RXB0D2 datasheet(43/92 Pages) MICROCHIP

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 2003-2012 Microchip Technology Inc.

DS21801G-page 43

MCP2515

5.3

Programming Time Segments

Some requirements for programming of the time

segments:

• PropSeg + PS1 >= PS2

• PropSeg + PS1 >= TDELAY

• PS2 > SJW

For example, assuming that a 125 kHz CAN baud rate

with FOSC = 20 MHz is desired:

TOSC = 50 ns, choose BRP<5:0> = 04h, then

TQ = 500 ns. To obtain 125 kHz, the bit time must be 16

TQ.

Typically, the sampling of the bit should take place at

about 60-70% of the bit time, depending on the system

parameters. Also, typically, the TDELAY is 1-2 TQ.

SyncSeg = 1 TQ and PropSeg = 2 TQ. So setting

PS1 = 7 TQ would place the sample at 10 TQ after the

transition. This would leave 6 TQ for PS2.

Since PS2 is 6, according to the rules, SJW could be a

maximum of 4 TQ. However, a large SJW is typically

only necessary when the clock generation of the differ-

ent nodes is inaccurate or unstable, such as using

ceramic resonators. So a SJW of 1 is usually enough.

5.4

Oscillator Tolerance

The bit timing requirements allow ceramic resonators

to be used in applications with transmission rates of up

to 125 kbit/sec as a rule of thumb. For the full bus

speed range of the CAN protocol, a quartz oscillator is

required. A maximum node-to-node oscillator variation

of 1.7% is allowed.

5.5

Bit Timing Configuration

Registers

The configuration registers (CNF1, CNF2, CNF3)

control the bit timing for the CAN bus interface. These

registers can only be modified when the MCP2515 is in

Configuration mode (see Section 10.0 “Modes of

Operation”).

5.5.1

CNF1

The BRP<5:0> bits control the Baud Rate Prescaler.

These bits set the length of TQ relative to the OSC1

input frequency, with the minimum TQ length being

2TOSC

(when

BRP<5:0>

‘b000000’).

The

SJW<1:0> bits select the SJW in terms of number of

TQs.

5.5.2

CNF2

The PRSEG<2:0> bits set the length (in TQ’s) of the

propagation segment. The PHSEG1<2:0> bits set the

length (in TQ’s) of PS1.

The SAM bit controls how many times the RXCAN pin

is sampled. Setting this bit to a ‘1’ causes the bus to be

sampled three times: twice at TQ/2 before the sample

point and once at the normal sample point (which is at

the end of PS1). The value of the bus is determined to

be the majority sampled. If the SAM bit is set to a ‘0’,

the RXCAN pin is sampled only once at the sample

point.

The BTLMODE bit controls how the length of PS2 is

determined. If this bit is set to a ‘1’, the length of PS2 is

determined by the PHSEG2<2:0> bits of CNF3 (see

Section 5.5.3 “CNF3”). If the BTLMODE bit is set to a

‘0’, the length of PS2 is greater than that of PS1 and the

information processing time (which is fixed at 2 TQ for

the MCP2515).

5.5.3

CNF3

The PHSEG2<2:0> bits set the length (in TQ’s) of PS2,

if the CNF2.BTLMODE bit is set to a ‘1’. If the

BTLMODE bit is set to a ‘0’, the PHSEG2<2:0> bits

have no effect.

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