MCP2515-ESLASHP datasheet(39/92 Pages) MICROCHIP

39 / 92 page

 2003-2012 Microchip Technology Inc.

DS21801G-page 39

MCP2515

5.0

BIT TIMING

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 times may vary from

node to node, the receiver must have some type of

Phase

Lock

Loop

(PLL)

synchronized

data

transmission 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 MCP2515 is implemented using a

DPLL that is configured to synchronize to the incoming

data, as well as provide 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.

5.1

The CAN Bit TIme

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 CAN bit time is made up of non-overlapping

segments. Each of these segments are made up of

integer units called Time Quanta (TQ), explained later

in this data sheet. The Nominal Bit Rate (NBR) is

defined in the CAN specification as the number of bits

per second transmitted by an ideal transmitter with no

resynchronization. It can be described with the

equation:

EQUATION 5-1:

Nominal Bit Time

The Nominal Bit Time (NBT) (tbit) is made up of non-

overlapping segments (Figure 5-1). Therefore, the

NBT is the summation of the following segments:

Associated with the NBT are the sample point,

Synchronization Jump Width (SJW) and Information

Processing Time (IPT), which are explained later.

SYNCHRONIZATION SEGMENT

The Synchronization Segment (SyncSeg) is the first

segment in the NBT and is used to synchronize the

nodes on the bus. Bit edges are expected to occur

within the SyncSeg. This segment is fixed at 1 TQ.

FIGURE 5-1:

CAN BIT TIME SEGMENTS

NBR

bit

-------

bit

SyncSeg

PropSeg

PS1

PS2

Nominal Bit Time (NBT), tbit

Sample

Point

SyncSeg

PropSeg

PhaseSeg1 (PS1)

PhaseSeg2 (PS2)

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