XC912BC32CFU8 Motorola Semiconductor Products, XC912BC32CFU8 Datasheet - Page 276

• Manufacturer Part Number: XC912BC32CFU8
• Description: M68HC12B Family Data Sheet
• Manufacturer: Motorola Semiconductor Products

msCAN12 Controller
16.8 Timer Link
16.9 Clock System
Data Sheet
276
The msCAN12 generates a timer signal whenever a valid frame has been received.
Because the CAN specification defines a frame to be valid if no errors occurred
before the EOF field has been transmitted successfully, the timer signal is
generated right after the EOF. A pulse of one bit time is generated. As the
msCAN12 receiver engine also receives the frames being sent by itself, a timer
signal also is generated after a successful transmission.
The previously described timer signal can be routed into the on-chip timer interface
module (TIM). This signal is connected to the timer n channel m input
control of the timer link enable (TLNKEN) bit in the CMCR0.
After timer n has been programmed to capture rising edge events, it can be used
under software control to generate 16 bit-time stamps which can be stored with the
received message.
Figure 16-7
this flexible clocking scheme the msCAN12 is able to handle CAN bus rates
ranging from 10 kbps to 1 Mbps.
The clock source bit (CLKSRC) in the msCAN12 module control register (CMCR1)
(see
connected to the output of the crystal oscillator (EXTALi) or to a clock twice as fast
as the system clock (ECLK).
The clock source has to be chosen so that the tight oscillator tolerance
requirements (up to 0.4 percent) of the CAN protocol are met. Additionally, for high
CAN bus rates (1 Mbps), a 50 percent duty cycle of the clock is required.
1. The timer channel being used for the timer link is integration dependent.
16.12.3 msCAN12 Bus Timing Register
shows the structure of the msCAN12 clock generation circuitry. With
SYSCLK
EXTALi
msCAN12 Controller
CLKSRC
Figure 16-7. Clocking Scheme
CGM
CGMCANCLK
0) defines whether the msCAN12 is
MSCAN12
PRESCALER
(1...64)
CLKSRC
M68HC12B Family — Rev. 8.0
TIME QUANTA
CLOCK
(1)
MOTOROLA
under the