Freescale Semiconductor, Inc.
Motorola Interconnect Bus (MI BUS)
The MC68HC11P2 contains two similar MI BUS modules. For ease of
reference, a full description of MI BUS2 is given, followed by a summary
of MI BUS3, detailing its differences.
The MI BUS uses a push-pull sequence to transfer data. The master
device, which, in this case, is the MC68HC11P2, sends a push field to
the slave devices connected to the bus. The push field contains data
plus an address that is recognized by one of the slaves. The slave
addressed returns data which the master pulls from the MI BUS over the
same wire. Specific details of the message format are covered later in
this section. The MCU (master) can take the bus at any time, with a start
bit that violates the rules of Manchester biphase encoding. Up to eight
slave devices may be addressed by the MI BUS. Other features of
MI BUS include message validation, error detection, and default value
setting.
On the MC68HC11P2 the MI BUS module shares the same pins on port
H as the SCI2 and SCI3 modules. Data is transmitted (or ‘pushed’) via
the TXD pin, and received (‘pulled’) via the RXD pin. While data is being
pushed RXD will be disconnected from the receiver circuitry. The
message frame is handled automatically in hardware. The MCU register
interface is similar to that for the SCI.
Alternate
function
Pin
PH4
PH5
PH6
PH7
RXD2
TXD2
RXD3
TXD3
MI BUS2 functions are enabled by MIE2 in S2CR1; MI BUS3 functions
by MIE3 in S3CR1.
Technical Data
MC68HC11P2 — Rev 1.0
Motorola Interconnect Bus (MI BUS)
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