PIC18F258-ESLASHSOSQTP datasheet(235/402 Pages) MICROCHIP

235 / 402 page

DS41159E-page 233

PIC18FXX8

19.7

Baud Rate Setting

All nodes on a given CAN bus must have the same

nominal bit rate. The CAN protocol uses Non-Return-

to-Zero (NRZ) coding which does not encode a clock

within the data stream. Therefore, the receive clock

must be recovered by the receiving nodes and

synchronized to the transmitters clock.

As oscillators and transmission time may vary from

node to node, the receiver must have some type of

Phase Lock Loop (PLL) synchronized to data transmis-

sion edges to synchronize and maintain the receiver

clock. Since the data is NRZ coded, it is necessary to

include bit stuffing to ensure that an edge occurs at

least every six bit times to maintain the Digital Phase

Lock Loop (DPLL) synchronization.

The bit timing of the PIC18FXX8 is implemented using

a DPLL that is configured to synchronize to the

incoming data and provides the nominal timing for the

transmitted data. The DPLL breaks each bit time into

multiple segments made up of minimal periods of time

called the

Time Quanta (TQ).

Bus timing functions executed within the bit time frame,

such as synchronization to the local oscillator, network

transmission delay compensation and sample point

positioning, are defined by the programmable bit timing

logic of the DPLL.

All devices on the CAN bus must use the same bit rate.

However, all devices are not required to have the same

master oscillator clock frequency. For the different clock

frequencies of the individual devices, the bit rate has to

be adjusted by appropriately setting the baud rate

prescaler and number of time quanta in each segment.

The

Nominal Bit Rate is the number of bits transmitted

per second, assuming an ideal transmitter with an ideal

oscillator, in the absence of resynchronization. The

nominal bit rate is defined to be a maximum of 1 Mb/s.

The

Nominal Bit Time is defined as:

TBIT = 1/Nominal Bit Rate

The nominal bit time can be thought of as being divided

into separate, non-overlapping time segments. These

segments (Figure 19-7) include:

• Synchronization Segment (Sync_Seg)

• Propagation Time Segment (Prop_Seg)

• Phase Buffer Segment 1 (Phase_Seg1)

• Phase Buffer Segment 2 (Phase_Seg2)

The time segments (and thus, the nominal bit time) are,

in turn, made up of integer units of time called time

quanta or TQ (see Figure 19-7). By definition, the

nominal bit time is programmable from a minimum of

8TQ to a maximum of 25 TQ. Also, by definition, the

minimum nominal bit time is 1

μs corresponding to a

maximum 1 Mb/s rate. The actual duration is given by

the relationship:

Nominal Bit Time = TQ * (Sync_Seg + Prop_Seg +

Phase_Seg1 + Phase_Seg2)

The time quantum is a fixed unit derived from the

oscillator period. It is also defined by the programmable

baud rate prescaler, with integer values from 1 to 64, in

addition to a fixed divide-by-two for clock generation.

Mathematically, this is

TQ (

μs) = (2 * (BRP + 1))/FOSC (MHz)

TQ (

μs) = (2 * (BRP + 1)) * TOSC (μs)

where FOSC is the clock frequency, TOSC is the

corresponding oscillator period and BRP is an integer

(0 through 63) represented by the binary values of

BRGCON1<5:0>.

FIGURE 19-7:

BIT TIME PARTITIONING

Input

Sync

Propagation

Segment

Phase

Segment 1

Phase

Segment 2

Sample Point

Nominal Bit Time

Bit

Time

Intervals

Signal

Segment

PIC18F258-E/SOSQTP Datasheet(PDF) 235 Page - Microchip Technology

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