User`s guide

The XMI
2.7.1 Causes of an Initialization
Three causes of XMI initialization are:
• Power-down/power-up
• System reset
• Node reset
2.7.2 Power-Up
On power-up, the XMI AC LO L, XMI DC LO L, and XMI RESET L lines
are sequenced to provide initialization of all nodes in the system. The XMI
initialization flowchart is shown in Figure 2–33.
During normal power-up, a node cannot access XMI-accessible memory
space locations until the deassertion of XMI AC LO L. However, memory
nodes clear memory locations following the deassertion of XMI DC LO L
if a cold start is indicated. During a system reset sequence, it is possible
for the resetting node to access memory prior to the deassertion of XMI
AC LO L, but no other node can access memory prior to the deassertion of
XMI AC LO L.
During brownout power conditions, XMI AC LO may assert and later
deassert without an assertion of XMI DC LO L. The XMI AC LO L signal
remains asserted for a period of time after the deassertion of XMI DC LO
L, allowing a node’s internal initialization signals to be removed before a
power restart interrupt is raised.
During power-down or reset, XMI AC LO L asserts followed by the
assertion of XMI DC LO L, which warns of the impending loss of DC
power and is used for initialization on power-up. The XMI DC LO L
signal is asserted after the assertion of XMI AC LO L, allowing the
power-fail routine to save processor state in memory and to halt. As the
machine comes back up, DC power and the XMI clock become valid before
the deassertion of XMI DC LO L. The result of any XMI transaction in
progress when XMI DC LO L asserts is indeterminate.
In a power outage, first AC power is lost, then (if not restored quickly), DC
power falls below acceptable levels, asserting first XMI AC LO L and then
XMI DC LO L.
During a power outage, the XMI side of the platform can be sustained
by an optional battery backup unit (BBU). After power is restored, the
memory is not reinitialized unless the BBU has been exhausted and
the data in memory is no longer reliable. Memory initialization is what
distinguishes warm starts from cold starts: memory need not be initialized
for warm starts; memory is initialized for cold starts. The XTC power
sequencer monitors the BBU signals and asserts the XMI RESET L line if
the battery was exhausted, thus initiating a cold start.
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