PIC18F25K80 MICROCHIP [Microchip Technology], PIC18F25K80 Datasheet - Page 455

• Manufacturer Part Number: PIC18F25K80
• Manufacturer: MICROCHIP [Microchip Technology]

FIGURE 27-7:
27.11 Programming Time Segments
Some requirements for programming of the time
segments:
• Prop_Seg + Phase_Seg 1  Phase_Seg 2
• Phase_Seg 2  Sync Jump Width.
For example, assume that a 125 kHz CAN baud rate is
desired, using 20 MHz for F
a baud rate prescaler value of 04h gives a T
To obtain a Nominal Bit Rate of 125 kHz, the Nominal
Bit Time must be 8  s or 16 T
Using 1 T
and 7 T
point at 10 T
Phase Segment 2.
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T
Q
Q
for Phase Segment 1 would place the sample
Q
for the Sync_Seg, 2 T
Q
Sync
after the transition. This leaves 6 T
SHORTENING A BIT PERIOD (SUBTRACTING SJW FROM PHASE SEGMENT 2)
Segment
Prop
OSC
Q
.
. With a T
Q
for the Prop_Seg
OSC
Actual Bit Length
Q
of 500 ns.
of 50 ns,
Segment 1
Phase
Q
Preliminary
Nominal Bit Length
for
Sample Point
PIC18F66K80 FAMILY
By the rules above, the Sync Jump Width could be the
maximum of 4 T
only necessary when the clock generation of the
different nodes is inaccurate or unstable, such as using
ceramic resonators. Typically, an SJW of 1 is enough.
27.12 Oscillator Tolerance
As a rule of thumb, the bit timing requirements allow
ceramic resonators to be used in applications with
transmission rates of up to 125 Kbit/sec. For the full bus
speed range of the CAN protocol, a quartz oscillator is
required. Refer to ISO11898-1 for oscillator tolerance
requirements.
Segment 2
Phase
Q
. However, normally a large SJW is
DS39977C-page 455
 SJW