33989 datasheet(47/66 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33989

TYPICAL APPLICATIONS

SUPPLEMENTAL APPLICATION NOTES

TX and RX Pins

The TX pin has an internal pull up to V2. The state of TX

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

supplied by V2. When V2 is set at 5V, and CAN is in normal

mode, RX reports the bus status. When V2 is off RX is low.

Normal Mode and Slew Rate Selection

The slew rate selection is done via the SPI. Four slew rates

are available. The slew rate affects the recessive to dominant

and dominant to recessive transitions. This affect is also the

delay time from the TX pin to the bus, and from the bus to RX.

The loop time is thus affected by the slew rate selection.

The following figure is an illustration of the slew rate on

CANH, CANL, TX and RX.

Figure 36. Normal Mode and Slew Rate Selection

Minimum Baud Rate

As TX permanent dominant is detected after TDOUT (min

200 µs), a minimum Baud rate is required in order to get good

behavior: once TX permanent dominant is detected the CAN

driver is off.

The maximum number of consecutive dominant bits in a

frame is 12 (6 bits of active error flag and its echo error flag).

200 µs/12 = 16.7 µs. The minimum Baud rate is 1/6.7 µs =

60 KBaud.

Termination

The MC33989 supports the two main types of bus

termination:

• Differential termination resistors between CANH and

CANL lines.

• Split termination concept, with mid point of the

differential termination connected to gnd through a

capacitor.

Figure 37. Bus Termination

CAN signal with slew rate 0 selected

CAN signal with slew rate 3 selected

R=60 ohms, CL = CH = 100pF

CANH

CANL

Gnd

CAN bus

MC33989

(partial drawing)

CANH

CANL

RT /2

Gnd

CAN bus

MC33989

(partial drawing)

Split termination concept

Differential termination concept

RT /2

33989 Datasheet(PDF) 47 Page - Freescale Semiconductor, Inc

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