SCD1284 datasheet(49/176 Pages) INTEL

49 / 176 page

IEEE 1284-Compatible Parallel Interface Controller — CD1284

Datasheet

The I/O cycle that activates the SVCACKP* input also removes the active SVCREQP* output. The

request output is inactive until after the CPU terminates the acknowledge routine by writing to the

EOSRR. As with the serial channels, this is a dummy operation and the data written is ‘don’t care’.

The purpose of the write is to clear the internal logic of the current request context and allow it to

generate another request when the need arises. Until this write occurs, no further service requests

are made from the parallel channel. When the MPU detects the write to the EOSRR, it zeros-out

the PIVR in preparation for the next service-request cycle.

5.4.2

Software-Activated Context Switch, Parallel

Software-activated acknowledges of the parallel channel differ somewhat from those of the serial

channels. The start of a software acknowledge of the parallel channel is the same as for the serial

channels: the CPU copies the contents of the PIR into the CAR (after first saving the current

contents of the CAR) to set the device context. However, at this point the methods (serial versus

parallel) diverge. The CPU can read either the LIVR or PIVR (or read the status from the two status

registers in the Parallel Port register set) to determine which of the parallel channel blocks is

requesting service, copy the PIR into the CAR (or just load it with ‘x’00’) to set the context, then

proceed to service that request. Once the CPU has satisfied the request needs of the parallel

channel, it must toggle the IntEn bit (PFCR[4]) or clear the PIR. Toggling IntEn clears the PPort

and Pipeline bits and the PPIreq bit (PIR[7]). This action informs the MPU to clean up the PIVR

and remove the external request. The software should then restore the CAR to its previous contents

and exit the service routine.

The PPIreq bit can be cleared at any time by the CPU. If the system design requires the request be

removed quickly, the procedure can be performed at the beginning of the polled service routine. If

the CPU waits until the end of the service routine, it clears the bit itself or terminates the service in

the manner described, letting the MPU do it.

5.5

Serial Data Reception and Transmission

The CD1284 has two serial channels, each with a receiver and a transmitter. Although a receiver

and a transmitter pair are associated with each channel, in many respects they operate

independently, sharing only parameter settings regarding character format including length, parity

type if any, and number of stop bits. Each receiver and transmitter has its own baud rate generation

function, allowing a channel to send at one rate and receive at another. Shared and independent

parameters are shown in the following diagram.

RECEIVER

TRANSMITTER

BAUD RATE

PARITY

CHARACTER LENGTH

STOP BITS

FIFO THRESH

PRESCALE PERIOD REGISTER

RCV TIMEOUT

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