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33897 Datasheet, PDF (13/21 Pages) Freescale Semiconductor, Inc – Single Wire CAN Transceiver
FUNCTIONAL DESCRIPTION
INTRODUCTION
FUNCTIONAL DESCRIPTION
INTRODUCTION
The 33897 Series is intended for use as a physical layer
device in a Single Wire CAN communications bus.
Communications takes place from a single terminal over a
single wire using a common ground for a current return path.
Two data rates are available, with the high rate used for
factory or assembly line communications and the lower for
actual system communications where the radiated EMI of the
higher rate could be an issue.
Two Pins control the mode of operation (sleep, low-speed,
high-speed, and high-voltage wake-up).
FUNCTIONAL PIN DESCRIPTION
The 33897 Series is intended to be used with an MCU to
control its operation and to process and generate the data for
the bus.
GROUND PINS (33897/A/C/T)
The four ground PINS are not only for electrical
conduction, their number and locations at each of the four
corners serve also to remove heat from the IC. The biggest
benefit of this is obtained by putting a lot of copper on the
PCB in this area and, if ground is an internal layer, by adding
numerous plated-through connections to it with the largest
diameter holes the layout can use.
TXD DATA
The data driven onto the SWCAN bus is inverted from the
TXD terminal. A “1” driven on TXD will result in an undriven
(recessive) state (bus at near zero volts). When the TXD
terminal is low, the output goes to a driven state. The voltage
and waveshaping in the driven state is determined by the
levels on the MODE0 and MODE1 Pins (refer to Table 6).
Table 6. Mode Control Logic Levels
Logic Level
MODE0
MODE1
Operation
0
0
Sleep Mode
0
1
High Voltage Wake-Up Mode
1
0
High Speed Mode
1
1
Normal Mode
MODE CONTROL
The MODE Pins control the transmitter filtering and BUS
voltage and the IC sleep mode operation. Table 6 shows the
mode versus the logic levels on MODE0 and MODE1.
The MODE0 and MODE1 Pins have a weak pulldown in
the IC so that in case the Pins are not driven, the device will
enter the sleep mode. This is usually the situation as the
MCU comes out of reset, before the driving signals have
been configured as outputs.
RXD Data
The data received on the bus is translated to logic levels
on this terminal. This terminal is a logic high when the bus is
in the recessive state (near zero volts) and is logic low when
the bus is in either the normal or high-voltage dominant state.
This is an open-drain type of output that requires an
external resistor to pull it up. When the device is in sleep
mode, the output will be off unless a high-voltage wake-up
level is detected on the bus. If the wake-up level is detected,
the output will be driven by the data on the bus. If the level of
the data returns to normal level, the output will return to off
after a short delay unless a non-sleep mode condition is set
by the MCU.
LOAD Switch
This switch is on in all operating modes unless a loss of
ground is detected. If this happens, the switch is opened and
the resistor normally attached to its terminal will no longer
pass current to or from the bus.
CNTL Output (33897/A/C/T ONLY)
This logic level signal is used to control a VCC regulator.
When the output is low, the VCC regulator is expected to
shutdown. This is normally used to shut down the MCU and
all the devices powered by VCC when the IC is in sleep mode.
This is done to save power. When the part is taken out of the
sleep mode by the higher-than-normal bus voltage, this
terminal is asserted high and the VCC regulator brings its
output up to the regulated level. This starts the MCU, which
controls the mode of the IC. The MCU must change the mode
signals to non-sleep mode levels in order to keep this
terminal from going low. There is a delay to allow the MCU to
fully wake up and take control after the high-voltage signaling
is removed before the level on this output returns low. After a
delay time, even if the bus is at high voltage, the IC will return
to sleep mode if both MODE Pins are low.
VBATT Input
This power input is not reverse battery protected and
should use an external diode to protect it from damage owing
to reverse battery if this protection is desired. The voltage
drop of the diode must be taken into consideration when the
operating range of the system is being determined. This
Analog Integrated Circuit Device Data
Freescale Semiconductor
33897/A/B/C/D/T
13