Page 144 - Programming Microcontrollers in C

Input/Output 129

similar to EEPROM and it is inexpensive enough to allow rather
large amounts of programmable memory on a microcontroller chip.
You will find chips with 30,000 bytes and more of FLASH and the
intent is to use these chips for production runs. The FLASH is
programmed as part of the production cycle. We will see many details
on programming FLASH memory in later chapters.
The architecture of Motorola microcontrollers is strictly Von
Neumann. That is, within the microcontroller chip, there is only one
data bus over which all program, data, and input/output must pass. In
a Harvard architecture system, each of these different data types will
have a dedicated bus over which the information will pass. Therefore,
the Harvard architecture microcontroller is able to access data, program,
and I/O simultaneously. The simultaneous availability of these different
data paths can result in a significant increase in overall processor speed.
It also increases the area of the microcontroller die and, hence, the cost
of the microcontroller. In general, most of the applications to which
the microcontrollers are directed do not require extreme speed. Thus,
the Von Neumann architecture is completely satisfactory.

Input/Output

The Motorola microcontrollers use an architecture called memory-
mapped I/O. Each I/O device input and output registers, its control
registers, and status registers are mapped into memory locations. I/O
transactions require no special computer instructions. It is merely
necessary to know the memory locations of the pertinent registers and
the uses of the register bits to be able to handle any I/O function. Listed
below are brief descriptions of several microcontroller I/O peripherals
found on Motorola microcontrollers. Not all of these peripheral systems
are found on each microcontroller. It is possible to pick and choose
between needs for the several peripheral systems and select a
microcontroller that has exactly those peripherals required.

Timer Subsystems
There are four popular timer systems that you will find on different
microcontrollers. The first is a general-purpose timer. Motorola refers
to the general-purpose timers as either 8- or 15-bit timers. These
timers are different. The 8-bit system contains a prescaler that counts
down from system clock. The output from the prescaler is fed into a