Renesas H8S Series Hardware Manual page 456

Section 15 Controller Area Network (HCAN)
Mailbox Transmit/Receive Settings: The HCAN has 16 mailboxes. Mailbox 0 is receive-only,
while mailboxes 1 to 15 can be set for transmission or reception. The Initial status of mailboxes 1
to 15 is for transmission. Mailbox transmit/receive settings are not initialized by a software reset.
Clearing a bit to 0 in the mailbox configuration register (MBCR) designates the corresponding
mailbox for transmission use, whereas a setting of 1 in MBCR designates the corresponding
mailbox for reception use. When setting mailboxes for reception, in order to improve message
reception efficiency, high-priority messages should be set in low-to-high mailbox order.
Mailbox (Message Control/Data) Initial Settings: Message control/data are held in RAM, and
so their initial values are undefined after power is supplied. Initial values must therefore be set in
all the mailboxes (by writing 0s or 1s).
Setting the Message Transmission Method: The following two kinds of message transmission
methods are available.
• Transmission order determined by message identifier priority
• Transmission order determined by mailbox number priority
Either of the message transmission methods can be selected with the message transmission method
bit (MCR2) in the master control register (MCR):
When messages are set to be transmitted according to the message identifier priority, if several
messages are designated as waiting for transmission (TXPR = 1), the message with the highest
priority in the message identifier is stored in the transmit buffer. CAN bus arbitration is then
carried out for the message stored in the transmit buffer, and the message is transmitted when the
transmission right is acquired. When the TXPR bit is set, the highest-priority message is found and
stored in the transmit buffer.
When messages are set to be transmitted according to the mailbox number proiority, if several
messages are designated as waiting for transmission (TXPR = 1), messages are stored in the
transmit buffer in low-to-high mailbox order. CAN bus arbitration is then carried out for the
message stored in the transmit buffer, and the message is transmitted when the transmission right
is acquired.
Rev. 6.00 Mar 15, 2006 page 420 of 570
REJ09B0211-0600