2000 Microchip Technology Inc.
Shortening a Bit Period
Programming Time Segments
Some requirements for programming of the time seg-
• Prop Seg + Phase Seg 1 >= Phase Seg 2
• Prop Seg + Phase Seg 1 >= TDELAY
• Phase Seg 2 > Sync Jump Width
For example, assuming that a 125 kHz CAN baud rate
with FOSC = 20 MHz is desired:
TOSC = 50 nsec, choose BRP<5:0> = 04h, then TQ =
500nsec. To obtain 125 kHz, the bit time must be 16
Typically, the sampling of the bit should take place at
about 60-70% of the bit time, depending on the system
parameters. Also, typically, the TDELAY is 1-2 TQ.
Sync Seg = 1 TQ; Prop Seg = 2 TQ; So setting Phase
Seg 1 = 7 TQ would place the sample at 10 TQ after the
transition. This would leave 6 TQ for Phase Seg 2.
Since Phase Seg 2 is 6, by the rules, SJW 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. So an SJW of 1 is typically
The bit timing requirements allow ceramic resonators
to be used in applications with transmission rates of up
to 125 kbit/sec, as a rule of thumb. 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.