19
CAN1 - 4 PORTS
These ports provide the CAN Bus input and output. Each port has a CAN Transceiver (Philips
PCA82C250) capable of up to 1M Baud data rate. Not all HC12 devices support all of the
channels, refer to the device data for capability. Each transceiver receive output has a COM
Switch position associated with it. For proper operation the COM Switch should be turned on
for each CAN channel that is used for CAN communication. The switch provides isolation for
the HC12 I/O port if the CAN operation is not supported by the device or desired by the user.
All HC12 I/O ports are available at the Port Headers for other I/O applications.
CAN 1 - 4 Port Connections
1
GND
2
CAN-H
3
CAN-L
4
+5V
The CAN1-4 connector provides an interface to the MSCAN12
channels 0 - 3 on the microcontroller.
CAN BUS TRANSMIT ENABLE
Each CAN port transceiver transmit driver is enabled for maximum drive and minimum slew
rate by default. The drive and slew rate may be adjusted by changing the value of RC10,
RC20, RC30, or RC40 for each CAN 1 - 4 port respectfully (see PCA82C250 data sheet for
more information). These 1206 size SMT resistors are located on the bottom of the CMD912x
board near the respective CAN port.
CAN Bus transceiver transmit enable control can be applied to each CAN 1 - 4 Port by the
RS1 - 4 tie pads respectfully. The user should select an available HC12 I/O port to perform
the transmit enable function and connect it from the MCU_PORTx pin to RSx pad as required.
The RC10, RC20, RC30, or RC40 resistors must be removed from the respective CAN
port to apply transmit enable control.
The transmit enable signal to the CAN transceivers is
active logic low.
CAN BUS TERMINATION
Each CAN port has a set of 1206 SMT size termination resistors on the bottom of the
CMD912x board that are not installed at the factory. The termination resistors provide
optional bias and termination impedance for the CAN bus connected to the CAN 1 - 4 ports.
Type of wire media, data rate, length of wire, and number of CAN bus nodes can all effect the
requirement or value of the termination for the CAN bus. User should refer to particular
application for termination requirements.
CAN-H Bias Resistors:
RC11, RC21, RC31, and RC41 provide bias to ground potential for
CAN 1- 4 ports respectfully.
CAN-L Bias Resistors:
RC13, RC23, RC33, and RC43 provide bias to +5V potential for CAN
1- 4 ports respectfully.
CAN Termination Resistors:
RC12, RC22, RC32, and RC42 provide termination between
CAN-H and CAN-L signals for CAN 1- 4 ports respectfully.
