MC33192 Datasheet, PDF (7/12 Pages) Motorola, Inc – MI-Bus Interface Stepper Motor Controller

Freescale SMeCm33i1c9o2nductor, Inc.
MI–Bus Interface Description
The MI–Bus Interface shown in Figure 9 is made up of a
single NPN transistor (Q1). The two main functions of this
NPN transistor are:
1) To drive the MI–Bus during the Push Field with
approximately 20 mA of current while also exhibiting low
saturation characteristics (VCE(sat)).
2) To protect the Input/Output (I/O) pin of the MCU against
any Electro–Magnetic Interference (EMI) captured on the
bus wire.
Without the NPN transistor, the MCU could be destroyed
as a result of receiving excessive EMI energy present on the
bus. In addition, the transistor blocks the MCU from receiving
EMI signals which could erroneously change the data
direction register of the MCU I/O.
The MCU input pin (Pin), used to read the Pull Field of the
MI–Bus, is protected by two diodes (D2 and D3) and two
resistors (R5 and R6). Any transient EMI generated voltage
present on the bus is clamped by the two diodes to a
windowed voltage value not to be greater than the VDD or
less than the VSS supply voltages of the MCU.
MI–Bus Levels
The MI–Bus can have one of two valid logic states,
recessive or dominant. The recessive state corresponds to a
Logic “1” and is obtained through use of a 10 kΩ pull–up
resistor (R9) to 5.0 V. The dominant state corresponds to a
Logic “0” which represents a voltage less than 0.3 V and
created by the VCE(sat) _{of Q1.}
MI–Bus Overvoltage Protection
An external zener diode (Z1) is incorporated in the
interface circuit so as to protect the MCU output pin (Pout)
from overvoltages commonly encountered in automotive
applications as a result of “Load Dump” and “Jump Start”
conditions. Load Dump is defined as the inductive transient
generated on the battery line as a result of opening the
battery connection while the alternator system is producing
charge current. Jump Start overvoltages are the result of
paralleling the installed automotive battery, through the use
of “jumper cables”, to an external voltage source in excess
of the vehicles nominal system voltage. For 12 V
automotive systems, it is common for 24 V “jump start”
voltages to be used.
When an overvoltage situation (>18 V) exists, due to a
load dump or jump start condition, the zener diode (Z1) is
activated and supplies base current to turn on the NPN
transistor Q1 causing the bus to be pulled to less than 0.3 V
producing a Logic “0” on the MI–Bus. After a duration
corresponding to 8ts (200 µs) of continuous Logic “0” on the
bus all MC33192 devices will disable their outputs. Normal
operation is resumed, following the overvoltage, by the MCU
sending out a “standard” message instruction.
MI–Bus Termination Network
The MI–Bus is resistively loaded according to the number
of MC33192 devices installed on the bus. Each MC33192
has an internal 10 kΩ pull–up resistor to 5.0 V. An external
pull–up resistor (R7) is recommended to be used to optimally
adjust termination of the bus for a load resistance of 600 Ω.
Figure 9. MI–Bus MCU Interface
5.0 V
VDD
Pout
MCU
VSS ^Pin
Program
Run
R1
(4.7 K)
Z1
(18 V)
R2
(1.2 k)
R3
(3.9 k)
Q1
D1
VDD
R4
(10 k)
R5
(10 k)
D2
R6
D3 (22 k)
Zin
MI–Bus
5.0 V
R7
(1.2 k)
#2
#3
#4
#5
#6
#7
#8
MC33192
5.0 V
R8
1 (20 Ω)
6.5 V
R9
(10 k)
12 V
7
VCC
Programming
Data In
Z2
(18 V)
Q2
Data
Out
Gnd
2
Additional MC33192 Devices
0V
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