MU9C4480A/L
OPERATIONAL CHARACTERISTICS Continued
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
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.
tEHMFV + (n-2)· tMIVMFV
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 the 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 the Enhanced mode, devices are enabled
to respond to some command and data writes, as noted in
Table 5b on page 11, but not command and data reads.
When the Control register is set to the 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 5b on page 11. In this
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. The Enhanced mode speeds up system
performance by eliminating the need to unlock the daisy chain
before Command or Data Write cycles.
Table 5a (Standard mode) and Table 5b (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 on page 6, 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 the Standard mode, only the Highest-Priority
Match device will respond (See Case 6 of Table 5a on page
11). 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
Rev. 3a
14
MUSIC [ MUSIC SEMICONDUCTORS ]
