mc9s12xf512 Freescale Semiconductor, Inc, mc9s12xf512 Datasheet - Page 999

• Manufacturer Part Number: mc9s12xf512
• Description: S12x Microcontrollers 16-bit Automotive Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

21.4.3.1
The MSCAN protects the user from accidentally violating the CAN protocol through programming errors.
The protection logic implements the following features:
21.4.3.2
Figure 21-42
The clock source bit (CLKSRC) in the CANCTL1 register (21.3.2.2/21-965) defines whether the internal
CANCLK is connected to the output of a crystal oscillator (oscillator clock) or to the bus clock.
The clock source has to be chosen such that the tight oscillator tolerance requirements (up to 0.4%) of the
CAN protocol are met. Additionally, for high CAN bus rates (1 Mbps), a 45% to 55% duty cycle of the
clock is required.
If the bus clock is generated from a PLL, it is recommended to select the oscillator clock rather than the
bus clock due to jitter considerations, especially at the faster CAN bus rates.
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The receive and transmit error counters cannot be written or otherwise manipulated.
All registers which control the configuration of the MSCAN cannot be modified while the MSCAN
is on-line. The MSCAN has to be in Initialization Mode. The corresponding INITRQ/INITAK
handshake bits in the CANCTL0/CANCTL1 registers (see
Register 0
— MSCAN control 1 register (CANCTL1)
— MSCAN bus timing registers 0 and 1 (CANBTR0, CANBTR1)
— MSCAN identifier acceptance control register (CANIDAC)
— MSCAN identifier acceptance registers (CANIDAR0–CANIDAR7)
— MSCAN identifier mask registers (CANIDMR0–CANIDMR7)
The TXCAN is immediately forced to a recessive state when the MSCAN goes into the power
down mode or initialization mode (see
Section 21.4.4.5, “MSCAN Initialization
The MSCAN enable bit (CANE) is writable only once in normal system operation modes, which
provides further protection against inadvertently disabling the MSCAN.
Oscillator Clock
Bus Clock
Protocol Violation Protection
Clock System
shows the structure of the MSCAN clock generation circuitry.
(CANCTL0)”) serve as a lock to protect the following registers:
MC9S12XF - Family Reference Manual, Rev.1.19
Figure 21-42. MSCAN Clocking Scheme
CLKSRC
Section 21.4.5.6, “MSCAN Power Down
Chapter 21 Freescale’s Scalable Controller Area Network (S12MSCANV2)
Mode”).
CANCLK
MSCAN
CLKSRC
Prescaler
(1 .. 64)
Section 21.3.2.1, “MSCAN Control
Time quanta clock (Tq)
Mode,” and
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