Renesas H8SX/1520 Series Hardware Manual page 527

(3)
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.
(4)
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).
(5)
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 priority, 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.
Downloaded from Elcodis.com electronic components distributor
Section 13 Controller Area Network (HCAN)
Rev. 3.00 Mar. 14, 2006 Page 489 of 804
REJ09B0104-0300