SBI Bus Serializer ASSP Telecom Standard Product Data Sheet
Released
Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC-2000168, Issue 5
315
13.2 LVDS Hot Swapping
The LVDS electrical interface differs from a standard CMOS interface; there is no inherent
problem in leaving the LVDS inputs floating. Note that the LVDS receiver consists of a
differential amplifier with a wide common-mode range. The power dissipation is independent
of the data transitions (that is, if the input is connected). There is an internal 100 termination
across the positive and negative input. Floating inputs will settle to an arbitrary voltage
(between VDD and VSS) determined by leakage paths. Regardless of this arbitrary voltage, the
input structure of the receiver will operate in its proper range and the receiver output will be
logic 1 or 0 depending on internal offsets. Noise events (power supply noise, crosstalk) may
induce the receiver to toggle randomly, generating "ambiguous" data. This ambiguous data will
not result in any problems but is not a desirable condition since it simply wastes power.
Unused links should be disabled in software. This will ensure that the power consumption for
those links will be reduced to nearly 0 mW. There is no requirement for how quickly the link
should be disabled. Disabling the link simply results in lower power dissipation since the
circuitry will be shut down. This action is not mandatory, but is good practice.
During a hot-swapping situation, there will be no electrical damage on the LVDS inputs
provided that maximum ratings are not exceeded (see absolute maximum ratings section 15)
.
There are no problems with hot-swapping. The “hot-swap” channel can be left enabled and the
device will sync up once the far end transmitter is connected. There are no effects on other
channels. Hot swapping of cards is still allowed by reprogramming of the links in software.
The framing algorithm used in the receive framer blocks will keep the receiver in an out-of-
frame state in the case of a link being hot swapped while still enabled. The device will pass on
the random data received on this link, or the user may choose to pass on an AIS indication via
register configuration. Despite passing on random data, control signals will be suppressed such
that there will be no false J0 indications on the outgoing SBI or Telecom Bus.
13.3 Selecting Between the Receive Working and Protection Links
The data from only one of the two receive links is propagated to the Outgoing Bus on the SBS.
The selection of which link, the working or protect, the SBS listens to is controlled by the
RWSEL input or the RWSEL_VAL bit in register 001H. If the RWSEL_SRC bit in register
001H is set to a logic 0, the RWSEL_VAL bit is used to select the active link. If RWSEL_SRC
is a logic 1, the RWSEL input is used to select the active link.
RWSEL is sampled at the C1 position of every frame, as marked by the RC1FP input, and when
a change is detected the switching between the links is synchronized to the first byte of the
following frame. This allows for a controlled switch between the working and protection links.
Note that both the working and protection PRBS monitors and in-band link controllers are
active regardless of which link is selected by RWSEL. This provides a method to monitor the
inactive link without disrupting the data on the active link.