Freescale Semiconductor, Inc.
MSCAN Controller
Message Storage
An overrun condition occurs when both the foreground and the
background receive message buffers are filled with correctly received
messages with accepted identifiers and another message is correctly
received from the bus with an accepted identifier. The latter message is
discarded and an error interrupt with overrun indication is generated if
enabled. The msCAN12 is still able to transmit messages with both
receive message buffers filled, but all incoming messages are
discarded.
17.4.3 Transmit Structures
The msCAN12 has a triple transmit buffer scheme in order to allow
multiple messages to be set up in advance and to achieve an optimized
real-time performance. The three buffers are arranged as shown in
Figure 17-2.
All three buffers have a 13 byte data structure similar to the outline of the
receive buffers (see Programmer’s Model of Message Storage). An
additional transmit buffer priority register (TBPR) contains an 8-bit so
called local priority field (PRIO) (see Transmit Buffer Priority Registers
(TBPR)).
In order to transmit a message, the CPU12 has to identify an available
transmit buffer which is indicated by a set transmit buffer empty (TXE)
flag in the msCAN12 transmitter flag register (CTFLG) (see msCAN12
Transmitter Flag Register (CTFLG)).
The CPU12 then stores the identifier, the control bits and the data
content into one of the transmit buffers. Finally, the buffer has to be
flagged as being ready for transmission by clearing the TXE flag.
The msCAN12 will then schedule the message for transmission and will
signal the successful transmission of the buffer by setting the TXE flag.
(1)
A transmit interrupt will be emitted when TXE is set and this can be
used to drive the application software to re-load the buffer.
1. The transmit interrupt is generated only if not masked. A polling scheme can be applied on
TXE also.
68HC(9)12D60 — Rev 4.0
MOTOROLA
Advance Information
MSCAN Controller
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