AN754

DS00754A-page 2

 2001 Microchip Technology Inc.

SAMPLE POINT

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

logic level is read and interpreted. The sample point is

located at the end of phase segment 1. The exception

to this rule is, if the sample mode is configured to sam-

ple three times per bit. In this case, the bit is still sam-

pled at the end of PS1, however, two additional

samples are taken at one-half TQ intervals prior to the

end of PS1 and the value of the bit is 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 sam-

pled bit. The IPT begins at the sample point, is mea-

sured in TQ and is fixed at 2TQ for the Microchip CAN

module. Since phase segment 2 also begins at the

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

required that 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 - 4TQ (as configured) to main-

tain synchronization with the transmitted message.

More on synchronization is covered later.

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

and TQ. The TQ length equals one TQ Clock period

ble prescaler named the Baud Rate Prescaler (BRP).

This is shown in the following equation:

Where: BRP equals the configuration as shown in

Figure 3.

Bit Timing Control Registers

The CAN Bit Timing Control (CNF) registers are the

three registers that configure the CAN bit time. Figure 3

details the function of the CNF registers.

of TQs in each segment, both the nominal bit time and

the sample point can easily be configured as desired.

PROGRAMMING THE TIMING SEGMENTS

The are several requirements for programming the

CAN bit timing segments.

1.

PropSeg + PS1

≥ PS2

2.

PropSeg + PS1

3.

PS2 > SJW

FIGURE 2:

TQ AND THE BIT PERIOD

IPT

2TQ

==

TQ

2 BRP

⋅

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

=

⋅⋅

=

T

BRPCLK

t

BIT

Sync

(fixed)

PropSeg

(Programmable)

PS2

(Programmable)

PS1

(Programmable)

TQ

(t

CAN Bit Time