Datasheet

 2003-2019 Microchip Technology Inc. DS20001801J-page 39
MCP2515
5.0 BIT TIMING
All nodes on a given CAN bus must have the same
Nominal Bit Rate (NBR). 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-Locked Loop (PLL) synchronized to 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-Locked 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 (T
Q
).
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 (BTL) 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 seg-
ments. Each of these segments is made up of integer
units, called Time Quanta (T
Q
), 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:
5.2 Nominal Bit Time
The Nominal Bit Time (NBT) (t
bit
) 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.
5.2.1 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 T
Q
.
FIGURE 5-1: CAN BIT TIME SEGMENTS
NBR f
bit
1
t
bit
-------==
t
bit
t
SyncSeg
t
PropSeg
t
PS1
t
PS2
+++=
Nominal Bit Time (NBT), t
bit
Sample
Point
SyncSeg PropSeg PhaseSeg1 (PS1) PhaseSeg2 (PS2)