BSP Developer's Guide

5
Driver Guidelines
word addresses. Thus, there are three different implementations with three
different addressing schemes.
Consider also that the designer who chose the long word implementation might
also have chosen to require the driver to use only long word read/write accesses.
The driver cannot even assume that a byte read operation can be used to read a
byte register.
One solution to this problem is to require that all accesses to the chip be restricted
to as few routines as possible. Further, each access to the chip should be declared
in a preprocessor macro that can be redefined so as to meet the special
requirements of any particular system.
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I/O-Mapped Chips
Most engineers had their first experiences on Motorola processors that use
memory mapped I/O exclusively. However, there is another world of processors
using a separate address space called I/O space. Unfortunately, the C language
does not provide a means to specify if an address is a memory address or an I/O
address. You have to use assembly language subroutines to access the special
instructions to reach data in I/O space. You cannot write a direct expression in C
to do that.
Fortunately, the solution to the memory-mapped problem above also solves this
problem. By defining the special hardware access macros to use a special I/O
subroutine, you can handle devices mapped into I/O space.
Multi-Function Chips
Designers are fond of combining more and more channels (devices) per chip. Then
they thought up the idea for ASIC chips combining multiple devices of different
types into a single piece of silicon. The designers are only limited by imagination
in the ways they can combine silicon building blocks together.
Rather than write a driver for a complete ASIC, you should strive to write drivers
as though each subsection is a separate device. It might be necessary, if
undesirable, for one driver to require support from another driver. If this is the
case, then this must be clearly documented in the dependent driver
documentation. A single large monolithic driver for an entire ASIC is opposed to
the goal of system scalability. The user should be able to exclude features that are
not needed by the application.
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