Datasheet

APPLICATION INFORMATION
DIAGNOSTIC LOOPBACK (SN65HVD233)
AUTOBAUD LOOPBACK (SN65HVD235)
Ω
120
Ω
120
CANH
CANL
TMS320LF243
SN65HVD251
D R
Rs
Vref
CANTX CANRX
Sensor, Actuator, or Control
Equipment
TMS320F2812
SN65HVD233
D R
0.1µF
Vcc
GND
Rs
LBK
CANTX CANRX
Sensor, Actuator, or Control
Equipment
TMS320LF2407A
SN65HVD230
D R
Rs
Vref
CANTX CANRX
Sensor, Actuator, or Control
Equipment
3.3 V
0.1µF
Vcc
GND
5 V
0.1
µ
F
Vcc
GND
3.3 V
Stub Lines -- 0.3 m max
Bus Lines -- 40 m max
GPIO
ISO 11898 COMPLIANCE OF SN65HVD230 FAMILY OF 3.3-V CAN TRANSCEIVERS
Introduction
SN65HVD233
SN65HVD234
SN65HVD235
SLLS557F – NOVEMBER 2002 – REVISED AUGUST 2008 .............................................................................................................................................. www.ti.com
The loopback (LBK) function of the HVD233 is enabled with a high-level input to pin 5. This forces the driver into
a recessive state and redirects the data (D) input at pin 1 to the received-data output (R) at pin 4. This allows the
host controller to input and read back a bit sequence to perform diagnostic routines without disturbing the CAN
bus. A typical CAN bus application is displayed in Figure 28 .
If the LBK pin is not used it may be tied to ground (GND). However, it is pulled low internally (defaults to a
low-level input) and may be left open if not in use.
The autobaud feature of the HVD235 is implemented by placing a logic high on pin 5 (AB). In autobaud, the
bus-transmit function of the transceiver is disabled, while the bus-receive function and all of the normal operating
functions of the device remain intact. With the autobaud function engaged, normal bus activity can be monitored
by the device. However, if an error frame is generated by the local CAN controller, it is not transmitted to the bus.
Only the host microprocessor can detect the error frame.
Autobaud detection is best suited to applications that have a known selection of baud rates. For example, a
popular industrial application has optional settings of 125 kbps, 250 kbps, or 500 kbps. Once the logic high has
been applied to pin 5 (AB) of the HVD235, assume a baud rate such as 125 kbps, then wait for a message to be
transmitted by another node on the bus. If the wrong baud rate has been selected, an error message is
generated by the host CAN controller. However, since the bus-transmit function of the device has been disabled,
no other nodes receive the error message of the controller.
This procedure makes use of the CAN controller's status register indications of message received and error
warning status to signal if the current baud rate is correct or not. The warning status indicates that the CAN chip
error counters have been incremented. A message received status indicates that a good message has been
received.
If an error is generated, reset the CAN controller with another baud rate, and wait to receive another message.
When an error-free message has been received, the correct baud rate has been detected. A logic low may now
be applied to pin 5 (AB) of the HVD235, returning the bus-transmit normal operating function to the transceiver.
Figure 28. Typical HVD233 Application
Many users value the low power consumption of operating their CAN transceivers from a 3.3 V supply. However,
some are concerned about the interoperability with 5-V supplied transceivers on the same bus. This report
analyzes this situation to address those concerns.
20 Submit Documentation Feedback Copyright © 2002 – 2008, Texas Instruments Incorporated
Product Folder Link(s): SN65HVD233 SN65HVD234 SN65HVD235