Freescale’s Scalable Controller Area Network (S12MSCANV3)
8.4.3.1
Protocol Violation Protection
The MSCAN protects the user from accidentally violating the CAN protocol through programming errors.
The protection logic implements the following features:
•
•
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 Section 8.3.2.1, “MSCAN Control
Register 0 (CANCTL0)”) serve as a lock to protect the following registers:
— 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 8.4.5.6, “MSCAN Power Down Mode,” and
Section 8.4.4.5, “MSCAN Initialization Mode”).
The MSCAN enable bit (CANE) is writable only once in normal system operation modes, which
provides further protection against inadvertently disabling the MSCAN.
8.4.3.2
Clock System
Figure 8-43 shows the structure of the MSCAN clock generation circuitry.
MSCAN
Bus Clock
Time quanta clock (Tq)
CANCLK
Prescaler
(1 .. 64)
CLKSRC
CLKSRC
Oscillator Clock
Figure 8-43. MSCAN Clocking Scheme
The clock source bit (CLKSRC) in the CANCTL1 register (8.3.2.2/8-259) 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.
S12P-Family Reference Manual, Rev. 1.13
Freescale Semiconductor
293
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