mc68hc05x4dw Freescale Semiconductor, Inc, mc68hc05x4dw Datasheet - Page 74

• Manufacturer Part Number: mc68hc05x4dw
• Description: Xc68hc705x4 Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

TBF – Transmit buffer
RBF – Receive buffer
Motorola CAN
MC68HC05X4
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.
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 CPU 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. This is because it cannot be
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Motorola CAN
6-mcan