Rainbow-electronics T89C51CC01 Manual do Utilizador descarregar pdf (Página 77)

T89C51CC01

Rev. D – 17-Dec-01

Figure 35. General structure of a bit period

example of bit timing determination for CAN baudrate of 500kbit/s:

Fosc = 12 MHz in X1 mode => FCAN = 6MHz

Verify that the CANbaud rate you want is an integer division of FCAN clock.

FCAN/CANbaudrate = 6MHz/500kHz = 12

The time quanta TQ must be comprised between 8 and 25: TQ = 12 and BRP=0

Define the various timing parameters: Tbit = Tsyns + Tprs + Tphs1 + Tphs2 =

12TQ

Tsyns = 1TQ and Tsjw =1TQ => SJW=0

If we chose a sample point at 66.6% => Tphs2 = 4TQ => PHS2 = 3

Tbit=12=4+1+Tphs1 + Tprs, let us choose Tprs = 3 Tphs1 = 4

PHS1 = 3 and PRS=2

BRP=0soCANBT1 = 00h

SJW=0andPRS=2soCANBT2 = 04h

PHS2=3andPHS1=3soCANBT3 = 36h

Bit Rate Prescaler

oscillator

1/ Fcan

Tscl

system clock

one nominal bit

Tsyns (*)

Tprs

Sample Point

(*) Synchronization Segment: SYNS

Tbit

Tsyns = 1xTscl (fixed)

data

Tbit Tsyns Tprs Tphs1 Tphs2++ +=

Tbit calculation:

Transmission Point

Tphs1 + Tsjw (3)

Tphs2 - Tsjw (4)

(1) Phase error £ 0

(2) Phase error Š 0

(3) Phase error > 0

(4) Phase error < 0

Tphs2 (2)

Tphs1 (1)

1 2 ... 72 73 74 75 76 77 78 79 80 81 82 ... 148 149

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Rainbow-electronics T89C51CC01 Manual do Utilizador Página 77