PIC18F2585_07 Datasheet, PDF (339/482 Pages) Microchip Technology – Enhanced Flash Microcontrollers with ECAN Technology, 10-Bit A/D and nanoWatt Technology

PIC18F2585/2680/4585/4680
FIGURE 23-6:
LENGTHENING A BIT PERIOD (ADDING SJW TO PHASE SEGMENT 1)
Input
Signal
Bit
Time
Segments
TQ
Sync
Prop
Segment
Phase
Segment 1
≤ SJW
Nominal Bit Length
Sample Point
Actual Bit Length
Phase
Segment 2
FIGURE 23-7:
SHORTENING A BIT PERIOD (SUBTRACTING SJW FROM PHASE SEGMENT 2)
Sync
Prop
Segment
TQ
Phase
Segment 1
Phase
Segment 2
Sample Point
Actual Bit Length
Nominal Bit Length
≤ SJW
23.11 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 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.
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.
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 differ-
ent nodes is inaccurate or unstable, such as using
ceramic resonators. Typically, an SJW of 1 is enough.
23.12 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. A maximum node-to-node oscillator variation
of 1.7% is allowed.
© 2007 Microchip Technology Inc.
Preliminary
DS39625C-page 337