Page 42 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

1.5 Basic Computer Structure 19

Figure 1.15. Expanded Wide Multiplexed Bus Mode of the MC68HC912B32

A significant advantage of the ' A4 or 'B32 is that either can be used in either the
single-chip or either narrow or wide expanded multiplexed bus mode. The former mode is
obviously useful when the resources within the microcontroller are enough for the
application—that is, when there are enough memory and I/O devices for the application.
The latter mode is required when more memory is needed, when a program is in an
EPROM and has to be used with the 'A4, or when more or different I/O devices are
needed than are in the 'A4.

1.5 Variable Word Width

We have glibly stated that a 6812 can have either 8-bit or 16-bit word widths. The
fastidious reader might wonder how this takes place. This optional section provides
details on how a 6812's word widths can be either 8 bits or 16 bits wide, as discussed.
The word width is a function of the instruction and of the bus mode discussed in the
last section. As noted in the last section, external memory can utilize a narrow or a wide
data bus. We first consider an external memory using the wide data bus and comment on
such a memory using a narrow bus at the end of our discussion. Figure 1.16 illustrates a
6812 system with an internal memory at locations $800 to $bff and an external memory
at locations $7000 to $7fff, each composed of two 8-bit-wide memory "banks."