PIC18F2480 MICROCHIP [Microchip Technology], PIC18F2480 Datasheet - Page 335

• 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]

23.9.1
The microcontroller clock frequency generated from a
PLL circuit is subject to a jitter, also defined as Phase
Jitter or Phase Skew. For its PIC18 Enhanced micro-
controllers, Microchip specifies phase jitter (P
being 2% (Gaussian distribution, within 3 standard
deviations, see parameter F13 in Table 27-7) and Total
Jitter (T
FIGURE 23-5:
Once these considerations are taken into account, it is
possible to show that the relation between the jitter and
the total frequency error can be defined as:
where jitter is expressed in terms of time and NBT is the
Nominal Bit Time.
 2004 Microchip Technology Inc.
jitter
Nominal Clock
Clock with Jitter
CAN Bit Time
with Jitter
) as being 2*P
EXTERNAL CLOCK, INTERNAL
CLOCK AND MEASURABLE JITTER
IN HS-PLL BASED OSCILLATORS
f
=
----------------------- -
10 NBT
T
EFFECTS OF PHASE JITTER ON THE MICROCONTROLLER CLOCK
AND CAN BIT TIME
jitter
jitter
.
=
----------------------- -
10 NBT
2 P
jitter
PIC18F2480/2580/4480/4580
jitter
) as
Preliminary
Phase Skew (Jitter)
CAN Bit Jitter
The CAN protocol uses a bit-stuffing technique that
inserts a bit of a given polarity following five bits with the
opposite polarity. This gives a total of 10 bits transmit-
ted without re-synchronization (compensation for jitter
or phase error).
Given the random nature of the jitter error added, it can
be shown that the total error caused by the jitter tends
to cancel itself over time. For a period of 10 bits, it is
necessary to add only two jitter intervals to correct for
jitter-induced error: one interval in the beginning of the
10-bit period and another at the end. The overall effect
is shown in Figure 23-5.
For example, assume a CAN bit rate of 125 Kb/s, which
gives an NBT of 8 µs. For a 16 MHz clock generated
from a 4x PLL, the jitter at this clock frequency is:
and resultant frequency error is:
2
-------------------------------------- -
10
2%
1.25
8
-------------------
16 MHz
10
10
6 –
1
–
9
=
=
3.125
-----------------
16
0.02
10
10
6
–
5
=
DS39637A-page 333
=
1.25ns
0.0031%