33989 datasheet(53/66 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33989

TYPICAL APPLICATIONS

SUPPLEMENTAL APPLICATION NOTES

Table 35. Current in Case of Bus Short Conditions

Notes

34.

For the failure case which leads to loss of communication and current flow for a very short time period as illustrated in Figure 45. So for

instance for CANH to gnd, the impact of the peak current on the V2 voltage regulator is very limited. The TX, RX, and CAN signal in the

figure are placed in a CANH to CANL short circuit condition.

Figure 45. CANH to CANL Short Circuit Condition

The sender node drives TX and the CAN bus, but doesn’t

receive anything on RX, so the CAN protocol handler inside

the MCU increases its TEC «transmit error counter» by 8.The

sender node keeps driving TX in dominant until it reaches the

error passive level (TEC=128).

When it is in error passive, it sends a passive error frame

(23 bits in recessive). Then the sender nodes drive the bus

and send only 1 dominant bit, and as nothing is received on

RX, the TEC is incremental by 8. After TX is driven 15 times,

the TEC reaches 255: then the node is in the BUS OFF state.

When the node is in the BUS OFF state, it needs 128

occurrences of 11 recessive bits (1.408 ms at 1MBauds) in

order to recover and be able to transmit again.

Condition

I term

current

I_H

current

I-L

current

Comment

peak current (mA)

No failure

Normal communication.

CANH line to gnd

150

No communication. Current flowing from V2 pin, during CAN driver dominant

state. (34)

CANH line to 5 V

-55

communication OK.

CANH line to +VBAT

150

-150

communication OK.

CANL line to gnd

communication OK.

CANL line to 5 V

-150

no communication (34)

CANL line to VBAT

-240

no communication (34)

CANH line shorted to

CANL line

-70

no communication (34)

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