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Cam Interface Operation; Normal (Nonextended) Match Mode - Motorola MC68838 User Manual

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The CAM interface provides actuation and data signals to an FSI CAM, a separate CAM,
or other frame recognition logic used for 48-bit destination and/or source address
recognition. Outputs from the MAC-CAM interface consist of the RCDATx, DA, and
LDADDR signals. The match input to the MAC from the CAM is the MATCH, TR
FWD, RABORT or RABORT2 signals. All signals are described and synchronized relative
to the BYTCLK signal provided by the FCG.
This section is written for ease of explanation from the point of view that the address
recognition logic consists mainly of a content addressable memory array. However, any
recognition logic may be employed that observes the signal requirements. The section
also refers mostly to a positive recognition logic implementation, i.e., an implementation
that asserts MATCH on TR
Another valid implementation is to put the MAC in a promiscuous copy mode, COPY
LLC and COPY_GRP_LLC are set to one, and then assert RABORT or RABORT2
to not copy, or flush, frames whose addresses are recognized.
The CAM interface presents frame information to a CAM for comparison purposes one
byte at a time from the ELM chip over the RCDATx lines at the same time the information
is being furnished to the MAC. Since the byte-wide data on RCDATx is two data symbols,
only the lower four bits of each symbol need be used; the upper four bits of each symbol
are always zero for data. Thus, the CAM uses RCDAT8–RCDAT5 and RCDAT3–
If the presented information matches an address stored in the CAM, the MATCH line to
the MAC should be asserted by the CAM. LDADDR and DA are additional signals driven
by the MAC that indicate to external logic or a CAM the position of data on the RCDATx
lines relative to the frame. BYTCLK and SYMCLK are used to clock the CAM interface and
to latch the address into the CAM.

9.1.1 Normal (Nonextended) Match Mode

Referring to Figure 9-1, the following sequence of events should occur between the MAC
and a CAM when an address is presented on the RCDATx lines and the MAC has
EXT_DA_MATCH = 0 in control register B:
1. LDADDR is pulsed for one BYTCLK cycle just before the first byte of both the DA
and SA fields is given to the MAC and the CAM from the ELM. The DA signal from
the MAC differentiates whether the current address is the DA or SA. LDADDR is
FWD to copy a frame when an address is recognized.


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