mc68hc912d60c Freescale Semiconductor, Inc, mc68hc912d60c Datasheet - Page 295

• Manufacturer Part Number: mc68hc912d60c
• Description: Hc12 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

MC68HC912D60A — Rev. 3.1
Freescale Semiconductor
The transistor serves both to drive the MI Bus during the push field and
to protect the MCU TX pin from voltage transients generated in the
wiring. Without the transistor, EMI could damage the TX pin. Similarly,
the input pin (RX) is protected from EMI by clamping it to the MCU supply
rails with two diodes. When a load dump occurs, the zener diode (18V)
is switched on and hence turns the transistor on; this generates the logic
‘0’ state on the MI Bus. After eight time slots (200ms) of continuous ‘0’
state, all devices on the MI Bus will have their outputs disabled.
The MI Bus line can take two states, recessive or dominant. The
dominant state (‘0’) is represented by a maximum 0.3V (V
transistor, T1). The recessive state (‘1’) is represented by 5V, through a
pull-up resistor of 10kΩ.
The bus load depends on the number of devices on the bus. Each device
has a pull-up resistor of 10kΩ. An external termination resistor is used
to stabilize the load resistance of the bus at 600Ω.
MCU
V
V
DD
SS
RX
TX
Freescale Interconnect Bus
Figure 16-4. A typical MI Bus interface
10kΩ
4.7kΩ
V
DD
18V
22kΩ
+12V
3.9kΩ
10kΩ
Freescale Interconnect Bus
T1
1.2kΩ
Interfacing to MI Bus
V
DD
CESAT
Technical Data
MI Bus
of the
295