AN2320 Freescale Semiconductor / Motorola, AN2320 Datasheet - Page 8

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

Clocking
2.2
2.2.1
A 24-MHz external oscillator is used to clock the CAN controller. This ensures that the CAN protocol can
be handled on both nodes at 1Mbps when the CAN controller is interfaced to an external host using the
maximum access rate. This assumes the built-in gateway and FIFO functions are not being used. For
systems that require these additional data handling capabilities, a higher frequency may be required unless
the external host-access rate is reduced. In the worst case, the data handling capability would be reduced to
500kbps for each node when both the FIFO and the gateway functions are running with a 24-MHz clock
input. Using a 24-MHz clock input also means the oscillator chip can be replaced directly by the USB clock
and clock divider with no software modification or interface timing implications if preferred. The integrated
USB controller on the MCF5272 requires a 48-Mhz oscillator which is provided externally on the M5272C3
board.
2.2.2
The 82C900 CAN controller serial interface baud rate is limited to
frequency. At 24 MHz this gives a maximum possible baud rate of 6 Mbps. The MCF5272 QSPI is
programmable and is set by the baud field in the QSPI mode register (QMR) as follows:
8
SPI
BAUDRATE
Serial Clk
Data Out
Name
Data In
CS
C
D
G
A
B
E
F
A
Clocking
CAN Controller Clock Input
SPI Baud Rate
8 CLKS
=
Addr
CLKIN
-------------------
2 B
Data byte transfer to data byte transfer, read access
Data byte transfer to data byte transfer, write access
Address transfer to data byte transfer, write access
Address transfer to data byte transfer, read access
B
Interfacing the MCF5272 to a Standalone CAN Controller
8 CLKS
Data
Last data byte transfer to CS inactive
Freescale Semiconductor, Inc.
CS active to SerialClk active
Figure 4. 82C900 SSC Timing Specification
For More Information On This Product,
CS inactive to CS active
READ ACCESS
C
Read Access
8 CLKS
Data
Parameter
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8 CLKS
Data
D
E
A
8 CLKS
f
----------- -
Addr
CAN
4
Write Access
WRITE ACCESS
where
Min Time, f
F
8 CLKS
Data
f
CAN
209ns
584ns
584ns
459ns
167ns
209ns
84ns
CAN
is the input clock
G
= 24 MHz
8 CLKS
MOTOROLA
Data