MU9C1965A/L LANCAM MP
OPERATIONAL CHARACTERISTICS Continued
device will respond (See Case 6 of Table 6a on page 12). 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.
The ripple delay of the flags when connected in a daisy
chain requires the extension of the /E HIGH time until the
logic in all devices has settled out. In a string of “n” devices,
the /E HIGH time should be greater than:
tEHMFV + (n-2)· tMIVMFV
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 exist, 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.
If the last device’s Match flag is required by external
logic or a state machine before the start of the next CAM
cycle, one additional tMIVMFV should be added to the
/E HIGH time along with the setup time and delays for
the external logic.
Locked Daisy Chain
In a locked daisy chain, the highest-priority device is the
one with /MI HIGH and /MF LOW. In Standard mode, only
this device will respond to command and data reads and
writes, until the daisy chain has been unlocked by taking
/EC HIGH. This allows reading the associated data field
from only the Highest-Priority Match location anywhere in
a string of devices, or the Match address from the Status
register of the device with the match. It also permits
updating the entry stored at the Highest-Priority Match
location. In Enhanced mode, devices are enabled to respond
to some Command and Data writes, as noted in Table 6b on
page 12, but not Command and Data reads.
Table 6a (Standard mode) and Table 6b on page 12
(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 5. 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.
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 on page 12. In the
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, 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.
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
Rev. 1a
16
MUSIC [ MUSIC SEMICONDUCTORS ]
