Freescale Semiconductor, Inc.
Motorola Interconnect Bus (MI BUS)
6.8 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 MC68HC11P2 for full
MI BUS operation.
V
DD
+12V
1.2k¾
4.7k¾
18V
MI BUS
V
DD
T1
TX
3.9k¾
10k¾
V
DD
MCU
22k¾
10k¾
RX
V
SS
Figure 6-4. A typical interface between the MC68HC11P2 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.
Technical Data
MC68HC11P2 — Rev 1.0
Motorola Interconnect Bus (MI BUS)
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