MC68HC08AZ60 Datasheet, PDF (310/452 Pages) Motorola, Inc – Advance Information

Freescale Semiconductor, Inc.
MSCAN Controller (MSCAN08)
Receive Structures
The received messages are stored in a 2-stage input first in first out
(FIFO). The two message buffers are mapped using a Ping Pong
arrangement into a single memory area (see Figure 2). While the
background receive buffer (RxBG) is exclusively associated to the
MSCAN08, the foreground receive buffer (RxFG) is addressable by the
CPU08. This scheme simplifies the handler software, because only one
address area is applicable for the receive process.
Both buffers have a size of 13 bytes to store the CAN control bits, the
identifier (standard or extended), and the data content (for details, see
Programmer’s Model of Message Storage on page 329).
The receiver full flag (RXF) in the MSCAN08 receiver flag register
(CRFLG) (see MSCAN08 Receiver Flag Register (CRFLG) on page
342), signals the status of the foreground receive buffer. When the buffer
contains a correctly received message with matching identifier, this flag
is set.
On reception, each message is checked to see if it passes the filter (for
details see Identifier Acceptance Filter on page 314) and in parallel is
written into RxBG. The MSCAN08 copies the content of RxBG into
RxFG¹, sets the RXF flag, and generates a receive interrupt to the
CPU². The user’s receive handler has to read the received message
from RxFG and to reset the RXF flag to acknowledge the interrupt and
to release the foreground buffer. A new message which can follow
immediately after the IFS field of the CAN frame, is received into RxBG.
The overwriting of the background buffer is independent of the identifier
filter function.
When the MSCAN08 module is transmitting, the MSCAN08 receives its
own messages into the background receive buffer, RxBG. It does NOT
overwrite RxFG, generate a receive interrupt or acknowledge its own
messages on the CAN bus. The exception to this rule is in loop-back
mode (see MSCAN08 Module Control Register 1 on page 338), where
the MSCAN08 treats its own messages exactly like all other incoming
messages. The MSCAN08 receives its own transmitted messages in the
1. Only if the RXF flag is not set.
2. The receive interrupt will occur only if not masked. A polling scheme can be applied on RXF
also.
MC68HC08AZ60 — Rev 1.0
310
MSCAN Controller (MSCAN08)
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