MU9C2480A/L
OPERATIONAL CHARACTERISTICS Continued
Priority Matching or Next Free location. When connected
in a daisy chain, the last device’s Full flag and Match flag
accurately report the condition for the whole string. A
system in which LANCAMs are vertically cascaded using
daisy-chaining of the flags is shown in Figure 1a on page 7.
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, but not command and data reads.
To operate the daisy chain, the Device Select registers are
set to FFFFH to enable all devices to execute Command
Write and Data Write cycles. In normal operation, read
cycles are enabled from the device with the Highest-Priority
match by locking the daisy chain (see the “Locked Daisy
Chain” section). An individual device in the chain may be
targeted for a read or write operation by temporarily setting
the Device Select registers to the Page address of the target
device. Setting the Device Select registers back to FFFFH
restores the operation of the entire daisy chain.
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.
Match Flag Cascading
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). 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 Match Flag daisy chain cascading is used for three
purposes: first, to allow operations on Highest-Priority
Match addresses to be issued globally over the whole
string; second, to provide a system wide match flag; third,
to lock out all devices except the one with the Highest-
Priority match for instructions such as Status reads after a
match. The Match flag logic causes only the highest-priority
device to operate on its Highest-Priority Match location
while devices with lower-priority matches ignore Highest-
Priority Match operations. The lock-out feature is enabled
by the match flag cascading and the use of the /EC control
signal, as shown in Tables 5a and 5b on page 12.
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
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 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.
/E
/EC
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
/EC (INT)
/MF
Figure 6: /EC(Int) Timing Diagram
Rev. 1a
14
MUSIC [ MUSIC SEMICONDUCTORS ]
