SH7059 Datasheet, PDF (566/1042 Pages) Renesas Technology Corp – 32-Bit RISC Microcomputer

17. Controller Area Network-II (HCAN-II)
• Internal Arbitration for Transmission
Figure 17.9 explains how the HCAN manages to schedule transmit-requested messages in the correct order based on the
CAN ID. “Internal arbitration” picks up the highest priority message among transmit-requested messages.
CAN bus
state
Frame-1
Frame-2
Frame-3
Bus idle SOF Message EOF Interm SOF Message EOF Interm SOF
Tx=arbitation for Tx=arbitation for
HCAN
frame-1
frame-2
Rx matching
scheduler state
Scheduler
start point
Tx=arbitation for
Tx=arbitation for
frame-3
Rx matching frame-4
TXPR/TXCR/
Error/Arbitation-lost
set point
1-1
2-1
2-2 3-1 3-2
3-3 3-4
Interm: Intermission Field
SOF: Start Of Frame
EOF: End Of Frame
Message: Arbitration + control + data + CRC + Ack field
Figure 17.9 Internal Arbitration for Transmission
The HCAN scheduler, which runs internal arbitration, has 2 states – Tx arbitration state and Rx matching state. The
HCAN scheduler is in the Rx matching state if the CAN bus is in the EOF or intermission cycles, or otherwise is in the Tx
arbitration state. When a transmit request or transmit abort request is made in the Tx arbitration state, the internal
arbitration starts running immediately. When a transmit request or transmit abort request is made in the Rx matching state,
the internal arbitration waits until the Rx matching state (i.e. intermission field) is finished, and then starts running as soon
as the HCAN scheduler state becomes the Tx arbitration.
There are 5 sources that can run internal arbitration, which are:
• TXPR is set
• TXCR is set (if TXCR is set for the message currently under transmission, the HCAN does not stop the transmission
but completes. If the message loses the bus arbitration or causes an error on the bus, the HCAN will cancel the transmit
request.)
• Error occurs on the CAN bus
• Message under transmission loses the arbitration on the CAN bus
• Mailbox with the setting MBC = 001 receives a remote frame
When these sources occur, the internal arbitration starts running to ensure that the highest priority message is always
transmitted first. The followings are examples set in Figure 17.9.
1-1: When a TXPR bit is set while the CAN bus is idle, the internal arbitration starts running immediately and the
transmission is started.
2-1, 2-2: During this period (Tx-arbitration for frame-2), when any of the above 5 sources occurs, the internal arbitration
starts running and the next frame (Frame-2) to be transmitted is scheduled.
3-1, 3-2: During this period (Rx matching), any internal arbitration is not allowed to run, but scheduling is performed at
the SOF of the next frame (Frame-2). If the transmit-requested message has the highest priority, the
transmission will be set for the Frame-3.
3-3, 3-4: This is the same case as 2-1, 2-2.
Rev.3.00 Mar. 12, 2008 Page 476 of 948
REJ09B0177-0300