NEC MuPD70F3187, User Manual

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Chapter 18 AFCAN Controller
User’s Manual U16580EE3V1UD00
18.9 Message Reception
18.9.1 Message reception
In all the operation modes, the complete message buffer area is analyzed to find a suitable buffer to
store a newly received message. All message buffers satisfying the following conditions are included in
that evaluation (RX-search process).
• Used as a message buffer
(MA0 bit of CnMCONFm register set to 1.)
• Set as a receive message buffer
(MT[2:0] bits of CnMCONFm register are set to 001B, 010B, 011B, 100B, or 101B.)
• Ready for reception
(RDY bit of CnMCTRLm register is set to 1.)
When two or more message buffers of the CAN module receive a message, the message is stored
according to the priority explained below. The message is always stored in the message buffer with the
highest priority, not in a message buffer with a low priority. For example, when an unmasked receive
message buffer and a receive message buffer linked to mask 1 have the same ID, the received mes-
sage is not stored in the message buffer linked to mask 1, even if that message buffer has not received
a message and a message has already been received in the unmasked receive message buffer. In
other words, when a condition has been set in two or more message buffers with different priorities, the
message buffer with the highest priority always stores the message; the message is not stored in mes-
sage buffers with a lower priority. This also applies when the message buffer with the highest priority is
unable to store a message (i.e., when DN = 1 indicating that a message has already been received, but
rewriting is disabled because OWS = 0). In this case, the message is not actually stored in the candi-
date message buffer with the highest priority, but neither is it stored in a message buffer with a lower pri-
ority.
18.9.2 Receive Data Read
To keep data consistency when reading CAN message buffers, perform the data reading according to
<~Reference>Figure 18-49 on page 854 to <~Reference>Figure 18-51 on page 856.
During message reception, the CAN module sets DN of the CnMCTRLm register two times: at the
beginning of the storage process of data to the message buffer, and again at the end of this storage
process. During this storage process, the MUC bit of the CnMCTRLm register of the message buffer is
set. (Refer to <~Reference>Figure 18-28 on page 811.)
The receive history list is also updated just before the storage process. In addition, during storage proc-
ess (MUC = 1), the RDY bit of the CnMCTRL register of the message buffer is locked to avoid the coin-
Priority Storing Condition If Same ID Is Set
1 (high) Unmasked message buffer DN bit = 0
DN bit = 1 and OWS bit = 1
2 Message buffer linked to mask 1 DN bit = 0
DN bit = 1 and OWS bit = 1
3 Message buffer linked to mask 2 DN bit = 0
DN bit = 1 and OWS bit = 1
4 Message buffer linked to mask 3 DN bit = 0
DN bit = 1 and OWS bit = 1
5 (low) Message buffer linked to mask 4 DN bit = 0
DN bit = 1 and OWS bit = 1