33889 Datasheet, PDF (31/60 Pages) Freescale Semiconductor, Inc – System Basis Chip with Low Speed Fault Tolerant CAN Interface

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
Receiver Function
In normal operation (no bus failures), RX is the image of
the differential bus voltage. The differential receiver inputs
are connected to CANH and CANL.
The device incorporates single ended comparators
connected to CANH and CANL in order to monitor the bus
state as well as detect bus failures. Failures are reported via
the SPI.
In normal operation when no failure is present, the
differential comparator is active. Under a fault condition, one
of the two CANH or CANL pins can be become non-
operational. The single ended comparator of either CANH or
CANL is activated and continues to report a bus state to Rx
pin. The device permanently monitors the bus failure and
recovery, and as soon as fault disappears, it automatically
switches back to differential operation.
CAN interface operation Mode
The CAN has 3 operation modes: TxRx (Transmit-
Receive), Receive Only, and Term-VBAT (Terminated to
VBAT). The mode is selected by the SPI. As soon as the
MC33889 mode is sleep or stop (selected via MCR register),
the CAN interface automatically enters Tem-Vbat mode.
Tx Rx mode:
In this mode, the CAN drivers and receivers are enabled,
and the device is able to send and receive messages. Bus
failures are detected and managed, this means that in case
of a bus failure, one of the CAN drivers can be disabled, but
communication continues via the remaining drivers.
Receive Only mode:
In this mode, the transmitter path is disabled, so the device
does not drive the bus. It maintains CANL and CANH in the
recessive state. The receiver function operates normally.
TermVbat mode:
In this mode, the transmitter and receiver functions are
disabled. The CANL pin is connected to V_SUP through the
RTL resistor and internal pull up resistor of 12.5kOhms. In
this mode, the device monitors the bus activity and if a wake
up conditions is encountered on the CAN bus, it will wakes up
the MC33889.
The device will enter into a normal request mode if low
power mode was in sleep, or generates an INT. It enters into
Normal request mode if low power mode was in stop mode.
If the device was in normal or stand by mode, the Rx pin will
report a wake up (feature not available on the MC33889B).
See Rx pin behavior.
Bus Failure Detection
General description:
The device permanently monitors the bus lines and
detects faults in normal and receive only modes. When a fault
is detected, the device automatically takes appropriate
actions 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 termination
resistors, or internal switches. These actions are detailed in
the following table.
The device permanently monitors the faults and in case of
fault recovery, it automatically switches back to normal
operation and reconnects the open functions. Fault detection
and recovery circuitry have internal filters and delays timing,
detailed in the AC characteristics parameters.
The failure list identification and the consequence on the
device operation are described in following table. The failure
detection, and recovery principle, the transceiver state after a
failure detected, timing for failure detection and recovery can
be found in the ISO11898-3 standard.
The following table is a summary of the failure
identifications and of the consequences on the CAN driver
and receiver when the CAN is in Tx Rx mode.
Analog Integrated Circuit Device Data
Freescale Semiconductor
33889
31