
Analog Integrated Circuit Device Data
32
Freescale Semiconductor
33889
FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
General description
CAN driver:
The CANH driver is a “high side” switch to the V2 voltage (5.0 V). The CANL driver is a “l(fā)ow side” switch to gnd.The turn on
and turn off time is controlled in order to control the slew rate, and the CANH and CANL driver have a current limitation as well
as an over temperature shutdown.
The CAN H or CANL driver can be disabled in case a failure is detected on the CAN bus (ex: CANH driver is disabled in case
CANH is shorted to VDD). The disabling of one of the drivers is controlled by the CAN logic and the communication continues via
the other drivers. When the failure is removed the logic detects a failure recovery and automatically reenables the associated
driver.
The CAN drivers are also disabled in case of a Tx failure detection.
Bus termination:
The bus is terminated by pull-up and pull-down resistors, which are connected to GND, VDD, or VBAT through dedicated RTL
and RTH pins and internal switches Srh, Srl, Stvbat. Each node must have a resistor connected between CANH and RTH and
between CANL and RTL. The resistor value should be between 500 and 16000 ohm.
Transmitter Function
CAN bus levels are called Dominant and Recessive, and correspond respectively to Low and High states of the TX input pin.
Dominant state:
The CANH and CANL drivers are on. The voltage at CANL is <1.4 V, the voltage at CANH is >3.6 V, and the differential voltage
between CANH and CANL line is >2.2 V (3.6 -1.4 V).
Recessive state:
This is a weak state, where the CANH and CANL drivers are off. The CANL line is pulled up to 5 V via the RTL pin and RTL
resistor, and the CANH line is pull down via the RTH and RTH resistor. The resultant voltage at CANL is 5.0 V and 0V at CANH.
The differential voltage is -5.0 V (0V - 5.0 V). The recessive state can be over written by any other node forcing a Dominant state.
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.