has an error bit set that character is sent to the

RxFIFO even though character stripping is active.

Flow Control (Xon/Xoff)

This section describes in–band flow control or Xon/Xoff signaling.

For the RTS/CTS hardware (out–of–band) control see MR1(7) and

MR2(4) descriptions.

The flow control is accomplished via the character recognition

system giving recognition information to the flow control processor.

Xon and Xoff are special characters used by a receiver to start and

stop the remote transmitter that is sending it data. As described

below several modes of manual and automatic flow control are

available by program control.

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 MR0. Bits 3 and 2

reset to zero resulting in all Xon/Xoff processing being disabled. If

MR0 [2] is set, Xon/Xoff characters received may gate the

transmitter. If MR0 [3] is set, the transmitter will transmit Xon and

Xoff when triggered by attainment of fixed fill levels in the channel

RxFIFO. The MR0 [7] bit also has a Xon/Xoff function control. If this

bit is set, a received Xon or Xoff character is not loaded into the

RxFIFO. If cleared, the power–on and reset default, the received

Xon or Xoff character is loaded onto the RxFIFO for examination by

the host CPU. The MR0 (7) function operates regardless of the

value in MR0 (3:2)

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 CRTX–– 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

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