PC87303 datasheet(59/114 Pages) NSC

59 / 114 page

50 FDC Functional Description (Continued)

Verify mode of the DMA controller If DACK is held active

during the entire burst the trailing edge of the RD or WR

strobe is used to deassert DRQ DRQ is deasserted within

50 ns of the leading edge of DACK RD orWR This quick

response should prevent the DMA controller from transfer-

ring extra bytes in most applications

Overrun Errors

An overrun or underrun error terminates the execution of

the command if the system does not transfer data within the

allotted data transfer time (see Section 37) which puts the

controller into the Result Phase During a read overrun the

P is required to read the remaining bytes of the sector

before the controller asserts IRQ6 signifying the end of ex-

ecution During a write operation an underrun error termi-

nates the Execution Phase after the controller has written

the remaining bytes of the sector with the last correctly writ-

ten byte to the FIFO and generated the CRC bytes Whether

there is an error or not an interrupt is generated at the end

of the Execution Phase and is cleared by reading the first

Result Phase byte

DACK asserted alone without a RD or WR strobe is also

counted as a transfer If RD or WR are not being strobed for

each byte DACK must be strobed for each byte so that the

floppy controller can count the number of bytes correctly A

new command the Verify command has been added to

allow easier verification of data written to the disk without

the need of actually transferring the data on the data bus

5323 Interrupt ModeFIFO Disabled

If the Interrupt (Non-DMA) mode is selected IRQ6 is assert-

ed instead of DRQ when each byte is ready to be trans-

ferred The Main Status Register should be read to verify

that the interrupt is for a data transfer The RQM and NON

DMA bits (D7 and D5) in the MSR are set The interrupt is

cleared when the byte is transferred to or from the Data

fer the data in or out of the Data Register (A2 – A0 must be

valid) CS asserted by itself is not significant CS must be

asserted with RD or WR for a read or write transfer to be

recognized

The mP should transfer the byte within the data transfer

service time (see Section 37) If the byte is not transferred

within the time allotted an Overrun Error will be indicated in

the Result Phase when the command terminates at the end

of the current sector

An interrupt is also be generated after the last byte is trans-

ferred This indicates the beginning of the Result Phase

The RQM and DIO bits (D7 and D6) in the MSR is set and

the non-DMA bit (D5) is cleared This interrupt is cleared by

reading the first Result Phase byte

5324 Interrupt ModeFIFO Enabled

The Interrupt (Non-DMA) mode with the FIFO enabled is

very similar to the Non-DMA mode with the FIFO disabled

In this case IRQ6 is asserted instead of DRQ under the

exact same FIFO threshold trigger conditions The MSR

should be read to verify that the interrupt is for a data trans-

fer The RQM and NON DMA bits (D7 and D5) in the MSR is

set CS and RD or CS and WR must be used to transfer the

data in or out of the Data Register (A2 – A0 must be valid)

CS asserted by itself is not significant CS must be asserted

with RD or WR for a read or write transfer to be recognized

The Burst mode may be used to hold the IRQ6 pin active

during a burst or the Non-Burst mode may be used to tog-

gle the IRQ6 pin for each byte of a burst The Main Status

ample during a read command after the last byte of data

has been read from the disk and placed in the FIFO the

MSR still indicates that the Execution Phase is active and

that data needs to be read from the Data Register Only

after the last byte of data has been read by the mP from the

FIFO does the Result Phase begin

The same overrun and underrun error procedures from the

DMA mode apply to the Non-DMA mode Also whether

there is an error or not an interrupt is generated at the end

of the Execution Phase and is cleared by reading the first

Result Phase byte

5325 Software Polling

If the Non-DMA mode is selected and interrupts are not

suitable the mP can poll the MSR during the Execution

Phase to determine when a byte is ready to be transferred

The RQM bit (D7) in the MSR reflects the state of the IRQ6

signal Otherwise the data transfer is similar to the Interrupt

Mode described above This is true for the FIFO enabled or

disabled

533 Result Phase

During the Result Phase the mP reads a series of bytes

from the data register These bytes indicate the status of the

command This status may indicate whether the command

executed properly or contain some control information (see

the Command Description and Status Register Description)

These Result Phase bytes are read in the order specified for

that particular command Some commands will not have a

result phase Also the number of result bytes varies with

each command All of the result bytes must be read from

the Data Register before the next command can be issued

Like the Command Phase the Main Status Register con-

trols the flow of result bytes and must be polled by the

software before reading each Result Phase byte from the

Data Register The RQM bit (D7) and DIO bit (D6) must both

be set before each result byte can be read After the last

result byte is read the COM PROG bit (D4) in the MSR is

cleared and the controller is ready for the next command

534 Idle Phase

After a hardware or software reset or after the chip has

recovered from the power-down mode the controller enters

the Idle Phase Also when there are no commands in prog-

ress the controller is in the Idle Phase The controller waits

for a command byte to be written to the Data Register The

RQM bit is set and the DIO bit cleared in the MSR After

receiving the first command (opcode) byte the controller

enters the Command Phase When the command is com-

pleted the controller again enters the Idle Phase The Data

Separator remains synchronized to the reference frequency

while the controller is idle While in the Idle Phase the con-

troller will periodically enters the Drive Polling Phase (see

Section 535)

PC87303 Datasheet(PDF) 59 Page - National Semiconductor (TI)

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