PIC18F258 Datasheet, PDF (232/384 Pages) Microchip Technology – High Performance, 28/40-Pin Enhanced FLASH Microcontrollers with CAN

PIC18FXX8
19.7.1 TIME QUANTA
As already mentioned, the Time Quanta is a fixed unit
derived from the oscillator period and baud rate pres-
caler. Its relationship to TBIT and the Nominal Bit Rate
is shown in Example 19-2.
EXAMPLE 19-2:
CALCULATING TQ,
NOMINAL BIT RATE AND
NOMINAL BIT TIME
TQ (µs) = (2 * (BRP+1)) / FOSC (MHz)
TBIT (µs) = TQ (µs) * number of TQ per bit interval
Nominal Bit Rate (bits/s) = 1 / TBIT
CASE 1:
For FOSC = 16 MHz, BRP<5:0> = 00h, and
Nominal Bit Time = 8 TQ:
TQ = (2*1) / 16 = 0.125 µs (125 ns)
TBIT = 8 * 0.125 = 1 µs (10^-6 s)
Nominal Bit Rate = 1 / 10^-6 = 10⁶ bits/s (1 Mb/s)
CASE 2:
For FOSC = 20 MHz, BRP<5:0> = 01h, and
Nominal Bit Time = 8 TQ:
TQ = (2*2) / 20 = 0.2 µs (200 ns)
TBIT = 8 * 0.2 = 1.6 µs (1.6 * 10^-6 S)
Nominal Bit Rate = 1 / 1.6 * 10^-6 s = 625,000 bits/s
(625 Kb/s)
CASE 3:
For FOSC = 25 MHz, BRP<5:0> = 3Fh, and
Nominal Bit Time = 25 TQ:
TQ = (2*64) / 25 = 5.12 µs
TBIT = 25 * 5.12 = 128 µs (1.28 * 10^-4 s)
Nominal Bit Rate = 1 / 1.28 * 10^-4 = 7813 bits/s
(7.8 Kb/s)
The frequencies of the oscillators in the different nodes
must be coordinated in order to provide a system wide
specified nominal bit time. This means that all oscilla-
tors must have a TOSC that is an integral divisor of TQ.
It should also be noted that although the number of TQ
is programmable from 4 to 25, the usable minimum is
8 TQ. A bit time of less than 8 TQ in length is not
guaranteed to operate correctly.
19.7.2 SYNCHRONIZATION SEGMENT
This part of the bit time is used to synchronize the var-
ious CAN nodes on the bus. The edge of the input sig-
nal is expected to occur during the sync segment. The
duration is 1 TQ.
19.7.3 PROPAGATION SEGMENT
This part of the bit time is used to compensate for phys-
ical delay times within the network. These delay times
consist of the signal propagation time on the bus line
and the internal delay time of the nodes. The length of
the Propagation Segment can be programmed from
1 TQ to 8 TQ by setting the PRSEG2:PRSEG0 bits.
19.7.4 PHASE BUFFER SEGMENTS
The Phase Buffer Segments are used to optimally
locate the sampling point of the received bit, within the
nominal bit time. The sampling point occurs between
phase segment 1 and phase segment 2. These seg-
ments can be lengthened or shortened by the resyn-
chronization process. The end of phase segment 1
determines the sampling point within a bit time. Phase
segment 1 is programmable from 1 TQ to 8 TQ in dura-
tion. Phase segment 2 provides delay before the next
transmitted data transition and is also programmable
from 1 TQ to 8 TQ in duration. However, due to IPT
requirements, the actual minimum length of phase seg-
ment 2 is 2 TQ, or it may be defined to be equal to the
greater of phase segment 1 or the Information
Processing Time (IPT).
19.7.5 SAMPLE POINT
The Sample Point is the point of time at which the bus
level is read and the value of the received bit is deter-
mined. The sampling point occurs at the end of phase
segment 1. If the bit timing is slow and contains many
TQ, it is possible to specify multiple sampling of the bus
line at the sample point. The value of the received bit is
determined to be the value of the majority decision of
three values. The three samples are taken at the sam-
ple point, and twice before, with a time of TQ/2 between
each sample.
19.7.6 INFORMATION PROCESSING TIME
The Information Processing Time (IPT) is the time seg-
ment, starting at the sample point that is reserved for
calculation of the subsequent bit level. The CAN spec-
ification defines this time to be less than or equal to
2 TQ. The PIC18FXX8 defines this time to be 2 TQ.
Thus, phase segment 2 must be at least 2 TQ long.
DS41159B-page 230
Preliminary
 2002 Microchip Technology Inc.