DS80C390_00 Datasheet, PDF (145/155 Pages) Dallas Semiconductor – High-Speed Microcontroller User’s Guide Supplement

DS80C390 High-Speed Microcontroller User’s Guide Supplement
3. The incoming message will overwrite the identifier and the data length code. The new data in the
message center will be:
DTBYC = 2, data field = 75 AF 43 2E 12 78 90 00
(Note that now only the first and second data bytes are recognized because DTBYC is now 2)
4. The outgoing response will be a data frame containing the following information:
DTBYC = 2, data field = 75 AF
Important Information Concerning ID Changes when Awaiting Data from a Previous Remote
Frame Request:
The use of acceptance filtering (MEME=1) in conjunction with remote frame requests can result in a
modification of the message center arbitration registers. When a message center is configured to transmit
a remote frame request (MTRQ = 1, EXTRQ = 0, T/ R = 0 and MSRDY = 1) it is possible for a second
Frame Request from an external node to modify the initial Arbitration Register value of the current
message center prior to the current message center receiving the requested data if arbitration masks are
used. When a remote frame request is received, the message ID is loaded into that message center’s
arbitration registers. When message identification masking is not used (MEME=0), the message ID will
always match the arbitration value, so the process will be transparent. If masking is used, however, the
message ID ANDed with the appropriate mask will be loaded into that message center’s arbitration
registers, resulting in a change of the arbitration values for that message center. To prevent this situation,
acceptance filtering should be disabled (MEME = 0) for any message center configured for remote frame
handling. An alternate solution would be to disable the overwrite feature for that message center, which
also prevents incoming messages from altering the message center ID.
Overwrite Enable/Disable Feature
The Write Over Enable bit (WTOE) located in each message center (CnMxAR3.0) enables or disables the
overwriting of unread messages in message centers 1 through 15. Programming WTOE = 1 following a
system reset or CRST bit-enabled reset allows newly received messages which pass arbitration to
overwrite unread (i.e., message centers with DTUP=1) messages. When an overwrite occurs the Receive
Overwrite (ROW) bit in the respective CAN Message Control Register will be set. When WTOE = 0,
message centers which have data waiting to be read (indicated by DTUP = 1) or transmitted (EXTRQ=1)
will not be overwritten by incoming data.
Special care must be taken when reading data from a message center with the overwrite feature enabled
(WTOE = 1). The caution is needed because the WTOE bit, when set, allows an incoming message to
overwrite the message center. If this overwrite occurs at the same time that software is attempting to read
several bytes from the message center (such as a multi-byte data field), it is possible that the read could
return a mix of information from the old and overwriting messages. To avoid this situation, software
should clear the DTUP bit to 0 prior to reading the message center, and then verify afterwards that the
DTUP bit remained at 0. If DTUP remains cleared after the read, no overwrite occurred and the returned
data was correct. If DTUP = 1 after the read then software again should clear DTUP = 0 and re-read the
message center, since a possible overwrite has occurred. The original message will be lost (as planned
since WTOE=1), but new message should be available on the next read.
One important use of the WTOE bit is to allow the microprocessor to program multiple message centers
with the same ID when operating in the receive mode, with WTOE=0. This allows the CAN module to
store multiple incoming messages in a series of message centers, creating a large storage area for high-
speed recovery of large amounts of data. The CPU is required to manage the use of these message centers
to keep track of the incoming data, but the use of multiple message centers and disabling of their
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