Maxim Integrated High-Speed Microcontroller Users Guide: Network Microcontroller Supplement - Manual (Page 146)

High-Speed Microcontroller User’s
Guide: Network Microcontroller
Supplement
146
CAN 0 Message Center y Format Register (C0MyF)
MOVX Address 1 7 6 5 4 3 2 1 0
Xxxxy6h DTBYC3 DTBYC2 DTBYC1 DTBYC0 T/R EX/ST MEME MDME
Special notes for message center 15. The ROW bit in message center 15 is associated with an over-
write of the shadow buffer for message center 15. The EXTRQ and DTUP bits are also shadow buffered
to allow the buffered message and the message center 15 value to take on different relationships. The
EXTRQ and DTUP values read by software are the current message center 15 values, rather than those
of the shadow buffer, as is the case with the ROW bit. The shadow buffer is automatically loaded into
message center 15 when both the DTUP bit and EXTRQ bit are cleared. If either DTUP = 1 or EXTRQ =
1 when clearing the other, any message in the shadow buffer is not transferred to the message 15 reg-
isters, and any incoming messages for message 15 are stored in the shadow buffer if WTOE = 1 or are
lost if WTOE = 0.
Special notes concerning remote frames. For remote frames, which can be received by transmit mes-
sage centers (1–14) in case of a matching identifier, WTOE and EXTRQ are evaluated. If WTOE = 1 or
WTOE = 0 and EXTRQ = 1, the respective transmit message center (1–14) arbitration bits can be over-
written.
DTBYC3–0
Bits 7–4
T/R
Bit 3
EX/ST
Bit 2
MEME
Bit 1
MDME
Bit 0
Data byte count. These bits indicate the number of bytes within the data field of the message. When
performing a transmit, software sets the DTBYC bits to establish the number of bytes that are to be trans-
mitted. When receiving a message, the DTBYC bits indicate the (binary) number of bytes of data in the
incoming message (i.e., 0000b = 0 data bytes and 1000b = 8 data bytes).
Transmit/receive select. This bit is programmed by the application software to indicate if the message
is to be transmitted (T/R = 1) or received (T/R = 0). This bit can only be modified when MSRDY = 0. This
bit does not exist for message center 15 and always returns 0 when read from message center 15.
Extended or standard identifier. This bit determines whether the respective message is to utilize the
extended 29-bit identification format (EX/ST = 1) or the standard 11-bit identification format (EX/ST = 0).
Message centers programmed for only one format receive/send extended messages in that format and
ignore the alternate format. This bit can only be modified when MSRDY = 0.
Message identification mask enable. The MEME bit enables (MEME = 1) or disables (MEME = 0) the
use of the message identification masking process, associated with the testing of the identification field
in the incoming message. This bit can only be modified when MSRDY = 0.
0 = The mask registers are ignored when evaluating the identification bits of the incoming message, and
the identification bits of the incoming message and the message center arbitration bits must match
exactly to allow receipt of the incoming message. This is equivalent to programming the mask with
all zeros. An exact match is also required before a remote data request is allowed.
1 = The mask registers are enabled, comparing only those bits message identification and arbitration
bits that correspond to a 1 in the mask register. Since the entire message is loaded on a success-
ful ID match, note that it is possible to overwrite the corresponding arbitration register bits that were
defined as “don’t cares” (0) in the standard or extended global ID mask.
Media identification mask enable. The MDME bit enables (MDME = 1) or disables (MDME = 0) the use of
the first 2 bytes of the data field as a message qualifier. This bit can be only modified when MSRDY = 0.
0 = The first 2 bytes of the data field are ignored and not compared.
1 = The first 2 data bytes are masked by the respective media mask ID register and then compared with
the media arbitration register 0 and 1 bytes. Only those bits in the first 2 data bytes and the arbi-
tration registers corresponding to a 1 in the mask register are compared. When MDME = 1 the test
is also performed before a remote request of data is accepted.
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