CD1865 datasheet(70/150 Pages) INTEL

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CD1865 — Intelligent Eight-Channel Communications Controller

Datasheet

7.2.4

Special Transmitter Commands

The CD1865 is capable of sending special characters preemptively (bypassing the FIFO): sending

break characters and inserting delays or pauses either between characters or to lengthen a break.

There are two basic mechanisms the CD1865 uses for these ‘Send Special Character’ and

‘Embedded Transmit Command’ functions.

7.2.5

Special Character Transmission by Send

Special Character Command

Selected special characters, or two-character sequences, may be transmitted preemptively by

setting the appropriate bits in the Channel Command register (CCR). The Send Special Character

(SEND SP CH) bit of the CCR, when set, initiates the Send Special Character Command. SSPC0–

2 bits of the CCR then specify which character or two-character sequence is used. The choice of a

single- or two-character sequence is determined by the XonCH and XoffCH bits of COR3.

When a Send Special Character Command is given, the CD1865 inserts the special character(s)

into the data stream immediately following the current character in the Transmit Holding register.

Thus, it is ensured that the special character begins transmitting within two-character times after

the command is issued. The Send Special Character Command overrides all other flow-control

modes, including the state of TXEN and CTS*. Generally this is the preferred case. However,

sample CTS* or CD* in some applications to determine if it is okay to send a character before

invoking the Send Special Character Command.

The CCR is reset by the CD1865 as an acknowledgment of the command. A new command must

not be issued if the CCR contents are non-zero. A send special character command is recognized

and cleared within 125

µs (at 15 MHz, proportionally longer at lower clock speeds), unless a break

is being sent. If a break is being sent, the special character is not sent until after the break time is

complete.

7.2.6

Embedded Transmit Commands

The CD1865 may be enabled to recognize certain ‘escape’ sequences as commands embedded in

the Transmit Data Stream. These commands are issued to introduce a time delay between

characters, to insert an idle period during the transmission, or to send a break on the line.

These capabilities are enabled on a per-channel basis by setting the Embedded Transmit Command

(ETC) bit in the Channel Option register 2 (COR2). The ‘null’ (00) character is used as the

controlling character to initiate the special action. To preserve data transparency, two mechanisms

are provided to allow the null character to be sent as data. If the host must transmit a null character

as data, either the ETC mode may be disabled, or the null character may be preceded by a null, that

is, ‘00 00’ causes one-null character to be sent. If the ETC bit is not set, the ‘00’ character has no

effect, and it may be sent as ordinary data. ETC mode may be enabled or disabled ‘on-the-fly’.

The CD1865 uses the Transmit Timer to generate time delays between characters in the output data

stream. It is also used to extend the duration of a line-break transmit condition when the delay is

inserted between the ‘Start Break’ and ‘Stop Break’ embedded-transmit commands. All of the

timers count ticks are determined by the Prescaler Counter. The two eight-bit Prescaler Period

registers (PPRH and PPRL) determine the real-time length of a tick. A tick is the period of the

CD1865 System Clock Input (CLK) multiplied by the Period registers’ contents.

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