MC68HC11PH8 Datasheet, PDF (116/264 Pages) Motorola, Inc – High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcomputer Unit

6.4
Interfacing to MI BUS
Physically the MI BUS consists of only a single wire. In the example shown in Figure 6-4, only a
single transistor and a few passive components are required to connect up the MC68HC11PH8
for full MI BUS operation.
V_DD
+12 V
4.7 kΩ
18 V
V_DD
TX
3.9 kΩ
1.2 kΩ
T1
MI BUS
6
MCU
V_DD
10 kΩ
RX
V_SS
10 kΩ
22 kΩ
Figure 6-4 A typical interface between the MC68HC11PH8 and the MI BUS
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 recessive state (‘1’) is
represented by 5V, through a pull-up resistor of 10kΩ. The dominant state (‘0’) is represented by
a maximum 0.3V (V_CESAT of the transistor, T1).
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.
MOTOROLA
6-6
MOTOROLA INTERCONNECT BUS (MI BUS)
TPG
MC68HC11PH8