Civil Engineering Reference
In-Depth Information
• Asingle-ended comparator for CAN_H (to detect CAN_H short to Vbatt), with
threshold voltage at 7.2 V
• Asingle-ended comparator for CAN_L (to detect CAN_L short to Vbatt) with
threshold voltage at 7.2 V
All receivers and comparators are also used to analyse failure cases on the bus. The
failure management logic decides when there is a failure on the bus and changes
from differential mode to single-ended mode. In the single-ended mode, the shorted
transmitter is switched off (for example, in the case of CAN_H short to ground the
CAN_H transmitter) and the single-ended CAN_L receiver is used. In the case of
a CAN_L short to VCC or Vbatt, the CAN_L transmitter is switched off and the
CAN_H single-ended receiver is used. The disadvantage of this single-ended mode
is the lower noise robustness and the lower possible ground shift between the sender
node and the receiver nodes.
In case of CAN_H short to Vbatt or VCC or a CAN_L short to ground, a high
current flows through the termination resistors in sleep mode. This is the reason
why termination resistor switches are implemented. In case of a short on the bus,
the termination resistors are switched off. Pin RTL is the switch for the CAN_L
termination resistor and RTH is the switch for the CAN_H termination resistor. In
sleep mode, normally the 5 V supply VCC is switched off, floating or 0 V. To have
a positive termination voltage, CAN_L will be high ohmic terminated to Vbatt.
When CANS communication is started, the transmitter of the transmitting node
pulls the CAN_L to ground and all other detects this as a remote wake-up event and
the transceivers change into standby mode. In standby mode, INH is activated and
switches on the voltage regulator and ramps up the microcontroller. After the suc-
cessful ramp-up of the microcontroller, the transceiver should be set to normal mode
for normal communication on the bus. In transceiver standby mode, the termination
switches RTL and RTH on and terminates the CAN_H and CAN_L wires.
2.1.4.1
High-Speed Physical Layer
An ideal high-speed CAN physical layer has a termination resistor of 120 Ω on
both ends of the wire. This reduces the echo on the wire to a minimum. All other
nodes are connected in between. This concept allows a data rate of up to 1 Mbaud.
The ringing, especially after the dominant to recessive edge, is minimized and the
high data rate is possible. After switching on the CAN_H and CAN_L output stages,
a current flows from CAN_H to CAN_L over the termination resistors. The result
is a voltage drop over both termination resistors between 1.5 and 3 V. This is called
dominant level. If both output stages are switched off, the voltage drop over the
termination resistors is zero. This is called recessive level. The receiver thresholds
are between 500 and 900 mV. A voltage drop higher than 900 mV will be detected as
dominant level on the bus and a voltage drop smaller than 500 mV will be detected
as recessive level. The common-mode range for the receiver is from −12 to +12V.
If the bus common-mode voltage is higher or lower, the receiver can detect wrong
signals. This concept is not proven against CAN_H shorts to ground and a CAN_L
short to battery voltage. In this case, the communication can be corrupted.
