Page 53 - The Art of Designing Embedded Systems

40 THE ART OF DESIGNING EMBEDDED SYSTEMS

Among the great money-makers for ICE vendors are complex hard-
ware breakpoints, used most often for chasing down errant changes to
global variables. If you like globals, figure on anteing up plenty for tools.
There’s yet one more waffle on my anti-global crusade: device han-
dlers sometimes must share data stored in common buffers and the like.
We do not write a serial receive routine in isolation. It’s part of a fabric of
handlers that include input, output, initialization, and one or more interrupt
service routines (ISRs).
This implies something profound about module design. Write pro-
grams with lots and lots of modules! Don’t lump code into a handful of
5000-line files. Assign one module per logical function: for example, have
a single module (file) that includes all of the serial device handlers-nd
nothing else. Structurally it looks like:
public serial-in, serial-out,
serial-init
serial-in: code
serial-out: code
serial-init: code
serial-isr: code
private data
buffer: data
status : data
The data items are filescopics-global to the module but private to
the rest of the system. I feel this tradeoff is needed in embedded systems
to reduce performance penalties of the noble but not-always-possible anti-
global tack.

Partit;on with CPUS

Given that firmware is the most expensive thing in the universe, given
that the code will always be the most expensive part of the development ef-
fort, given that we’re under fire to deliver more complex systems to market
faster than ever, it makes sense in all but the most cost-sensitive systems to
have the hardware design fall out of software considerations. That is, design
the hardware in a way to minimize the cost of software development.
It’s time to reverse the conventional design approach, and let the
sofware drive the hardware design.
Consider the typical modern embedded system. A single CPU has the
metaphorical role of a mainframe computer: it handles all of the inputs and
outputs, runs application code, and services interrupts. Like the main-