AMIS−42671
http://onsemi.com
6
FUNCTIONAL DESCRIPTION
Operating Modes
The behavior of AMIS−42671 under various conditions is
illustrated in Table 5 below. In case the device is powered,
one of two operating modes can be selected through
Pin AUTB.
Table 5. FUNCTIONAL TABLE OF AMIS−42671 WHEN NOT CONNECTED TO THE BUS; x = don’t care
VCC Pin TxD Pin AUTB Pin CANH Pin CANL Bus State Pin RxD
4.75 to 5.25 V 0 0
(or floating)
High Low Dominant 0
4.75 to 5.25 V 1
(or floating)
1 V
CC
/2 V
CC
/2 Recessive 1
4.75 to 5.25 V 1
(or floating)
x V
CC
/2 V
CC
/2 Recessive 1
V
CC
< PORL
(unpowered)
x x 0 V < CANH <
V
CC
0V < CANL <
V
CC
Recessive 1
PORL < V
CC
< 4.75 V >2 V x 0 V < CANH <
V
CC
0V < CANL <
V
CC
Recessive 1
High−Speed Mode
If pin AUTB is pulled low (or left floating), the transceiver
is in its high−speed mode and is able to communicate via the
bus lines. The signals are transmitted and received to the
CAN controller via the pins TxD and RxD. The slopes on the
bus line outputs are optimized to give extremely low
electromagnetic emissions.
Autobaud Mode
If Pin AUTB is pulled high, AMIS−42671 is in Autobaud
mode. The transmitter is disabled while the receiver remains
active. All other IC functions also continue to operate.
Normal bus activity can be monitored at the RxD pin and
transmit data on TxD is looped back to RxD without
influencing the CAN communication.
AUTB
TxD
RxD
CANH
CANL
PC20071002.4
Figure 3. Simplified Schematic Diagram of Autobaud Function
In Autobaud mode the local CAN controller is able to
detect the used communication speed of other transmitting
network nodes. Bus communication is received and via the
RxD pin sent to the CAN controller. If the CAN controller
operates at the wrong baud rate, it will transmit an error
frame. This message will be looped back to the CAN
controller which will increment its error counter. The CAN
controller will be reset with another baud rate. When an
error−free message is received, the correct baud rate is
detected. A logic low may now be applied to Pin AUTB,
returning to the high−speed mode.
Overtemperature Detection
A thermal protection circuit protects the IC from damage
by switching off the transmitter if the junction temperature
exceeds a value of approximately 160°C. Because the
transmitter dissipates most of the power, the power
dissipation and temperature of the IC is reduced. All other
IC functions continue to operate. The transmitter off−state
resets when pin TxD goes high. The thermal protection
circuit is particularly necessary when a bus line
short−circuits.
