MC33388 Datasheet, PDF (22/28 Pages) Motorola, Inc – Fault Tolerant CAN Interface

MC33388
DEVICE DESCRIPTION
Battery Fail Flag
When the VBAT supply drops below “battery voltage for
setting power on flag” threshold, this information is internally
latched. This means that the system power supply has been
lost (disconnected and reconnected for instance).
This flag can be read by the microcontroller by switching
the device into receive only mode, where NERR pin reports
the VBAT power on flag. This flag is reset by entering into the
Normal mode.
Bus Failure Detection
The device permanently monitors the bus lines and
detects faults in normal and receive only modes. Below is the
list of failures detected at the bus level :
1- CANH wire interruption
2- CANL wire interruption
3- CANH short-circuit to battery
4- CANL short-circuit to ground
5- CANH short-circuit to ground
6- CANL short-circuit to battery
7- CANL and CAN H mutually shorted
8- CANH short-circuit to Vdd
TX Permanent Dominant Detection
In addition to the above list, the MC33388 detects a
permanent low state at TX input which results of a permanent
dominant bus state. The MC33388 detects if TX is Low for
more than 2ms typical and then disables the bus output driver
in order to switch into recessive state. This avoid to block
communication between other nodes of the network.
Behaviour Under Faults Condition
When a fault is detected, the device automatically takes
appropriate action to minimize the system current
consumption and to allow communication on the network.
Depending on the type of fault, the mode of operation and the
fault detected, the device automatically switches off one or
more of the following functions: CANL or CANH line driver, Rtl
or Rth pull up or down resistors or internal switches. These
actions are detailed in table 2.
The device permanently monitors the faults and in case of
fault recovery, it automatically switches back to normal
operation and reconnects the open functions. Faults detection
and recovery circuitry have internal filters and delays timing
detailed in the AC characteristics parameters section.
Failures 3 and 6 are detected by comparators respectively
connected to CANH and CANL. If the comparator threshold is
exceeded for a certain time, the device is switched to single
wire mode. This time is needed to avoid false triggering by
external RF fields. Recovery from these failures is detected
automatically after a certain time-out (filtering) and no
transmission is lost.
Failures 4, 7 and 8 initially result in a permanent dominant
level at the internal comparator outputs. If failure 4 and 7
appear, the CANL driver and the RTL pin are switched off
after a time out, only a weak pull up at RTL remains.
Reception continues by switching to single wire mode through
CANH. When the failures 4 or 7 are removed, the recessive
bus levels are restored. If the receiver voltages remain in the
recessive state for a certain time, reception and transmission
switch back to the differential mode.
If failure 8 is recognized, the CANH driver is switched off
after a time out and the reception is switched to single wire
mode through CANL. If the receiver voltages remain in the
recessive state for a certain time, reception and transmission
switch back automatically to the differential mode.
If any of the 8 wiring failure occurs, the output NERR will
be switched low. When the error recovers, NERR will be
switched back to high state.
Wake Up Events
Wake-up requests are recognized by the MC33388 in
Sleep or VBAT Stanby modes, either when a dominant state
is detected on CANL or CANH bus lines (remote wake-up) or
if the WAKE pin changes state (local wake-up).
Under power-up conditions when VBAT is higher than 5V,
the state voltage on the WAKE pin is considered to be the
reference state for the wake-up function. On leaving normal
mode, the current WAKE pin state becomes the new
reference state.
In sleep mode, on a wake-up request the transceiver sets
the INH output high, to activate the external voltage regulator,
used for VDD supplied. In VBAT standby INH is already set
high. When VDD is set, the wake-up request can be read on
the NERR or RX outputs by the microcontroller.
To prevent false wake-up due to transients or RF fields,
wake-up threshold levels have to be maintained for a certain
time. In the low power modes, failure detection circuit remains
partly active to prevent increased power consumption in
cases of error 3, 4, 7 and 8.
Detailed Description of Error Detections
The differential receiver threshold is set at -2,8V. This
assures a proper reception in the normal operating modes
and in case of failures 1, 2 and 5 noise margin as high as
possible. These failures or their recovery do not destroy
ongoing transmissions.
Fault Operation Table
Table 2 shows the device operation in normal and low
power modes and the internal actions happening under fault
condition. Please refer to simplified block diagram page 1 for
device internal switch reference.
MC33388
MOTOROLA
8