MI bus

Motorola Interconnect

MI bus Description

MI [Motorola Interconnect] is a serial communications interface using a single line from one Master to as many as 8 slave devices.
The MI bus is used to control Smart Switches, Motors, Sensors, and Actuators.
The MI bus bus may also be used as an Automotive Bus to drive Mirrors, Seats, Window lifts or Head light levellers.
The MI interface uses only 1-wire to send and receive data at a data rate of 20kHz.

MI Bus interconnect
MI Bus Master driving 3 Slaves devices

The MI bus utilizes a push/pull sequence to transfer data between the master and [1 of 8] slaves.
The Master sends a push field to the slave devices connected to the bus. This field contains data, plus the address of one of the slaves.
The Slave addressed then responds to the received data, transmitting the Pull field on the MI Bus to the Master.
The data returned by the slave is likely to be status bits representing internal or external information.

MI Protocol Field
MI Bus Master driving 3 Slaves devices

The Push Field shown above left delivers the following:

Master Push Field
Bit Field Bit Length
Start bit field 3 time slots held low
Synchronization bit field 1 Bi-Phase encoded 0
Data bit field 5 bits of Bi-Phase encoded data
Address bit field 3 bits of Bi-Phase encoded data

The Pull Field shown below right delivers the following:

Slave Pull Field
Bit Field Bit Length
Synchronization bit field 1 Bi-Phase encoded 1 which is initiated by the Master
Data bit field 3 bits of Non Return to Zero encoded data
End of Frame bit field 3 cycles of a 20KHz square wave

MI Interface Bus Circuit, Electrical Interface Schematic
MI Hardware Interface Bus Circuit Schematic

The MI interface circuit is shown above. The MI bus uses only one wire.
The transistor serves to drive the bus and protect and isolate the MCU from voltage transients on the line.
The MCU's RX pin is protected by two resistors and two diodes which protect against transient currents.
The MI bus has two states; Dominant [State 0] represented by a maximum of 0.3v, and
Recessive [state 1] represented by +5v [via a 10k pull-up resistor].
Each device on the bus has a 10K pull-up. A termination resistor of 600 ohms is used to stabilize the bus.
The schematic is from a 68HC11 data sheet, which would represent the MCU [MicroController Unit].

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The circuit schematic is from a Motorola MC68HC11 Microcomputer Unit data sheet [now freescale].
Editor note; the MI Bus started being used as an interface in the mid 1990's, not sure if it's still employed on current microcontrollers.

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Modified 6/13/15
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