PIC18F258-ESLASHLSQTP datasheet(236/402 Pages) MICROCHIP

236 / 402 page

PIC18FXX8

DS41159E-page 234

19.7.1

TIME QUANTA

As already mentioned, the time quanta is a fixed unit

derived from the oscillator period and baud rate

prescaler. 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

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

8TQ. A bit time of less than 8 TQ in length is not

ensured to operate correctly.

19.7.2

SYNCHRONIZATION SEGMENT

This part of the bit time is used to synchronize the

various CAN nodes on the bus. The edge of the input

signal 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

physical 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

1TQ 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

segments can be lengthened or shortened by the

resynchronization 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 duration. 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 Segment 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

segment, starting at the sample point, that is reserved

for calculation of the subsequent bit level. The CAN

specification 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.

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 (10-6s)

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 * 10-6s)

Nominal Bit Rate = 1/1.6 * 10-6s =

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 * 10-4s)

Nominal Bit Rate = 1/1.28 * 10-4 =

7813 bits/s

(7.8 Kb/s)

PIC18F258-E/LSQTP Datasheet(PDF) 236 Page - Microchip Technology

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