Teledyne LeCroy SAS_SATA InFusion - Users Manual - Manual (Page 39)

SAS InFusion User Manual Version 2.0
LeCroy Corporation
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Spread Spectrum Clock to Target. Default setting is Disabled. When
disabled, InFusion uses a normal (fixed frequency) clock to transmit traffic to
the Target. When enabled,
InFusion uses a spread spectrum clock, where
the clock frequency varies over time through a small range of frequencies.
The intent of a spread spectrum clock is to reduce interference with other
devices that might be sensitive to the fixed clock frequency.
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Out of Band Sensitivity to Initiator. Default is Normal. When set to
Decrease by 30mV or Decrease by 60 mV, the sensitivity of the idle detection
circuits are decreased, to enable correct detection of marginal signals.
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Out of Band Sensitivity to Target. Default is Normal. When set to
Decrease by 30mV or Decrease by 60 mV, the sensitivity of the idle detection
circuits are decreased, to enable correct detection of marginal signals.
SATA Hold Handling in SAS InFusion
Under normal operation, SAS InFusion transmits data traffic in both directions without
any modifications at all, other than the specific modifications defined in scenarios and/or
test cases which the user has scripted and are currently active. The only normal effect
of inserting a SAS InFusion box into a data stream is the slight delay as traffic is received
by InFusion and then retransmitted.
However, there are certain unusual situations where even the slight delay in
retransmitting the traffic might cause problems in communication between initiator and
target, and one of these is the flow control mechanism defined as a HOLD condition.
SATA devices may pause data transfer from another device by issuing a HOLD primitive,
which requires the sending device to (very) quickly cease sending traffic and instead
transmit HOLDA primitives. Note that, under normal circumstances, the HOLD condition
applies to traffic in only one direction, and traffic in the other direction proceeds as usual.
In order to manage this issue, under these conditions InFusion will act like an expander
(which has to deal with the same issue), and as soon as InFusion receives a HOLD
primitive from a device, InFusion ceases to transmit data to that device and instead
transmits HOLDA primitives. To avoid losing data, InFusion will simultaneously begin
transmitting HOLD primitives to the sending device, and will buffer the small amount of
data received until the sending device can respond and cease sending data. When data
flow is restored, InFusion reestablishes communication including transmitting any data
traffic which was buffered before flow could be stopped.
Although highly unlikely, there are some special case scenarios that could develop. If
both devices begin sending HOLD primitives at the same time, InFusion cannot simply
send HOLDA primitives to both devices, since the device can continue to transmit data
while receiving HOLDA primitives. Instead, InFusion transmits HOLD primitives to one
device and HOLDA primitives to the other.
Another unlikely case is when the stream that gets interrupted (the stream flowing
towards the sender of HOLD) is already in a CONT condition. SATA has a mechanism
to prevent the need for repeated transmission of the same primitive. Instead of repeating
the same primitive hundreds or thousands of times, a device can send the primitive twice
followed inmmediately by a CONT (continue) primitive, which is a signal for the receving
device to assume the primitive repeated twice is continuing until a new primitive is
received. Any traffic that might occur between the CONT and the next valid primitive can
be treated as "junk data" and discarded by the receiver.