Table of Contents
Advertisement
Message Reception and Interrupts:
• Message reception CRC check
When a message is received, a CRC check is performed automatically (by hardware). If the
result of the CRC check is normal, ACK is transmitted in the ACK field irrespective of
whether or not the message can be received.
• Data frame reception
If the received message is confirmed to be error-free by the CRC check, etc., the identifier in
the mailbox (and also LAFM in the case of mailbox 0 only) and the identifier of the receive
message are compared, and if a complete match is found, the message is stored in the mailbox.
The message identifier comparison is carried out on each mailbox in turn, starting with
mailbox 0 and ending with mailbox 15. If a complete match is found, the comparison ends at
that point, the message is stored in the matching mailbox, and the corresponding receive
complete bit (RXPR0–RXPR15) is set in the receive complete register (RXPR). However,
when a mailbox 0 LAFM comparison is carried out, even if the identifier matches, the mailbox
comparison sequence does not end at that point, but continues with mailbox 1 and then the
remaining mailboxes. It is therefore possible for a message matching mailbox 0 to be received
by another mailbox (however, the same message cannot be stored in more than one of
mailboxes 1 to 15). If the corresponding bit (MBIMR0–MBIMR15) in the mailbox interrupt
mask register (MBIMR) and the receive message interrupt mask (IMR1) in the interrupt mask
register (IMR) are set to the interrupt enable value at this time, an interrupt can be sent to the
CPU.
• Remote frame reception
Two kinds of messages—data frames and remote frames—can be stored in mailboxes. A
remote frame differs from a data frame in that the remote reception request bit (RTR) in the
message control register (MC[x]5) and the data field are 0 bytes. The data length to be returned
in a data frame must be stored in the data length code (DLC) in the control field.
When a remote frame (RTR = recessive) is received, the corresponding bit is set in the remote
request wait register (RFPR). If the corresponding bit (MBIMR0–MBIMR15) in the mailbox
interrupt mask register (MBIMR) and the remote frame request interrupt mask (IRR2) in the
interrupt mask register (IMR) are set to the interrupt enable value at this time, an interrupt can
be sent to the CPU.
• Unread message reception
When the identifier in a mailbox matches a receive message, the message is stored in the
mailbox. If a message overwrite occurs before the CPU reads the message, the corresponding
bit (UMSR0–UMSR15) is set in the unread message register (UMSR). In overwriting of an
unread message, when a new message is received before the corresponding bit in the receive
complete register (RXPR) has been cleared, the unread message register (UMSR) is set. If the
unread interrupt flag (IRR9) in the interrupt mask register (IMR) is set to the interrupt enable
579
Advertisement
Table of Contents
