AMIS−41682, AMIS−41683

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5

FUNCTIONAL DESCRIPTION

Description

AMIS−41682 is a fault tolerant CAN transceiver which

works as an interface between the CAN protocol controller

and the physical wires of the CAN bus (see Figure 2). It is

primarily intended for low speed applications, up to 125 kB,

in passenger cars. The device provides differential transmit

capability to the CAN bus and differential receive capability

to the CAN controller.

The AMIS−41683 has open−drain outputs (RXD and

ERR Pins), which allow the user to use external pullup

resistors to the required supply voltage; this can be 5 V or

3.3 V.

To reduce EME, the rise and fall slope are limited.

Together with matched CANL and CANH output stages,

this allows the use of an unshielded twisted pair or a parallel

pair of wires for the bus lines.

The failure detection logic automatically selects a suitable

transmission mode, differential or single−wire transmission.

Together with the transmission mode, the failure detector

will configure the output stages in such a way that excessive

currents are avoided and the circuit returns to normal

operation when the error is removed.

A high common−mode range for the differential receiver

guarantees reception under worst case conditions and

together with the integrated filter the circuit realizes an

excellent immunity against EMS. The receivers connected

to pins CANH and CANL have threshold voltages that

ensure a maximum noise margin in single−wire mode.

A timer has been integrated at Pin TXD. This timer

prevents the AMIS−41682 from driving the bus lines to a

permanent dominant state.

Failure Detector

The failure detector is fully active in the normal operating

mode. After the detection of a single bus failure the detector

switches to the appropriate mode. The different wiring

failures are depicted in Figure 4. The figure also indicates

the effect of the different wiring failures on the transmitter

and the receiver. The detection circuit itself is not depicted.

The differential receiver threshold voltage is typically set

noise margin as high as possible in the normal operating

mode and in the event of failures 1, 2, 5, and 6a. These

failures, or recovery from them, do not destroy ongoing

transmissions. During the failure, reception is still done by

the differential receiver and the transmitter stays fully

active.

To avoid false triggering by external RF influences the

single−wire modes are activated after a certain delay time.

When the bus failure disappears for another time delay, the

transceiver switches back to the differential mode. When

one of the bus failures 3, 3a, 4, 6, and 7 is detected, the

defective bus wire is disabled by switching off the affected

bus termination and the respective output stage. A wake−up

from sleep mode via the bus is possible either by way of a

dominant CANH or CANL line. This ensures that a

wake−up is possible even if one of the failures 1 to 7 occurs.

If any of the wiring failure occurs, the output signal on pin

ERR will become low. On error recovery, the output signal

on pin ERR will become high again.

During

all

single−wire

transmissions,

the

EMC

performance (both immunity and emission) is worse than in

the differential mode. The integrated receiver filters

suppress any HF noise induced into the bus wires. The

cut−off frequency of these filters is a compromise between

propagation delay and HF suppression. In the single−wire

mode, LF noise cannot be distinguished from the required

signal.