MCP2510-SLASHST datasheet(35/80 Pages) MICROCHIP

35 / 80 page

DS21291F-page 35

MCP2510

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 syn-

chronized to the transmitters 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 MCP2510 is implemented using a

DPLL that is configured to synchronize to the incoming

data, and provide the nominal timing for the transmitted

data. The DPLL breaks each bit time into multiple seg-

ments 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 seg-

ment.

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.

Nominal Bit Time is defined as:

TBIT = 1 / NOMlNAL BlT RATE

The nominal bit time can be thought of as being divided

into separate non-overlapping time segments. These

segments are shown in Figure 5-1.

- Synchronization Segment (Sync_Seg)

- Propagation Time Segment (Prop_Seg)

- Phase Buffer Segment 1 (Phase_Seg1)

- Phase Buffer Segment 2 [Phase_Seg2)

Nominal Bit Time = TQ * (Sync_Seg + Prop_Seg +

Phase_Seg1 + Phase_Seg2)

The time segments and also the nominal bit time are

made up of integer units of time called time quanta or

TQ (see Figure 5-1). By definition, the nominal bit time

is programmable from a minimum of 8 TQ to a maxi-

mum of 25 TQ. Also, by definition the minimum nominal

bit time is 1 µs, corresponding to a maximum 1 Mb/s

rate.

FIGURE 5-1:

BIT TIME PARTITIONING

Input Signal

Sync

Prop

Segment

Phase

Segment 1

Phase

Segment 2

Sample Point

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