MC68HC05X16 Datasheet, PDF (86/232 Pages) Freescale Semiconductor, Inc – High-density complementary metal oxide semiconductor HCMOS) microcontroller unit

Freescale Semiconductor, Inc.
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.
5.4.1 Single wire operation
5
In the event of a bus fault occurring, limited operation of the MCAN bus may still be possible,
depending on the nature of the fault. If the fault is due to a short circuit between the two bus lines
or between one of the lines and ground, battery voltage or some other potential, it is possible to
identify (using a special software procedure) the line on which the fault exists and to switch the
corresponding comparator input from the faulty line to the V_DD/2 reference supply. At the same
time the driver transistors to the faulty line should also be switched off. This will allow
communication to continue on the bus. One result of this mode of one wire transmission is a
significant reduction in the common mode range of the input comparator.
Switching to one wire operation is achieved using the control bits RX0-passive and RX1-passive
in the MCAN command register, located at address $21. Setting either of these bits will result in
the corresponding input being disconnected from the bus and connected to V_DD/2.
5.5
Sleep mode
If the SLEEP bit in the MCAN command register is set by the processor the MCAN will go to sleep,
unless it is active. If there is activity on the MCAN bus lines, or there is an interrupt pending, the
MCAN is deemed to be active and will not go to sleep; a wake-up interrupt will be generated by
the MCAN in these circumstances. The SLEEP bit may also be cleared by the processor, in which
case no wake-up interrupt will be generated. Note that this bit is write-only by the CPU, and it is
not possible therefore to check whether sleep mode has been entered by reading it. However, the
CAF bit in the EEPROM control register is set when the MCAN is asleep, and cleared when it is
woken up (see Section 3.5.1).
In order to minimize power consumption, the active comparator is switched off and the sleep
comparator circuitry is used to detect activity on the bus. When in sleep mode the MCAN stops its
own clocks, leaving the MCU in normal run mode. (Similarly a STOP instruction will stop the
processor clocks, leaving the MCAN in run mode.) The on-chip oscillator will stop only if the MCAN
is in sleep mode and the MCU executes a STOP instruction. There is a time delay between the
STOP instruction being executed and the oscillator stopping. During this time it is possible that the
MCAN will come out of sleep mode, and hence prevent the oscillator from stopping.
MOTOROLA CAN MODULE (MCAN)
MC68HC05X16
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Rev. 1
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