SCD1284 datasheet(55/176 Pages) INTEL

55 / 176 page

IEEE 1284-Compatible Parallel Interface Controller — CD1284

Datasheet

The transmitter is capable of performing several special functions such as break generation, inter-

character delays, and automatic flow control. These functions are discussed in Section 5.6, Section

5.7, and Section 5.8.

As with the receiver, the transmitter has a timer associated with it. This timer generates the timing

for embedded transmit commands that send line breaks and inter-character delays. Whenever the

MPU detects an embedded transmit command specifying the delay command, this timer is loaded

with the value contained in the parameter byte. Then the timer is decremented on each tick of the

PPR (prescaler timer) until it reaches zero. At that time, the delay terminates unless the next

character in the FIFO is the beginning of another delay command sequence.

5.6

Flow Control

In all data communications applications, data is sent from one system to another by a protocol.

Most systems have a method of buffering data for transmission and reception.

In asynchronous protocol, there is no way at the protocol level to determine the length of a data

transmission. Therefore, it is not normally possible to designate a buffer area to handle the entire

length of the transmission. Also, the hardware receiving the data generally has a limited amount of

buffer area — usually a FIFO — and, if the CPU does not unload data fast enough, the buffer or

FIFO can overflow. For these reasons, two methods are provided to stop the remote from sending

data until there is space to receive data. This is known as flow control.

Flow control can be in-band or out-of-band. In-band flow control uses special characters that can

be sent to the CPU to stop data transmission. Out-of-band flow control are signals outside the serial

data channel that perform the same function: the RTS* (Request To Send) and CTS* (Clear To

Send) signal set, and the DSR* (Data Set Ready) and DTR* (Data Terminal Ready) signals.

The CD1284 supports manual flow control and has built-in capabilities for automatic and/or semi-

automatic (depending on direction and options) implementation without CPU intervention.

5.6.1

In-Band Flow Control

In-band flow control is implemented by special characters imbedded in the serial data stream; one

to request that transmission stop and one to request that data transmission resume. Any character

can be selected, although conventionally, the XON or DC1 (x’11) and XOFF or DC3 (x’13)

characters are selected if the ASCII character set is being used.

XOFF designates the character used to stop data transmission. XON determines the character used

to resume transmission. Whether these characters are used, the CD1284 allows the two characters

to be set to any value appropriate to the system design by the value programmed in SCHR1 and

SCHR2 (Special Character register 1 and 2).

SCHR1 defines the XON character and SCHR2 defines the XOFF character. These registers must

be initialized by the CPU; the default value loaded during device reset is ‘x’00’.

5.6.2

Receiver In-Band Flow Control

When the CPU senses that the sender requires flow-control due to the receive buffer filling too fast

to service, it can request the remote stop transmission by the transmitter sending an XOFF

character. This is accomplished by issuing a send special character 2 command through the CCR.

The CD1284 then transmits the character programmed in SCHR2.

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