33742 datasheet(38/70 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33742

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

Figure 20. CAN Interface Levels

TXD AND RXD PINS

The TXD pin has an internal pullup to V2. The state of TXD

depends on the V2 status. RXD is a push-pull structure,

supplied by V2. When V2 is set at 5.0V and CAN is TXRX

mode, RXD reports bus status. For details, refer to Table 7,

page 29, Table 9, below, and Table 10, page 39.

The TXD pin is a push-pull structure between the V2 pin

and GND. The circuitry has a parasitic diode between RXD

and V2. It is illustrated in Figure 25. This parasitic diode is

reversed biased in normal operation (TXD voltage is lower or

equal to V2). In case the TXD voltage is greater than V2, a

current will flow into the diode.

If the V2 pin is low (e.g. in sleep mode, or in stop with a

ballast transistor), the current leakage at V2 is low enough

(10

μA max) to ensure than the RXD pin can be pulled up by

an external resistor (i.e. the MCU RXD pin internal pullup).

The states of the RXD pin in the following Table 9 and 10

is dependant upon external circuitry connected to the V2 and

RXD pins.

CAN TXRX MODE AND SLEW RATE SELECTION

The slew rate selection is done via CAN register (refer to

Tables 21 through 23 on page 50). Four slew rates are

available and control the recessive-to-dominant and

dominant-to-recessive transitions. The delay time from TXD

pin to CAN bus, from CAN bus to RXD, and from the TXD to

RXD loop time is affected by the slew rate selection.

CANH

TXD

RXD

Typ 2.5V

CANL

CAN Recessive State

CAN Dominant State

CAN Recessive State

Typ 2.5V

VCANH-VCANL > 900mV

VCANH-VCANL < 500mV

33742 数据表(PDF) 38 Page - Freescale Semiconductor, Inc

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