Analog Integrated Circuit Device Data

Freescale Semiconductor

17

33902

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

Figure 11 shows the meaning of the ERR pin versus the device state, the state transition and the events on TXD.

Figure 11. ERR versus device state

CAN INTERFACE DESCRIPTION:

CAN Interface Supply

The supply voltage for the CAN driver is the VDD pin. The

CAN interface also has a supply path from the battery line,

through the VSUP pin. This path is used in CAN Sleep mode

to allow wake-up detection.

During CAN communication (transmission and reception),

the CAN interface current is sourced from the VDD pin.

During CAN low power mode, the current is sourced from the

VSUP pin.

CAN Driver Operation in Normal Mode

The CAN driver will be enabled as soon as the device is in

Normal mode and the TXD pin is recessive.

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 at

VDD divided by 2, approx. 2.5 V.

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

and the 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 provide a 2.5 V biasing to the

SPLIT output.

Normal Mode and Slew Rate Selection

The CAN signal slew rate selection is done via the P_SPI.

By default, and if no P_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, which are also dependent on CANH and CANL

capacitance. This also affects the delay time from the TXD

pin to the bus, and from the bus to 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 leads

to a single bit time of: 300

μs / 12 = 25 μs.

So the minimum Baud rate is 1 / 25

μs = 40 kBaud.

WAKE-UP EVENT

CAN Wake-up or Local Wake-up =>

LOCAL FAILURE

VDD low, TXD-PD, RXD-PR, TXD short to RXD => ERR low

BATFAIL

BUS FAILURE,

VDDlow, Bus dom,

4 dominant

WAKE-UP SOURCE

Local Wake-up => ERR low; CAN Wake-up => ERR high

GO TO SLEEP

STANDBY

SLEEP

LISTEN ONLY

NORMAL

STBY = 0

EN = X

STBY = 1

EN = 0

STBY = 1

EN = 1

STBY = 0

EN = X

STBY = 0

EN = X

STBY = 0

EN = X

pulses at TXD

STBY = 1

EN = 0

STBY = 1

EN = 1

STBY = 1

EN = 1

STBY = 1

EN = 0

DEVICE MODE

ERR MEANING

STBY = 1

EN = 1

STBY = 1

EN = 0

STBY = 0

EN = X

CANH or CANL short to GND