PIC18F2480/2580/4480/4580
FIGURE 24-6:
LENGTHENING A BIT PERIOD (ADDING SJW TO PHASE SEGMENT 1)
Input
Signal
Bit
Time
Segments
Prop
Segment
Phase
Segment 1
Phase
Segment 2
Sync
≤ SJW
TQ
Sample Point
Nominal Bit Length
Actual Bit Length
FIGURE 24-7:
SHORTENING A BIT PERIOD (SUBTRACTING SJW FROM PHASE SEGMENT 2)
Prop
Segment
Phase
Segment 1
Phase
Segment 2
Sync
≤ SJW
TQ
Sample Point
Actual Bit Length
Nominal Bit Length
By the rules above, the Sync Jump Width could be the
maximum of 4 TQ. However, normally a large SJW is
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.
24.11 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.
24.12 Oscillator Tolerance
For example, assume that a 125 kHz CAN baud rate is
desired, using 20 MHz for FOSC. With a TOSC of 50 ns,
a baud rate prescaler value of 04h gives a TQ of 500 ns.
To obtain a Nominal Bit Rate of 125 kHz, the Nominal
Bit Time must be 8 μs or 16 TQ.
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.
Using 1 TQ for the Sync_Seg, 2 TQ for the Prop_Seg
and 7 TQ for Phase Segment 1 would place the sample
point at 10 TQ after the transition. This leaves 6 TQ for
Phase Segment 2.
© 2009 Microchip Technology Inc.
DS39637D-page 343
MICROCHIP [ MICROCHIP ]
