Microchip Technology DM164134 Data Sheet

PIC18FXX8

DS41159E-page 236

FIGURE 19-9:

SHORTENING A BIT PERIOD (SUBTRACTING SJW FROM PHASE SEGMENT 2)

19.9

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

OSC

. With a T

OSC

of 50 ns,

a baud rate prescaler value of 04h gives a T

of 500 ns.

To obtain a nominal bit rate of 125 kHz, the nominal bit
time must be 8

μs or 16 T

Using 1 T

for the Sync Segment, 2 T

for the Propa-

gation Segment and 7 T

for Phase Segment 1 would

place the sample point at 10 T

after the transition.

This leaves 6 T

for Phase Segment 2.

By the rules above, the Sync Jump Width could be the
maximum of 4 T

. 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.

19.10 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.

19.11 Bit Timing Configuration

Registers

The Configuration registers (BRGCON1, BRGCON2,
BRGCON3) control the bit timing for the CAN bus
interface. These registers can only be modified when
the PIC18FXX8 is in Configuration mode.

19.11.1

BRGCON1

The BRP bits control the baud rate prescaler. The
SJW<1:0> bits select the synchronization jump width in
terms of multiples of T

19.11.2

BRGCON2

The PRSEG bits set the length of the Propagation Seg-
ment in terms of T

. The SEG1PH bits set the length of

Phase Segment 1 in T

. The SAM bit controls how

many times the RXCAN pin is sampled. Setting this bit
to a ‘1’ causes the bus to be sampled three times; twice
at T

/2 before the sample point and once at the normal

sample point (which is at the end of Phase Segment 1).
The value of the bus is determined to be the value read
during at least two of the samples. If the SAM bit is set
to a ‘0’, then the RXCAN pin is sampled only once at
the sample point. The SEG2PHTS bit controls how the
length of Phase Segment 2 is determined. If this bit is
set to a ‘1’, then the length of Phase Segment 2 is
determined by the SEG2PH bits of BRGCON3. If the
SEG2PHTS bit is set to a ‘0’, then the length of Phase
Segment 2 is the greater of Phase Segment 1 and the
information processing time (which is fixed at 2 T

for

the PIC18FXX8).

19.11.3

BRGCON3

The PHSEG2<2:0> bits set the length (in T

) of Phase

Segment 2 if the SEG2PHTS bit is set to a ‘1’. If the
SEG2PHTS bit is set to a ‘0’, then the PHSEG2<2:0>
bits have no effect.

Sync

Prop

Segment

Phase

Segment 1

Phase

Segment 2

≤ SJW

Sample Point

Nominal Bit Length

Actual Bit Length