177
7647H–AVR–03/12
Atmel ATmega16/32/64/M1/C1
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
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
5. Tsjw = (1 to 4) x Tscl = (SJW[1..0]+ 1) x Tscl
Notes: 1. The total number of Tscl (Time Quanta) in a bit time must be from 8 to 25.
2. PHS2[2..0] 2 is programmable to be
≤ PHS1[2..0] and ≥ 1.
16.4.4 Fault Confinement
(c.f. Section 16.7 “Error Management” on page 183).
16.4.5 Overload Frame
An overload frame is sent by setting an overload request (OVRQ). After the next reception, the
CAN channel sends an overload frame in accordance with the CAN specification. A status or
flag is set (OVRF) as long as the overload frame is sent.
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
x
Tscl (
fixed
)
Notes: 1. Phase error < 0
2. Phase error > 0
3. Phase error > 0
4. Phase error < 0
or or
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