MC9S12KT256 Datasheet, PDF (309/594 Pages) Freescale Semiconductor, Inc – Microcontrollers

9.4.3.1 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 9.3.2.1, “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 pin is immediately forced to a recessive state when the MSCAN goes into the power
down mode or initialization mode (see Section 9.4.6.6, “MSCAN Power Down Mode,” and
Section 9.4.6.5, “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.
9.4.3.2 Clock System
Figure 9-40 shows the structure of the MSCAN clock generation circuitry.
Bus Clock
MSCAN
Oscillator Clock
CLKSRC
CANCLK
Prescaler
(1 .. 64)
CLKSRC
Time quanta clock (Tq)
Figure 9-40. MSCAN Clocking Scheme
The clock source bit (CLKSRC) in the CANCTL1 register (9.3.2.2/9-274) 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.
MC9S12KT256 Data Sheet, Rev 1.15
Freescale Semiconductor
309