PIC18F2480 MICROCHIP [Microchip Technology], PIC18F2480 Datasheet - Page 339

• Manufacturer Part Number: PIC18F2480
• Description: 28/40/44-Pin Enhanced Flash Microcontrollers with ECAN Technology, 10-Bit A/D and nanoWatt Technology
• Manufacturer: MICROCHIP [Microchip Technology]

FIGURE 23-6:
FIGURE 23-7:
23.11 Programming Time Segments
Some requirements for programming of the time
segments:
• Prop_Seg + Phase_Seg 1
• Phase_Seg 2
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.
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.
 2004 Microchip Technology Inc.
Input
Signal
Bit
Time
Segments
T
Q
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
Sync
Q
. However, normally a large SJW is
Sync Jump Width.
LENGTHENING A BIT PERIOD (ADDING SJW TO PHASE SEGMENT 1)
SHORTENING A BIT PERIOD (SUBTRACTING SJW FROM PHASE SEGMENT 2)
Segment
Segment
Prop
OSC
Prop
Q
Phase_Seg 2
.
. With a T
Q
for the Prop_Seg
Nominal Bit Length
OSC
Actual Bit Length
Q
Segment 1
of 500 ns.
PIC18F2480/2580/4480/4580
of 50 ns,
Segment 1
Phase
Phase
Q
Preliminary
for
Nominal Bit Length
Actual Bit Length
Sample Point
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.
SJW
Sample Point
Segment 2
Phase
Segment 2
Phase
DS39637A-page 337
SJW