MC68HC12 Datasheet, PDF (103/478 Pages) Motorola, Inc – The MC68HC912DT128A microcontroller unit (MCU) is a 16-bit device composed of standard on-chip peripherals including a 16-bit central processing unit

Technical Data — MC68HC912DT128A
Section 7. Bus Control and Input/Output
7.1 Contents
7.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
7.3 Detecting Access Type from External Signals . . . . . . . . . . . .103
7.4 Registers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .104
7.2 Introduction
Internally the MC68HC912DT128A has full 16-bit data paths, but
depending upon the operating mode and control registers, the external
multiplexed bus may be 8 or 16 bits. There are cases where 8-bit and
16-bit accesses can appear on adjacent cycles using the LSTRB signal
to indicate 8- or 16-bit data.
It is possible to have a mix of 8 and 16 bit peripherals attached to the
external multiplexed bus, using the NDRF bit in the MISC register while
in expanded wide modes.
7.3 Detecting Access Type from External Signals
The external signals LSTRB, R/W, and A0 can be used to determine the
type of bus access that is taking place. Accesses to the internal RAM
module are the only type of access that produce LSTRB = A0 = 1,
because the internal RAM is specifically designed to allow misaligned
16-bit accesses in a single cycle. In these cases the data for the address
that was accessed is on the low half of the data bus and the data for
address + 1 is on the high half of the data bus.
MC68HC912DT128A — Rev 4.0
MOTOROLA
Bus Control and Input/Output
Technical Data
103