70353C datasheet(61/76 Pages) MICROCHIP

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DS70353C-page 21-61

Section 21. Enhanced Controller Area Network (ECAN™)

21.9.4.2 STEP 2: CALCULATE THE BAUD RATE PRESCALER

(BRP<5:0> (CiCFG1<5:0>))

Equation 21-3 shows the formula for computing the baud rate prescaler.

Equation 21-3:

Baud Rate Prescaler

21.9.4.3 STEP 3: SELECT THE INDIVIDUAL BIT TIME SEGMENTS

The Individual Bit Time Segments are selected using the CiCFG2 register:

Bit Time = Sync Segment + Propagation Segment + Phase Segment 1 + Phase Segment 2

Example 21-8 shows the procedure to calculate the FTQ, to obtain a CAN Bus speed of 1 Mbps

when the dsPIC33E/PIC24E device is operating at 40 MHz.

Example 21-8:

CAN Bit Timing Calculation Example

A code example for configuring the ECAN bit timing parameters is shown in Example 21-9.

Note 1: (Propagation Segment + Phase Segment 1) must be greater than or equal to the

length of Phase Segment 2.

2: Phase Segment 2 must be greater than the Synchronous Jump Width.

BRP<5:0> (CiCFG1<5:0>)

FCAN

FTQ

()

-------------------------

–

Step 1: Calculate the FTQ.

If FBAUD = 1 Mbps, and the number of time quanta ‘N’ = 20, then FTQ = 20 MHz

Step 2: Calculate the baud rate prescaler.

BRP<5:0> (CiCFG1<5:0>) = [40000000/(2 * FTQ)] - 1 = 1 - 1 = 0

Step 3: Select the individual bit time segments.

Synchronization Segment = 1 TQ (constant)

Based on system characteristics, if Propagation Delay = 5 TQ, if the sample point is to be at

70% of Nominal Bit Time, then:

Phase Segment 2 = 30% of Nominal Bit Time = 6 TQ

Phase Segment 1 = 20 TQ - (1 TQ + 5 TQ + 6 TQ) = 8 TQ

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