Rainbow Electronics DS31256 - Manual (Page 134)

DS31256
134 of 181
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 Envoy 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
Envoy 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
1 2 3 4 5 6 7 8 9 10
PCLK
PFRAME
PSTOP
PDEVSEL
1 2 3 4 5 6 7 8 9 10
PCLK
PFRAME
PSTOP
PDEVSEL
PTRDY
PIRDY