AM79C971VCW datasheet(41/265 Pages) AMD

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Am79C971

Figure 16.

Disconnect With Data Transfer

Disconnect Without Data Transfer

Figure 17 shows a target disconnect sequence during

which no data is transferred. STOP is asserted on clock

4 without TRDY being asserted at the same time. The

Am79C971 controller terminates the access with the

deassertion of FRAME on clock 5 and of IRDY one

clock cycle later. It finally releases the bus on clock 7.

The Am79C971 controller will again request the bus

after two clock cycles to retry the last transfer. The

starting address of the new transfer will be the address

of the last non-transferred data.

Target Abort

Figure 18 shows a target abort sequence. The target

asserts DEVSEL for one clock. It then deasserts

DEVSEL and asserts STOP on clock 4. A target can

use the target abort sequence to indicate that it can-

not service the data transfer and that it does not want

the transa ctio n to be retr ied. Ad ditio nally, the

Am79C971 controller cannot make any assumption

about the success of the previous data transfers in the

current transaction. The Am79C971 controller termi-

nates the current transfer with the deassertion of

FRAME on clock 5 and of IRDY one clock cycle later.

It finally releases the bus on clock 6.

Since data integrity is not guaranteed, the Am79C971

controller cannot recover from a target abort event. The

Am79C971 controller will reset all CSR locations to

their STOP_RESET values. The BCR and PCI config-

uration registers will not be cleared. Any on-going net-

wo r k tran sm ission is te r minat ed in an orde r ly

sequence. If less than 512 bits have been transmitted

onto the network, the transmission will be terminated

immediately, generating a runt packet. If 512 bits or

more have been transmitted, the message will have the

current FCS inverted and appended at the next byte

boundary to guarantee an FCS error is detected at the

receiving station.

FRAME

CLK

IRDY

TRDY

C/BE

DEVSEL

REQ

GNT

PAR

DEVSEL is sampled

ADDRi

0000

0111

PAR

0111

3456789

PAR

DATA

STOP

ADDRi+8

DATA

20550D-19

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