WidePort LANCAM® Family
OPERATIONAL CHARACTERISTICS Continued
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 “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.
/E
/EC
/EC (INT)
/MF
Match Flag Cascading
Figure 6: /EC(Int) Timing Diagram
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 6a and 6b.
COMPARE OPERATIONS
During a Compare operation, the data in the Comparand
register is compared to all locations in the Memory array
simultaneously. Any mask register used during compares must
be selected beforehand in the Control register. There are two
ways compares are initiated: Automatic and Forced compares.
Automatic compares perform a compare of the contents of the
Comparand register against Memory locations that are tagged
as “Valid,” and occur whenever the following happens:
Ø The Destination Segment counter in the Segment
Control register reaches its end limit during writes to
the Comparand or mask registers.
Ø After a command write of a TCO CT is executed (except
for a software reset), so that a compare is executed
with the new settings of the Control register.
The ripple delay of the flags when connected in a daisy
chain may require 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
Forced compares are initiated by CMP instructions
using one of the four validity conditions, V, R, S, and E. The
forced compare against “Empty” locations automatically
masks all 64 bits of data to find all locations with the validity
bits set to “Empty”, while the other forced compares are
only masked as selected in the Control register.
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 any required setup time and delays
for the external logic.
VERTICAL CASCADING
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 Tables 6a
and 6b, but not command and data reads.
WidePort LANCAMs can be vertically cascaded to
increase system depth. Through the use of flag daisy-
chaining, multiple devices will respond as an integrated
system. The flag daisy chain allows all commands to be
issued globally, with a response only in the device
containing the Highest-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 WidePort LANCAMs
are vertically cascaded using daisy-chaining of the flags is
shown in Figure 1a.
Rev. 2
14
MUSIC [ MUSIC SEMICONDUCTORS ]
