Rainbow-electronics ATmega64C1 Manuel d'utilisateur télécharger pdf (Page 175)

100

101

175

7647A–AVR–02/08

ATmega32/64/M1/C1

The total number of TQ in a bit time has to be programmed at least from 8 to 25.

Figure 16-7. Sample and Transmission Point

Figure 16-8. General Structure of a Bit Period

16.4.3 Baud Rate

With no baud rate prescaler (BRP[5..0]=0) the sampling point comes one time quantum too

early. This leads to a fail according the ISO16845 Test plan. It is necessary to lengthen the

Phase Segment 1 by one time quantum and to shorten the Phase Segment 2 by one time quan-

tum to compensate.

The baud rate selection is made by T

bit

calculation:

Tbit

(1)

= Tsyns + Tprs + Tphs1 + Tphs2

1. Tsyns = 1 x Tscl = (BRP[5..0]+ 1)/clk

(= 1TQ)

2. Tprs = (1 to 8) x Tscl = (PRS[2..0]+ 1) x Tscl

3. Tphs1 = (1 to 8) x Tscl = (PHS1[2..0]+ 1) x Tscl

4. Tphs2 = (1 to 8) x Tscl = (PHS2[2..0]

(2)

+ 1) x Tscl

Bit Timing

Sample

Point

Transmission

Point

Prescaler BRP

PRS (3-bit length)

SJW (2-bit length)

PHS1 (3-bit length)

PHS2 (3-bit length)

CLK

Fcan (Tscl)

Time Quantum

Bit Rate Prescaler

CLK

CAN

Data

Tscl (TQ)

CLK

one nominal bit

Tsyns(5)

Tphs2+Tsjw (

)Tphs1+Tsjw (

)

Tbit

Tphs2 (

)Tphs1 (

)Tprs

Sample

Point

Transmission

Point

5. Synchronization Segment: SYNS

Tsyns=1

Tscl (

fixed

)

Notes: 1. Phase error < 0

2. Phase error > 0

3. Phase error > 0

4. Phase error < 0

or or

1 2 ... 170 171 172 173 174 175 176 177 178 179 180 ... 365 366

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Rainbow-electronics ATmega64C1 Manuel d'utilisateur Page 175

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