33903 datasheet(55/104 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33903/4/5

CAN INTERFACE

CAN INTERFACE DESCRIPTION

When the CAN interface is in Normal Mode, the driver has

two states: recessive or dominant. The driver state is

controlled by the TXD pin. The bus state is reported through

the RXD pin.

When TXD is high, the driver is set in the recessive state,

and CANH and CANL lines are biased to the voltage set with

5 V-CAN divided by 2, or approx. 2.5 V.

When TXD is low, the bus is set into the dominant state,

and CANL and CANH drivers are active. CANL is pulled low

and CANH is pulled high.

The RXD pin reports the bus state: CANH minus the CANL

voltage is compared versus an internal threshold (a few

hundred mV).

If “CANH minus CANL” is below the threshold, the bus is

recessive and RXD is set high.

If “CANH minus CANL” is above the threshold, the bus is

dominant and RXD is set low.

The SPLIT pin is active and provides a 2.5 V biasing to the

SPLIT output.

TXD/RXD Mode and Slew Rate Selection

The CAN signal slew rate selection is done via the SPI. By

default and if no SPI is used, the device is in the fastest slew

rate. Three slew rates are available. The slew rate controls

the recessive to dominant, and dominant to recessive

transitions. This also affects the delay time from the TXD pin

to the bus and from the bus to the RXD. The loop time is thus

affected by the slew rate selection.

Minimum Baud Rate

The minimum baud rate is determined by the shortest TXD

permanent dominant timing detection. The maximum number

of consecutive dominant bits in a frame is 12 (6 bits of active

error flag and its echo error flag).

The shortest TXD dominant detection time of 300

μs lead

to a single bit time of: 300

μs / 12 = 25 μs.

So the minimum Baud rate is 1 / 25

μs = 40 kBaud.

Sleep Mode

Sleep Mode is a reduced current consumption mode.

CANH and CANL drivers are disabled and CANH and CANL

lines are terminated to GND via the RIN resistor, the SPLIT

pin is high-impedance. In order to monitor bus activities, the

CAN Wake-Up receiver can be enabled. It is supplied

internally from VSUP/2.

Wake-up events occurring on the CAN bus pin are

reporting by dedicated flags in SPI and by INT pulse, and

results in a device Wake-Up if the device was in LP Mode.

When the device is set back into Normal Mode, CANH and

CANL are set back into the recessive level. This is illustrated

in Figure 36.

CANL

CANH

TXD

RXD

2.5 V

CANL-DOM

CANH-DOM

CANL/CANH-REC

2.5 V

High ohmic termination (50 kohm) to GND

SPLIT

Dominant state

Recessive state

Bus Driver

Receiver

(bus dominant set by other IC)

High-impedance

Normal or Listen Only Mode

Go to sleep,

CANH-CANL

Sleep or Stand-by Mode

Figure 36. Bus Signal in TXD/RXD and LP Mode

Wake-up

When the CAN interface is in Sleep Mode with Wake-Up

enabled, the CAN bus traffic is detected. The CAN bus Wake-

Up is a pattern Wake-Up. The Wake-Up by the CAN is

enabled or disabled via the SPI.

33903 Datasheet(PDF) 55 Page - Freescale Semiconductor, Inc

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