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or more mailbox of the mailboxes 1 to 15. On receiving a message, a CPU interrupt request
may be generated according to the settings of the mailbox interrupt mask register (MBIMR)
and interrupt mask register (IMR).
• Remote frame reception
A mailbox can store two kinds of messages: data frames and remote frames. A remote frame
differs from a data frame in the value of the remote transmission request bit (RTR) in the
message control register and its 0-byte data field. The data length to be returned in a data frame
must be stored in the data length code (DLC) in the message control.
When a remote frame (RTR = recessive) is received, the corresponding bit in the remote
request wait register (RFPR) is set. Interrupts can be sent to the CPU according to the settings
of the corresponding bit (MBIMR0 to MBIMR15) in the mailbox interrupt mask register
(MBIMR) and the remote frame request interrupt mask (IRR2) in the interrupt mask register
(IMR).
Unread message overwrite: If the receive message identifier matches the mailbox identifier, the
receive message is stored in the mailbox regardless of whether the mailbox contains an unread
message or not. If a message overwrite occurs, the corresponding bit (UMSR0 to UMSR15) in the
unread message register (UMSR) is set. In overwriting an unread message, the unread message
register (UMSR) is set when a new message is received before the corresponding bit in the receive
complete register (RXPR) has been cleared. If the unread interrupt flag (IRR9) in the interrupt
mask register (IMR) is set to enable interrupts at this time, an interrupt can be sent to the CPU.
Figure 15.12 shows a flowchart for unread message overwriting.
Rev. 1.0, 09/02, page 395 of 568
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