MCP25020ESLASHP datasheet(8/66 Pages) MICROCHIP

8 / 66 page

MCP2502X/5X

DS21664D-page 8

The base TQ is defined as twice the oscillator period.

Adding the BRP into the equation yields:

By definition, the nominal bit time is programmable

from a minimum of 8 TQ to 25 TQ. Also, the minimum

nominal bit time is 1 µs, which corresponds to 1 Mb/s.

2.4.2

TIME SEGMENTS

Time segments make up the nominal bit time. The

nominal bit time can be thought of as being divided into

separate non-overlapping time

segments.

These

segments are shown in Figure 2-3.

• Synchronization Segment (SyncSeg)

• Propagation Segment (PropSeg)

• Phase Buffer Segment 1 (PS1)

• Phase Buffer Segment 2 (PS2)

Rules for Programming the Segments

There are a few rules to follow when programming the

time segments:

• PropSeg + PS1

≥ PS2

• PS2 > Sync Jump Width

• PS2

≥ Information Processing Time

2.4.2.1

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 SyncSeg. The

duration is fixed at 1 TQ.

2.4.2.2

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 delay

is calculated as being the round-trip time from

transmitter to receiver (twice the signal's propagation

time on the bus line), the input comparator delay and

the output driver delay. The length of the Propagation

Segment can be programmed from 1 TQ to 8 TQ by

setting the PRSEG2:PRSEG0 bits of the CNF2

2.4.2.3

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

PS1 and PS2. These segments can be automatically

lengthened or shortened by the resynchronization

process. Thus, the variation of the values of the phase

buffer segments represent the DPLL functionality.

Phase Segment 1 (PS1): The end of PS1 determines

the sampling point within a bit time. PS1 is programma-

ble from 1 TQ - 8 TQ in duration.

Phase Segment 2 (PS2): PS2 provides delay before

the next transmitted data transition and is also pro-

grammable from 1 TQ - 8 TQ in duration. However, due

to IPT requirements, the actual minimum length of

phase segment 2 is 2 TQ. It may also be defined to be

equal to the greater of PS1 or the information process-

ing time (IPT).

2.4.3

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

determined. The sampling point occurs at the end of

PS1. If desired, 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 sample point, and twice before, with a

time of TQ/2 between each sample.

2.4.4

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 MCP2502X/5X defines this time to be

2TQ. Thus, PS2 must be at least 2 TQ long.

2.4.5

SYNCHRONIZATION JUMP WIDTH

compensate

for

phase

shifts

and

oscillator

tolerances between the nodes in the system, each

CAN controller must be able to synchronize to the

relevant signal edge of the incoming signal. When a

recessive-to-dominant edge in the transmitted data is

detected, the logic will compare the location of the edge

to the expected time (SyncSeg). The circuit will then

adjust the values of PS1 and PS2, as necessary, using

the programmed Synchronization Jump Width (SJW).

This adjustment is made for resynchronization during a

message and not hard synchronization, which occurs

only at the message Start-of-Frame (SOF).

2*T

OSC * BRP

()

where BRP = binary value represented by

CNF1.BRP<5:0>

Nominal Bit Time

Q* Sync_Seg

PropSeg

Phase_Seg1

Phase_Seg2

(

)

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