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ManualsBrandsNXP SEMICONDUCTORS ManualsMicrocontrollers, Microprocessors & QuartzesEmbedded microcontroller LQFP 100 (14x14) 32-Bit 72 MHz I/O number 66

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concurrent memory access due to the CPU or other internal

FlexCAN sub-blocks.

When doing matching and arbitration, FlexCAN needs to scan the whole Message Buffer

memory during the available time slot. In order to have sufficient time to do that, the

following requirements must be observed:

• A valid CAN bit timing must be programmed, as indicated in Table 45-80

• The peripheral clock frequency can not be smaller than the oscillator clock

frequency, that is, the PLL can not be programmed to divide down the oscillator

clock; see Clock domains and restrictions

• There must be a minimum ratio between the peripheral clock frequency and the CAN

bit rate, as specified in the following table:

Table 45-81. Minimum ratio between peripheral clock frequency and CAN

bit rate

Number of Message Buffers RFEN

Minimum number of peripheral

clocks per CAN bit

16 and 32 0 16

64 0 25

16 1 16

32 1 17

64 1 30

A direct consequence of the first requirement is that the minimum number of time quanta

per CAN bit must be 8, therefore the oscillator clock frequency should be at least 8 times

the CAN bit rate. The minimum frequency ratio specified in the preceding table can be

achieved by choosing a high enough peripheral clock frequency when compared to the

oscillator clock frequency, or by adjusting one or more of the bit timing parameters

(PRESDIV, PROPSEG, PSEG1, PSEG2) contained in the Control 1 Register (CTRL1).

In case of synchronous operation (when the peripheral clock frequency is equal to the

oscillator clock frequency), the number of peripheral clocks per CAN bit can be adjusted

by selecting an adequate value for PRESDIV in order to meet the requirement in the

preceding table. In case of asynchronous operation (the peripheral clock frequency

greater than the oscillator clock frequency), the number of peripheral clocks per CAN bit

can be adjusted by both PRESDIV and/or the frequency ratio.

Functional description

K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012

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Preliminary

Freescale Semiconductor, Inc.

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