AN2320 Freescale Semiconductor / Motorola, AN2320 Datasheet - Page 4

• Manufacturer Part Number: AN2320
• Description: Interfacing the MCF5272 to a Standalone CAN Controller
• Manufacturer: Freescale Semiconductor / Motorola

Freescale Semiconductor, Inc.
CAN Transceiver
Regarding the interface, this has been touched on before. Few standalone CAN controllers on the market
today have non-multiplexed bus interfaces and none offer a glueless interface to the MCF5272 external bus.
Design complexity and additional cost in using a parallel interface resulted in SPI being the preferred
choice.
In terms of data rate support, CAN data rates can vary between 10kbit/s and 1Mbit/s, depending on the
length of the bus line and on the degree of fault tolerance required. A bus length of less than 40m makes
1Mbit/s achievable. CAN controllers vary in the data rates they support; most support up to 0.5Mbit/s or
1Mbit/s. The Infineon controller can handle 1Mbit/s which is desirable for many of today’s real-time
industrial applications.
Finally, standalone CAN controllers vary in the extent to which the CPU is required to take over message
transmission. The simplest controller, known formerly as BasicCAN, has hardware logic dedicated to
creating and verifying the bitstream according to protocol. Administration of data sets to be sent and
received and comprehensive acceptance filtering is carried out by the CPU, placing increased overhead on
the processor. Full CAN controllers, like the Infineon 82C900, include extra logic to provide object storage,
support additional prioritisation capabilities, and implement comprehensive acceptance filtering. This,
along with the additional on-chip FIFO and gateway mechanisms, ensures CPU overhead is kept to a
minimum. In the end, this means real-time performance is optimised, which is often the most important
criteria in the types of industrial-control and automation applications for which this is intended.
1.3
CAN Transceiver
The Philips PCA82C250 high-speed transceiver was chosen as the interface between the CAN controller
and the CAN physical bus because it supports data rates up to a maximum of 1Mbps. Alternate transceivers
such as fault-tolerant and single-wire transceivers limit the maximum data rate to 125 kbit/s and 33.33
kbit/s, respectively. High-speed transceivers typically support data rates in excess of 500 kbit/s.
2.0 Hardware Design
The MCF5272 CAN reference design is developed around the M5272C3 evaluation board using a daughter
card for the CAN circuitry. The daughter card connects to the evaluation board using expansion connectors
already provided.
The M5272C3 board provides the 10/100 Ethernet interface, RS232 interface, BDM interface, 4MB
SDRAM, and 2MB Flash ROM for system development. For additional detailed information on the
evaluation board, including full schematics, refer to the M5272C3 user’s manual on the M5272C3 CAN
webpage.
Figure 2 outlines the hardware design of the CAN daughtercard. The main features and key issues
(interface, reset, clocking, power supply, and more) are explained in the remainder of this section. Full
schematics and schematic summary can be downloaded from the ColdFire website.
4
Interfacing the MCF5272 to a Standalone CAN Controller
MOTOROLA
For More Information On This Product,
Go to: www.freescale.com