MC68HC05X16 Datasheet, PDF (66/232 Pages) Freescale Semiconductor, Inc – High-density complementary metal oxide semiconductor HCMOS) microcontroller unit

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5.1
TBF – Transmit buffer
The transmit buffer is an interface between the CPU and the bit stream processor (BSP) and is
able to store a complete message. The buffer is written by the CPU and read by the BSP. The CPU
may access this buffer whenever transmit buffer access is set to released. On requesting a
transmission (by setting transmission request in the MCAN command register to present) transmit
buffer access is set to locked, giving the BSP exclusive access to this buffer. The transmit buffer is
released after the message transfer has been completed or aborted.
The TBF is 10 bytes long and holds the identifier (1 byte), the control field (1 byte) and the data
field (maximum length 8 bytes). The buffer is implemented as a single-ported RAM, with mutually
exclusive access by the CPU and the BSP.
5
5.2
RBF – Receive buffer
The receive buffer is an interface between the BSP and the CPU and stores a message received
from the bus line. Once filled by the BSP and allocated to the CPU (by the IML), the receive buffer
cannot be used to store subsequent received messages until the CPU has acknowledged the
reading of the buffer’s contents. Thus, unless the CPU releases a receive buffer within a protocol
defined time frame, future messages to be received may be lost.
To reduce the requirements on the CPU, two receive buffers (RBF0 and RBF1) are implemented.
While one receive buffer is allocated to the CPU, the BSP may write to the other buffer. RBF0 and
RBF1 are each 10 bytes long and hold the identifier (1 byte), the control field (1 byte) and the data
field (maximum length 8 bytes). The buffers are implemented as single-ported RAMs with mutually
exclusive access from the CPU and the BSP. The BSP signals the MCU to read the receive buffer
only when the message being received has an identifier that passes the acceptance filter. Note
that a message being transmitted will be automatically written to the receive buffer if the identifier
passes the acceptance filter. This is because it cannot be known, until after the first byte has been
stored, whether or not the transmitting node will lose arbitration to another node.
5.3
Interface to the MC68HC05X16 CPU
The MCAN handles all the communication transactions flowing across the serial bus. For example,
the CPU merely places a message to be transmitted into the transmit buffer and sets the TR bit.
The MCAN will begin transmitting the message when it has determined that the bus is idle. In the
event of a transmission error, the MCAN will initiate a repeated transmission automatically.
MOTOROLA CAN MODULE (MCAN)
MC68HC05X16
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