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AN754 Datasheet PDF : 13 Pages
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AN754
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:
PS2min = IPT = 2TQ
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.
FIGURE 2: TQ AND THE BIT PERIOD
tosc
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
(tOSC). The base TQ equals twice the oscillator period.
Figure 2 shows how the bit period is derived from TOSC
and TQ. The TQ length equals one TQ Clock period
(tBRPCLK), which is programmable using a programma-
ble prescaler named the Baud Rate Prescaler (BRP).
This is shown in the following equation:
TQ = 2 ⋅ BRP ⋅ TOSC= 2---F--⋅--O-B---S-R--C--P--
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.
By adjusting the length of the TQ (tTQ) and the number
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 ≥ tPROP
3. PS2 > SJW
TBRPCLK
tBIT
Sync
(fixed)
PropSeg
(Programmable)
TQ
(tTQ)
PS1
(Programmable)
PS2
(Programmable)
CAN Bit Time
DS00754A-page 2
2001 Microchip Technology Inc.
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