Maxim Integrated High-Speed Microcontroller Users Guide: DS80C390 Supplement - Manual (Page 144)

High-Speed Microcontroller User’s Guide: DS80C390 Supplement
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to the remote frame request. Higher numbered message centers (lower priority) that are not reviewed
prior to the match will not be evaluated during the current message test. Depending on the state of the
transmit/receive bit for that message center, the CAN module will perform one of two responses.
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 by 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.