RXB0D2 datasheet(40/92 Pages) MICROCHIP

40 / 92 page

MCP2515

DS21801G-page 40

 2003-2012 Microchip Technology Inc.

PROPAGATION SEGMENT

The

Propagation

Segment

(PropSeg)

exists

compensate for physical delays between nodes. The

propagation delay is defined as twice the sum of the

signal’s propagation time on the bus line, including the

delays associated with the bus driver. The PropSeg is

programmable from 1-8 TQ.

PHASE SEGMENT 1 (PS1) AND PHASE

SEGMENT 2 (PS2)

The two phase segments, PS1 and PS2, are used to

compensate for edge phase errors on the bus. PS1 can

be lengthened (or PS2 shortened) by resyncronization.

PS1 is programmable from 1-8 TQ and PS2 is

programmable from 2-8 TQ.

SAMPLE POINT

The sample point is the point in the bit time at which the

logic level is read and interpreted. The sample point is

located at the end of PS1. The exception to this rule is

if the sample mode is configured to sample three times

per bit. In this case, while the bit is still sampled at the

end of PS1, two additional samples are taken at one-

half TQ intervals prior to the end of PS1, with the value

of the bit being determined by a majority decision.

INFORMATION PROCESSING TIME

The Information Processing Time (IPT) is the time

required for the logic to determine the bit level of a

sampled bit. The IPT begins at the sample point, is

measured in TQ and is fixed at 2 TQ for the Microchip

CAN module. Since PS2 also begins at the sample

point and is the last segment in the bit time, it is

required that the PS2 minimum is not less than the IPT.

Therefore:

SYNCHRONIZATION JUMP WIDTH

The Synchronization Jump Width (SJW) adjusts the bit

clock as necessary by 1-4 TQ (as configured) to

maintain

synchronization

with

the

transmitted

message. Synchronization is covered in more detail

later in this data sheet.

Time Quantum

Each of the segments that make up a bit time are made

up of integer units called Time Quanta (TQ). The length

of each Time Quantum is based on the oscillator period

(tOSC). The base TQ equals twice the oscillator period.

Figure 5-2 shows how the bit period is derived from

TOSC and TQ. The TQ length equals one TQ clock

period (tBRPCLK), which is programmable using a

programmable

prescaler,

called

the

Baud

Rate

Prescaler (BRP). This is illustrated in the following

equation:

EQUATION 5-2:

FIGURE 5-2:

TQ AND THE BIT PERIOD

PS2

min

IPT

2TQ

2 BRP T

OSC



2BRP



OSC

-------------------

Where: BRP equals the configuration as shown in

tOSC

TBRPCLK

tBIT

Sync

(fixed)

PropSeg

(Programmable)

PS2

(Programmable)

PS1

(Programmable)

(tTQ)

CAN Bit Time

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