MC33388 Datasheet, PDF (21/28 Pages) Motorola, Inc – Fault Tolerant CAN Interface

MC33388
DEVICE DESCRIPTION
Introduction
The MC33388 is a low speed CAN fault tolerant physical
interface designed for automotive multiplexed electronic
systems. The MC33388 addresses the low speed body
electronics application, in which the speed of communications
is between 10 and 125kBauds, on two wires bus
configurations. It is designed to operate in the harsh
automotive environment.
The MC33388 can control the external voltage regulator of
the system through the dedicated INH pin. It allows the
application to be switched into low power mode. Wake up can
be done either from bus activity or local wake up switch. The
MC33388 is tolerant to faults occurring at the CAN bus in
normal operating mode and low power mode.
Packaging
The device is assembled in a SO14 narrow body package.
Thermal performances allow the device to operate in the
automotive ambient temperature range, from –40°C to 125°C.
Transmitter Function
CAN bus levels are called Dominant and Recessive, and
correspond respectively to Low and High state of TX input pin.
The Recessive state is a weak state, where bus line are driven
through pull up and pull down resistors. Recessive state can
be over written by any other node forcing a Dominant state, in
which bus line are driven through active switches.
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 circuitry.
The bus line slew rates are controlled in order to minimize
the RFI and this allows use of unshielded cables for the bus.
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 through integrated filters. The
filtered input signals are also used for the single wire
receivers.
The device incorporates comparators connected to CANH
and CANL in order to monitor and report the bus state to the
microcontroller as well as detect bus failures. Failures are
reported back to the microcontroller through NERR pin.
In normal operation when no failure is present the
differential comparator is active. Under fault condition, one of
the two CANH or CANL pin can be non operational then the
single ended comparator of either CANH or CANL is activated
and continue to report bus state to the microcontroller. The
MC33388 permanently monitors bus failure and recovery,
and as soon as fault disappears, it automatically switches
back to differential operation.
Noise Filtering
The device is optimized for dual wires operations. During
all single wire transmissions, the EMC performances in both
immunity and emission are worse than in differential mode.
Integrated receiver filters suppress any HF noise induced into
the bus wires. The cut-off frequency of these filters is a
compromise between propagation delay and HF suppression.
In single wire mode, low frequency noise can not be
distinguished from the active signal at the bus line.
Device Operation Mode
The device has four operation modes : Normal, Receive
Only, Standby VBAT and Sleep. Each of these modes is
controlled by the state of EN and STB pins.
The state machine figure 3 and the truth table 1 indicate
how to configure the device into each mode and the pins
functions in each operation mode.
Operation Mode
Normal mode: In this mode, all functions are available and
NERR pin reports bus failure
Receive Only mode: In this mode, the transmitter path is
disabled, so the device do not drive the bus. It maintains
CANL and CANH in recessive state. The receiver function
operates normally. As the device can not drive the bus, an
incoming CAN message could not be acknowledge by the
node. NERR output signals the VBAT power-on flag and RX
reports bus state. Failure detection and management are the
same as in normal mode.
Sleep mode: In this mode, the transmitter and receiver
functions are disabled. CANL pin is connected to VBAT
through Rtl resistor and internal pull up resistor of 12.5kOhms.
INH pin is switched in high impedance state. The external
voltage regulator connected to INH will be switched off and no
VDD is supplied to MC33388. In this mode the device is still
supplied by the VBAT. Supply current from VBAT is 15uA
typical. The MC33388 monitors the bus activity and the state
of WAKE pin and VBAT level. If wake up conditions are
encountered, the device wakes up to Standby VBAT mode
and INH is switched on.
Standby VBAT mode: This mode is similar to Sleep mode,
but the INH pin is in high state in order to maintain the external
5V regulator activated. Wake up events are directly reported
to NERR and RX thanks to the 5V available at VDD. CANL is
in the same configuration as in Sleep mode.
Standby and Sleep modes are active when STB and EN
are low. Selection of Standby or Sleep is done through the
sequence of activation of EN and STB pins. Sleep mode is
entered through an intermediate steps (go to sleep) where
STB, EN are 0, 1. (Refer to truth table 1).
System Power On
When the supply is first applied to the system, VBAT and
VDD rise from zero up to their nominal value and the device
automatically enters into VBAT standby mode. At this time,
INH is switched in high state in order to activate the external
voltage regulator and an internal flag is set (batt fail flag). EN
and STB pins are internally forced in low state to maintain the
device into VBAT standby mode.
The VDD “forced Vbat standby mode (fail safe)” circuit will
maintain the device in VBAT standby mode until VDD is
higher that 3V, whatever the external state of EN and STB.
As soon as VDD reaches the “forced VBAT standby mode
(fail safe)” threshold, the device can enter into other mode,
depending upon EN and STB state.
VDD Reset Function
If during operation VDD drops below “forced VBAT standby
mode (fail safe)” threshold, the device is automatically switched
into VBAT standby mode to provide fail safe functionality.
MC33388
MOTOROLA
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