DS80C390_00 Datasheet, PDF (142/155 Pages) Dallas Semiconductor – High-Speed Microcontroller User’s Guide Supplement

DS80C390 High-Speed Microcontroller User’s Guide Supplement
Message centers enabled for transmission (T/ R = 1) will set the EXTRQ and MTRQ bits in the
corresponding message center when a remote frame request is successfully received, to mark the message
as a ‘to be sent’ message. The CAN module will attempt to automatically transmit the requested if the
message center is fully enabled to do so (MSRDY = 1, TIH = 0, DTUP = 1). After the transmission, the
TXS bit in the status register is set, the EXTRQ and MTRQ bits are reset to a 0 and a message center
interrupt of the corresponding message center is asserted if enabled by the respective ETI bit. If the
transmit inhibit bit is set (TIH = 1), the message center will receive the RFR, modifying the DTBYC
and/or arbitration bits if necessary, but the return data will not be transmitted until TIH = 0.
If software wants to modify the data in a message center configured for transmission of an answer to a
remote request (EXTRQ set to a 1), the microcontroller must set the TIH = 1 and DTUP = 0. The
microcontroller may then access the data byte registers 0 -7, data byte count (DTBYC3–0), the extended
or standard mode bit (EX/ ST ), and the mask enables (MEME and MDME) of the message center to load
the required settings. Following the set up, the software should reset TIH to a 0 and sets DTUP to a 1 bit
to signal the CAN that the access is finished. Until the DTUP = 1 and TIH = 0, the transmission of this
mailbox is not permitted. Thus, the CAN will transmit the newest data and reset EXTRQ = 0 after the
transmission is complete. The message center must first be disabled to change the identifier or the
direction control (T/ R ).
Message centers enabled for reception (T/ R = 0) will not automatically transmit the requested data. The
Remote Frame Request will, however, continue to store the requested number of return bytes in DTBYC
and set EXTRQ = 1. No data bytes are received or stored from a remote frame request. The message
center can then be configured via software to either function as transmitter (T/ R = 1) and transmit the
requested data, or the microcontroller can configure another message center in a transmit mode (T/ R = 1)
to send the requested data. Note that the MTRQ bit is not set by the loading of a matching remote frame
request, when T/ R = 0. RXS must be previously cleared by software or a system reset.
When a remote frame is received the CAN module can be configure to either automatically transmit data
back to the remote node or to allow the microcontroller to intervene and establish the conditions of the
transmitting of the return message. The following examples outline various options to respond to remote
frame requests.
Case 1)
Automatic Reply
CAN Controller receives a remote frame Request (RFR) and automatically transmits data
without additional software intervention.
1. Software sets T/ R = 1, MSRDY = 0, DTUP = 0, and TIH = 1.
2. Software loads data into respective message center.
3. Software sets MSRDY = 1, DTUP = 1 and TIH = 0 in same instruction.
Note: Software does not change MTRQ = 0 from previous completed transmission
4. CAN does not transmit data (MTRQ = 0), but waits for RFR.
5. CAN successfully receives RFR.
6. CAN forces MTRQ = 1 and EXTRQ = 1
7. CAN loads DTBYC from RFR and ID into arbitration registers.
8. CAN automatically transmits data in respective message center.
9. CAN clears EXTRQ = 0 and MTRQ = 0.
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