DESIGN
GUIDELINES
FOR
PICs
Auto Configuration
All
PlCs implement the auto-configuration protocol. The auto config
protocol
is
designed so that system auto-config software can inter-
rogate the PlCs ID locations, build a system table of the installed
PICs, and place the PlCs in the
68000
memory space.
General Description of
If
it
is difficult to imagine how to implement this protocol while
it's
Auto Configuration
being described, don't worry. The design requires one PAL, one latch,
and one address match circuit. Complete details are given in the
example design.
Upon reset, all PlCs come up in the unconfigured state. In the
uncon-
figured state, the PIC responds to the
64
kilobyte address space
starting at location
E80000, if CONFIGIN*
is
active to the Plc. If
CONFIGIN* is not active, the PIC does not respond to any bus cycles.
The processor comes out and reads nibbles of ID data on D 1 5-D 12
from the
Plc. The table of ID data and the locations of control
latches
is
detailed later in
this
section. This data includes such things
as size of address space required, manufacturer's product number.
and whether to add the PIC to the free memory pool (if
it
is
a
memory PIC.)
Under normal conditions, the processor determines how much ad-
dress space the PIC requires and then loads the
PIC's address latch
with an appropriate base address. This permanently relocates the
PIC at
its
new address (until Reset), and passes CONFIGOUT* out to
the next
PIC's CONFIGIN*, whereupon the process
is
enacted again
until all PlCs are configured.
The smallest unit of memory that a PIC can ask for is
64
kilobytes.
The largest
is
eight megabytes.
All
PlCs should be designed to be
based on boundaries that match their space requirements; for exam-
ple, one megabyte PlCs should be designed to reside on one mega-
byte boundaries (match circuit matches
A23-A20). There are two ex-
ceptions to this rule, however. Four megabyte PlCs must be capable
of being placed on four megabyte boundaries, as well as at hex
200000
and at hex 600000. Eight megabyte PlCs should be capable
of being placed on eight meg boundaries and at hex 200000. This
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