WidePort LANCAM® Family
OPERATIONAL CHARACTERISTICS Continued
which occur only in the device with the highest match; and
data writes or move instructions involving NF, which occur
only in the device with /FI LOW and /FF HIGH. Enhanced
mode speeds up system performance by eliminating the need
to unlock the daisy chain before Command or Data Write
cycles.
Table 6a (Standard mode) and Table 6b (Enhanced mode) show
when a device will respond to reads or writes and when it will
not, based on the state of /EC(int), the internal match condition,
and other control inputs. /EC is latched by the falling edge of
/E. /EC(int) is registered from the latched /EC signal off the
rising edge of /E, so it controls what happens in the next cycle,
as shown in Figure 6. When /EC is first taken LOW in a string
of LANCAM devices (and assuming the Device Select
registers are set to FFFFH), all devices will respond to that
command write or data write.
Full Flag Cascading
The Full Flag daisy chain cascading is used for three
purposes: first, to allow instructions that address Next Free
locations to operate globally; second, to provide a system
wide Full flag; third, to allow the loading of the Page
Address registers during initialization using the SFF
instruction. The full flag logic causes only the device
containing the first empty location to respond to Next Free
instructions such as “MOV NF,CR,V”, which will move the
contents of the Comparand register to the first empty
location in a string of devices and set that location Valid,
so it will be available for the next automatic compare. With
devices connected as in Figure 1a, the /FF output of the
last device in a string provides a full indication for the
entire string.
From then on the daisy chain will remain locked in each
subsequent cycle as long as /EC is held LOW on the falling
edge of /E in the current cycle. When the daisy chain is locked
in Standard mode, only the Highest-Priority Match device will
respond (See Case 6 of Table 6a). If, for example, all of the
CAM memory locations were empty, there would be no match,
and /MF would stay HIGH. Since none of the devices could
then be the Highest-Priority Match device, none will respond
to reads or writes until the daisy chain is unlocked by taking
/EC HIGH and asserting /E for a cycle.
If there is a match between the data in the Comparand
register and one or more locations in memory, then only the
Highest-Priority Match device will respond to any cycle,
such as an associated data or Status Register read. If there
is not a match, then a NOP with /EC HIGH needs to be
inserted before issuing any new instructions, such as Write
to Next Free Address instruction to learn the data. Since
Next Free operations are controlled by the /FI–/FF daisy
chain, only the device with the first empty location will
respond. If an instruction is used to unlock the daisy chain
it will work only on the Highest-Priority Match device, if
one exists. If none exists, the instruction will have no effect
except to unlock the daisy chain. To read the Status
registers of specific devices when there is no match requires
the use of the TCO DS command to set DS=PA of each
device. Single chip systems can tie /EC HIGH and read the
Status register or the /MA and /MM pins to monitor match
conditions, as the daisy chain lock-out feature is not needed
in this configuration. This removes the need to insert a
NOP in the case of a no-match.
IEEE 802.3/802.5 Format Mapping
To support the symmetrical mapping between the address
formats of IEEE 802.3 and IEEE 802.5, the WidePort
LANCAM provides a bit translation facility. Formally
expressed, the nth input bit, D(n), maps to the xth output
bit, Q(x), through the following expressions:
D(n) = Q(7-n) for 0 ≤ n ≤ 7,
D(n) = Q(23-n) for 8 ≤ n ≤ 15
D(n) = Q(39-n) for 16 ≤ n ≤ 23
D(n) = Q(55-n) for 24 ≤ n ≤ 31
Control Register bits CT10 and CT9 select whether to
persistently translate, or persistently not to translate, the
data written onto the 64-bit internal bus. The default
condition after a Reset command is not to translate the
incoming data. Figure 2 shows the bit mapping between
the two formats.
INITIALIZING THE WIDEPORT LANCAM
Initialization of the WidePort LANCAM is required to
configure the various registers on the device. Since a
Control register reset establishes the operating conditions
shown in Table 5, restoration of operating conditions better
suited for the application may be required after a reset,
whether using the Control Register reset, or the /RESET
pin. When the device powers up, the memory and registers
are in an unknown state, so the /RESET pin must be asserted
to place the device in a known state.
When the Control register is set to Enhanced mode, you
can continue to write data to the Comparand register or
issue a Move to Next Free Address instruction without
first having to issue a NOP with /EC HIGH to unlock the
daisy chain after a Compare cycle with no match, as
indicated in cases 4 and 5 of Table 6b. In Enhanced mode,
data write cycles as well as command write cycles are
enabled in all devices even when /EC is LOW. Exceptions
are data writes, moves, or VBC instructions involving HM,
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