Hardware Reference
In-Depth Information
As a result of resynchronization, phase_seg1 may be lengthened or phase_seg2
may be short-
ened. The amount by which the phase buffer segments may be altered may not be greater than
the
resynchronization jump width
, which is programmable to be between 1 and the smaller of
4 and the phase_seg1 time quanta.
Clocking information may be derived from transitions from 1 bit value to the other. The
property that only a fixed maximum number of successive bits have the same value provides
the possibility of resynchronizing a bus node to the bitstream during a frame.
The maximum length between two transitions that can be used for resynchronization is
29 bit times.
The
phase error
of an edge is given by the position of the edge relative to sync_seg,
measured in time quanta. The sign of phase error is defined as follows:
e
, 0 if the edge lies after the sample point of the previous bit
e
5 0 if the edge lies within sync_seg
e
. 0 if the edge lies before the sample point
The Freescale implementation of the CAN 2.0A/B protocol for the HCS12 microcontroller
family is referred to as MSCAN12. The MSCAN12 provides the following features:
•
Full implementation of the CAN 2.0A/B protocol
•
Five receive buffers with FIFO storage scheme
•
Three transmit buffers with internal prioritization using a local priority concept
•
Maskable identifier filter supporting two full-size extended identifier filters (two
32-bit) or four 16-bit filters or eight 8-bit filters
•
Programmable wake-up functionality with integrated low-pass filter
•
Programmable loopback mode supporting self-test operation
•
Programmable listen-only mode for monitoring of CAN bus
•
Separate signaling and interrupt capabilities for all CAN receiver and transmitter
error states (warning, error passive, bus off)
•
Clock source coming from either E-clock or oscillator clock
•
Internal time for time stamping of received and transmitted message
•
Three low-power modes: sleep, power-down, and MSCAN12 enable
•
Global initialization of configuration registers
The block diagram of the MSCAN12 module is shown in Figure 13.13. As shown in the
figure, each MSCAN module uses two signal pins: TxCAN and RxCAN.
All members of the automotive subfamily in the HCS12 family have implemented one
or more CAN modules. The HCS12DP256 implements five CAN modules. For other HCS12
devices without an on-chip CAN module, a CAN controller chip such as the MCP2510 from
Microchip or the PCA82C200 from Philips can be used to interface with the CAN bus. Most
CAN controllers do not have enough driving capability and hence require a dedicated CAN bus
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