DS80C390_00 Datasheet, PDF (144/155 Pages) Dallas Semiconductor – High-Speed Microcontroller User’s Guide Supplement

DS80C390 High-Speed Microcontroller User’s Guide Supplement
Case 4)
Software-Initiated Reply (Reply via same different center, using TIH as Gating Control)
CAN Controller wishes to receive an RFR in a message center (denoted MC1) configured
also be able to receive data (T/ R = 0) and to wait for software to select another message
center (denoted MC2) to send data back to remote request node.
1. Software sets T/ R = 0, MSRDY = 1, and DTUP = 0 in MC1 and awaits either data frame or RFR.
Note: Software does not change MTRQ = 0 in MC1 from previous completed transmission.
2. CAN successfully receives RFR in MC1.
3. CAN forces EXTRQ = 1 and DTUP = 1 in MC1.
MTRQ can not be written to a 1 by the CAN when T/ R = 0 and is left as MTRQ = 0
4. CAN loads DTBYC from RFR and ID into arbitration registers in MC1.
5. CAN waits for Software to read message center and determine the fact that EXTRQ = 1.
6. Software disables in MC1 to transfer information to MC2.
7. Software clears MSRDY = 0 to disable MC1. Software leaves EXTRQ = 1.
8. Software clears MSRDY = 0 in a MC2.
9. Software forces MC2 to transmit mode T/ R = 1.
10. Software loads ID and DTBYC value from MC1 into ID and DTBYC value for MC2.
11. Software writes MSRDY = 0, DTUP = 0 and TIH = 1 in MC2 in preparation to load data to MC2.
12. Software loads data into MC2.
13. Software writes MSRDY = 1, MTRQ = 1 EXTRQ = 0, DTUP = 1 and TIH = 0 in MC2 in same
instruction.
Note that CAN has not set EXTRQ in MC2, and is not required to be set for transmission of data
from MC2.
14. CAN will automatically transmit data (as per RFR DTBYC) in MC2.
15. CAN clears MTRQ =0 (leaving previous EXTRQ = 0 cleared).
16. Software sets T/ R = 0, MSRDY = 1, EXTRQ = 0, and DTUP = 0 in MC1 and awaits either next
RFR or data frame.
Note that MTRQ is still cleared in MC1 since MC1 has not been set to a transmit mode.
Remote Frame Handling in Relation to the DTBYC Bits
The DTBYC bits in the CAN Message Format Register perform a slightly modified function when
Remote Frames are used. When remote frames are used, the data length code will be overwritten by the
data length code field of the incoming remote request frame. These requested data bytes will be sent in
the data frame which answers the remote request. The following example demonstrates how the DTBYC
bits are modified by a received remote frame request.
1. Assume the microcontroller has programmed the following into a message center:
DTBYC = 5, data field = 75 AF 43 2E 12 78 90 00
(Note that only the first through the fifth data bytes will be recognized because DTBYC=5)
2. When the CAN module successfully receives a remote frame with the following data:
Identifier = ID, DTBYC = 2, RTR = 1.
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