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
at 3 V (V
CC
= 5 V). This ensures correct reception with a
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.
