242
7679H–CAN–08/08
AT90CAN32/64/128
The total number of TQ in a bit time has to be programmed at least from 8 to 25.
Figure 19-7. Sample and Transmission Point
Figure 19-8. General Structure of a Bit Period
19.4.3 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
IO
(= 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
IO
Fcan (Tscl)
Time Quantum
Bit Rate Prescaler
CLK
IO
F
CAN
Data
Tscl (TQ)
1
/
CLK
IO
one nominal bit
Tsyns(5)
Tphs2+Tsjw (
4)Tphs1+Tsjw (3)
Tbit
Tphs2 (
2)Tphs1 (1)Tprs
Sample
Point
Transmission
Point
5. Synchronization Segment: SYNS
Tsyns=1
xTscl (fixed)
Notes: 1. Phase error < 0
2. Phase error > 0
3. Phase error > 0
4. Phase error < 0
or or
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