Page 167 - Programming Microcontrollers in C

152 Chapter 4 Small 8-Bit Systems

Another useful directive pair is the #asm and the #endasm.
The code enclosed in a block that starts with #asm and ends with
#endasm must be in standard assembly language. Variables defined
in the C program can be used safely.
C can accomplish almost everything that the assembly lan­
guage program can. You will find that the C6805 compiler will create
tight, efficient code that is probably as good as can be written by a
competent assembly programmer. There are, however, some items that
are absolutely foreign and inaccessible to a compiler. A compiler cre­
ates code for an abstract machine that does not exist in reality. The
usual registers found in the real machine are nonexistent in the abstract
machine. For example, it is not possible to access the status register of
the microcontroller with compiled code. Usually, status register con­
tents are not directly important to the conduct of the program. But
later we’ll see an example where the ability to manipulate the carry bit
of the status register can save many bytes of code. Therefore, it is
important to be able to use some assembly code as well as C.
This chapter will concentrate on small 8-bit microcontrollers.
Subsystems such as timers, analog-to-digital converters, computer
operating properly (COP) timers, etc., found on the 8-bit systems
will be outlined and their programming discussed. While the main
details of the central processor in the microcontroller are important
to the assembly language programmer, they are of little interest to
the C programmer. This observation is true at least at the C level. If it
becomes necessary to enter an assembly language program for opti­
mization of code size or other considerations, then the programmer
is required to have detailed knowledge of the programming model
and the internal architecture of the computer.
Let’s start by discussing important microcontroller peripheral
components that you can expect to find. We’ll begin with what is
probably the most important single consideration in the selection of
a microcontroller to do a job—the device memory. This discussion
will be followed by sections on other important peripherals such as
timers, analog-to-digital and digital-to-analog converters, serial com­
munications devices, and simple digital input/output lines.