SCD1284 datasheet(58/176 Pages) INTEL

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CD1284 — IEEE 1284-Compatible Parallel Interface Controller

Datasheet

5.6.3

Out-of-Band Flow Control

Flow control can also be accomplished through the modem handshake signal pairs RTS/CTS and

DSR/DTR. These are called out-of-band because they are external to the data channel. The

CD1284 can be programmed to automatically respond to and generate out-of-band flow control

through these signals.

5.6.3.1

Receiver Out-of-Band Flow Control

Along with the receiver FIFO threshold that sets the level where the CD1284 posts a service

request, another threshold can be set to determine when it automatically asserts/deasserts DTR*.

This is the DTR threshold and is enabled in the DTRth[3:0] bits (MCOR1[3:0]). The level can be

set for any number of characters from 0 to 12. A threshold of zero disables the function and DTR*

is not controlled by the device. If the function and the receiver are enabled, the CD1284

automatically asserts the DTR* output whenever the number of characters in the receive FIFO is

less than the programmed number. Once the level reaches the threshold, DTR* is deasserted. DTR*

is held in the deasserted state until the CPU removes enough characters from the FIFO to lower the

level below the threshold.

For the receiver to operate properly, the DTR threshold must be set to a value equal to, or higher

than the receiver service-request threshold. If the levels were reversed, normal character reception

could not be completed because DTR* would always be deasserted before the receive FIFO

threshold is reached. The CPU would then not get a receive data service request until the receive

FIFO timeout is reached. This would result in a serial data transmission performance limitation.

The DTR* output can also be manually controlled through MSVR2[1]. Setting this bit to ‘1’ asserts

the DTR* output.

5.6.3.2

Transmitter Out-of-Band Flow Control

Transmitter out-of-band flow control is implemented with three modem control signals: the RTS*

output and the CTS* and DSR* inputs. The RTS* output can be programmed to be automatically

asserted whenever there is data in the transmit FIFO and the transmitter is cleared to send. CTS*

and DSR* can be enabled to automatically control the transmitter.

RTS Automatic Output is enabled in the RtsAO bit (COR2[2]). If RtsAO is set, the CD1284

automatically asserts the RTS* output when there is data in the FIFO to send. When the data is sent

and the FIFO is empty, RTS* is deasserted until the CPU places more data in the FIFO. If RTSAO

is not set and if required by the remote, the CPU must manually control the RTS* output through

MSVR1[0].

Table 17. COR Control Bits

Bit Name

Function

SCD12

COR3

Enables recognition of special

characters 1 and 2

FCT

COR3

Enables transparent flow control

TxIBE

COR2

Enables automatic transmitter in-

band flow control

IXM

COR2

Enables implied XON mode

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