MC68HC05X4 Datasheet, PDF (98/156 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Motorola CAN
Freescale Semiconductor, Inc.
Interface to the MCAN bus
Physically, the MCAN bus may be composed of two wires. The bus can
take on one of two values: dominant or recessive. During simultaneous
transmission of dominant and recessive bits by two or more CAN
modules the resulting bus value will be dominant. (For example, with a
wired-AND implementation of the bus, the dominant level would
correspond to a logic 0, and the recessive level to a logic 1.)
The two wires of the MCAN bus are designated CANH and CANL. The
voltage levels appearing on these lines are designated V_CANH and V_CANL.
A simple termination network is required for each wire. Figure 21 shows
the physical interface circuitry within the MCAN module, and its
connection to the MCAN bus with a typical low speed (<125 kbaud)
hardware interface. (Note that the suggested values shown in the
diagram are subject to change in the future.)
For the voltage and resistor values shown in Figure 21 the voltages on
the MCAN bus are:
– Recessive level: V_CANH = 3.25 V
– Dominant level: V_CANH = 1.00 V
V_CANL = 1.75 V
V_CANL = 4.00 V
If several CAN modules are driving a dominant level on the bus at the
same time then the values for V_CANH and V_CANL can go to 0.3 and 4.7 volts
respectively. The residual 0.3 V is due to the voltage drop across the
diodes and driver transistors in the transmission circuit.
The receiver part of the network uses two identical voltage divider
networks, with a divide ratio of 6:1 (resistor values of 150kΩ and 30kΩ)
referenced to V_DD/2. This increases the common mode range of the input
comparator on the physical bus lines. If the common mode range of the
comparator at its inputs is 1.5 to 3.5 volts then, for V_DD = 5.0 V, the
common mode range will be increased to –3.5 to +8.5 volts on the bus
lines.
MC68HC05X4
Motorola CAN
For More Information On This Product,
Go to: www.freescale.com
30-mcan