MPC5200B Users Guide, Rev. 1
19-32 Freescale Semiconductor
Functional Description
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 Section
19.5.3, MSCAN Control Register 0 (CANCTL0)—MBAR + 0x0900 / 0x980 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 (CANIDAR0-7)
MSCAN Identifier Mask Registers (CANIDMR0-7)
The TXCAN pin is immediately forced to a recessive state when the MSCAN goes into the Power Down Mode or Initialization
Mode (see Section 19.7.8.6, MSCAN Power Down Mode and Section 19.7.8.5, MSCAN Initialization Mode).
The MSCAN enable bit (CANE) is only writable once in normal modes as further protection against inadvertently disabling the
MSCAN.

19.7.5 Clock System

Figure 19-7 shows the structure of the MSCAN clock generation circuitry. With this flexible clocking scheme, the MSCAN is able to handle
CAN bus rates ranging from 10 Kbps up to 1 Mbps.

Figure 19-7. MSCAN Clocking Scheme

The clock source bit (CLKSRC) in the CANCTL1 register Section 19.5.4, MSCAN Control Register 1 (CANCTL1)—MBAR + 0x0901 / 0x981
defines whether the internal CANCLK is connected to the output of the system oscillator clock (SYS_XTAL_IN) or to the IP bus clock.
NOTE
Both MSCAN modules can have different selected clock sources. To select the oscillator clock the
CLKSRC bit in the CANCTL1 register must be set.
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%-55% duty cycle of the clock is required.
Because 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.
A programmable prescaler generates the time quanta (Tq) clock from CANCLK. A time quantum is the atomic unit of time handled by the
MSCAN.
A bit time is subdivided into three segments1 2 (reference Figure 19-8):
SYNC_SEG: This segment has a fixed length of one time quantum. Signal edges are expected to happen within this section.
Time Segment 1: This segment includes the PROP_SEG and the PHASE_SEG1 of the CAN standard. It can be programmed by
setting the parameter TSEG1 to consist of 4 to 16 time quanta.
1. For further explanation of the under-lying concepts please refer to ISO/DIS 11519-1, Section 10.3.
2. Reference the Bosch CAN 2.0A/B protocol specification dated September 1991 for bit timing.
IP bus clock
Oscillator Clock
MSCAN
CANCLK
CLKSRC
CLKSRC
Prescaler
(1 .. 64)
Time quanta clock (Tq)
fTq
fCANCLK

Prescaler valueÞ()

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