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PIC18F6585/8585/6680/8680

DS30491C-page 336

2004 Microchip Technology Inc.

23.9

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 transmitter’s 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 PIC18F6585/8585/6680/8680 is

implemented using a DPLL that is configured to syn-

chronize 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:

EQUATION 23-1:

The Nominal Bit Time can be thought of as being

divided into separate, non-overlapping time segments.

These segments (Figure 23-4) 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 23-4). By definition, the Nom-

inal Bit Time is programmable from a minimum of 8 TQ

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

Nominal Bit Time is 1

s, corresponding to a maximum

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

relationship:

EQUATION 23-2:

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:

EQUATION 23-3:

where FOSC is the clock frequency, TOSC is the corre-

sponding oscillator period, and BRP is an integer (0

through 63) represented by the binary values of

BRGCON1<5:0>.

FIGURE 23-4:

BIT TIME PARTITIONING

TBIT = 1/Nominal Bit Rate

Nominal Bit Time = TQ * (Sync_Seg + Prop_Seg +

Phase_Seg1 + Phase_Seg2)

TQ (

s) = (2 * (BRP+1))/FOSC (MHz)

TQ (

s) = (2 * (BRP+1)) * TOSC (s)

Input

Sync

Propagation

Segment

Phase

Segment 1

Phase

Segment 2

Sample Point

Nominal Bit Time

Bit

Time

Intervals

Signal

Segment

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