DS80C390_00 Datasheet, PDF (141/155 Pages) Dallas Semiconductor – High-Speed Microcontroller User’s Guide Supplement

DS80C390 High-Speed Microcontroller User’s Guide Supplement
Message center 15 is a special receive-only, FIFO-buffered message center, designed to receive messages
not accepted by the other message centers. The ROW bit in message center 15 is associated with the
overwrite of the shadow buffer for message center 15. The EXTRQ and DTUP bits are 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 the microcontroller are not those of the shadow buffer as is the case
with the ROW bit, but are the current values associated with message center 15. The shadow buffer is
automatically loaded into message center 15 when both the DTUP bit and the EXTRQ bit are cleared. If
either DTUP or EXTRQ are left set 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).
Transmitting Remote Frame Requests
Starting with the lowest numbered message center (highest priority) each CAN module sequentially scans
each message center. When it finds a message center properly enabled to transmit a remote frame (T/ R =
0, MSRDY = 1, and MTRQ = 1), the contents of the respective message center is then transferred to the
transmit buffer and the CAN module attempts to transmit the message. If successful the appropriate
MTRQ bit will be cleared to 0, indicating that the message was successfully sent. Following a successful
transmission, loss of arbitration, or an error condition, the CAN module will again search for a properly
configured message center, starting with the lowest numbered message center. This search relationship
will always allow the highest priority message center to be transmitted, independent of the last successful
(MTRQ = 0) or unsuccessful (MTRQ = 1) message transmission. The state of the TIH bit does not effect
the transmission of a remote frame request.
Receiving/Responding to Remote Frame Requests
The remote frame request is handled like a data frame with data length zero and the EXTRQ and RXS
bits are set. Each incoming Remote Frame Request (RFR) message is compared sequentially with each
enabled (MSRDY = 1) message center starting with the lowest numbered message center (highest
priority) and proceeding to the highest numbered message center. Testing continues until a match is found
(incorporating masking functions as required), at which time the incoming RFR message is stored in the
respective message center, the DTBYC bits are updated to indicate the requested number of return bytes
(DTBYC=0 for a remote frame request), and EXTRQ and MTRQ are both set to 1. When the message is
successfully received and stored, an interrupt of the corresponding message center will be asserted if
enabled by the ERI bit. The EXTRQ bit can be left set if the message center is reconfigured to perform a
transmit (T/ R = 1) and used in the standard reply of a remote frame operating with transmit message
centers. EXTRQ can also be cleared by software if the current message center is not being used to reply
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.
If the microcontroller wishes to request data from another node, it first clears the respective MSRDY bit
to 0 and then writes the identifier and control bits in this message center, configures the message center as
a receive message center (T/ R = 0) and then sets the MTRQ bit. After a successful transmission, the
CAN module will clear MTRQ = 0 and set TXS = 1. In addition to the TXS bit, if the ETI bit is set, the
successful transmission will also set the corresponding INTRQ bit. Requesting data from another node is
possible in message centers 1 to 14. As seen above the CAN module sends a remote frame request and
receives the data frame in the same message center that sent the request. Therefore, only one mailbox is
necessary to do a remote request.
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