DS31256 datasheet(134/183 Pages) MAXIM

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DS31256 256-Channel, High-Throughput HDLC Controller

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10.1.5 PCI Target Retry

Targets can terminate the requested bus transaction before any data is transferred because the target is

busy and temporarily unable to process the transaction. Such a termination is called a target retry and no

data is transferred. A target retry is signaled to the initiator by the assertion of

PSTOP and not asserting

PTRDY on the initial data phase (Figure 10-6). When the DS31256 is a target, it only issues a target

retry when the host is accessing the local bus. This occurs when the local bus is operating in the

arbitration mode. It is busy at the instant the host requests access to the local bus. See Section 11.1 for

more details about the operation of the local bus.

Figure 10-6. PCI Target Retry

10.1.6 PCI Target Disconnect

A target can prematurely terminate a transaction by asserting

PSTOP (Figure 10-7). Depending on the

current state of the ready signals when

PSTOP is asserted, data may or may not be transferred. The target

always deasserts

PSTOP when it detects that the initiator has deasserted PFRAME. When the DS31256

is a target, it disconnects with data after the first data phase is complete, if the master attempts a burst

transaction. This is because the device does not support burst transactions when it is a target. When it is

an initiator and experiences a disconnect from the target, it attempts another bus transaction (if it still has

the bus granted) after waiting either one (disconnect without data) or two clock cycles (disconnect with

data).

Figure 10-7. PCI Target Disconnect

12345678910

PCLK

PFRAME

PSTOP

PDEVSEL

123456

78910

PCLK

PFRAME

PSTOP

PDEVSEL

PTRDY

PIRDY

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