Maxim Integrated MAXQ7666 - Manual (Page 149)

MAXQ7665/MAXQ7666 User’s Guide
4-19
Special Notes for Message Center 15: The ROW bit in message center 15 is associated with an overwrite 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 will not be transferred to the message 15 registers, and any incoming messages for message 15 will be stored in the
shadow buffer if WTOE = 1, or will be lost if WTOE = 0.
Special Notes Concerning Remote Frames: For remote frames, which can be received by transmit message 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 mes-
sage center (1–14) arbitration bits can be overwritten.
CAN 0 Message Center y Format Register (C0MyF)
Bits 7 to 4: Data Byte Count (DTBYC3 to DTBYC0). 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 transmitted. Any value above
8 acts as if 8 had been programmed. 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).
Bit 3: Transmit/Receive Select (T/ R). This bit is programmed by the application software to indicate if the message is to be transmit-
ted (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.
Bit 2: Extended or Standard Identifier (EX/ ST). This bit determines whether the respective message is to use the extended 29-bit iden-
tification format (EX/ ST = 1) or the standard 11-bit Identification format (EX/ST = 0). Message centers programmed for one format will only
receive/send extended messages in that format and will ignore the alternate format. This bit can only be modified when MSRDY = 0.
Bit 1: Message Identification Mask Enable (MEME). The MEME bit enables (MEME = 1) or disables (MEME = 0) the use of the mes-
sage 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 successful ID match, note that it is possible to overwrite the corre-
sponding arbitration register bits that were defined as don’t cares (0) in the standard or extended global ID mask.
Bit 0: Media Identification Mask Enable (MDME). 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 only be 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 reg-
ister 0 and 1 bytes. Only those bits in the first 2 data bytes and the arbitration 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 from a remote node is accepted.
Bit # 7 6 5 4 3 2 1 0
Name DTBYC3 DTBYC2 DTBYC1 DTBYC0 T/ R EX/ST MEME MDME
Reset X X X X X X X X
Access rw rw rw rw rw rw rw rw
Dual Port
Address
(8y+3)h(L)
X = Don’t care
r = read, w = write (via C0DP/C0DB)
Maxim Integrated