Page 90 - Build Your Own Quadcopter_ Power Up Your Designs with the Parallax Elev-8

chapter 4

Programming the

Parallax Propeller

Chip

Introduction

The Parallax Propeller chip that was introduced in Chapter 1 will be discussed in this chapter
from hardware and software perspectives. I will also refer to the chip as the Prop from now
on for brevity’s sake.
The Prop was developed by Parallax engineers solely for an extended project that started
in the late 1990s and ran to its market introduction in 2006. It is still the only available
multicore processor designed specifically for the technical/hobbyist market. Intel and AMD
multicore processors are distinctly different technologies designed for PC and server
applications. The Prop also has a much lower clock speed than Intel processors, which is not
an issue because it serves a very different purpose than the Intel/AMD multicores do. Props
are also very inexpensive, typically less than $10 USD, making them ideal for experimentation
and prototyping. First, I will describe the Prop’s unique architecture before going on to cover
the software and programming details.

Prop Architecture

In Figure 4.1 is the Prop P8X32X block diagram showing the eight separate cores along with
other key elements.
One of the first things that you might have noticed is that the cores are called cogs in the
Prop diagram. Each cog is an independent processing element with its own 2-kB memory.
The Prop also has 32-bit data and address busses, which means the “word” size is also
32 bits. There are 32 general-purpose input/output (GPIO) pins that each cog can access. These
are shared GPIO pins; however, some or all may be dedicated to a specific cog or group of
cogs if you want. A GPIO pin so dedicated is also referred to as being mutually exclusive
indicating that the pin is being controlled by a designated cog.
All the cogs communicate with a hub that contains a common or shared 64-kB memory
that is divided as 32 kB of RAM and 32 kB of ROM. The hub also contains configuration,
power management, reset, and clock circuits.
The clock circuits are quite flexible and may use either an internal resistor-capacitor (RC)
oscillator or a crystal-controlled oscillator, which in turn requires an external crystal. Most
Prop boards that I have used operate with a crystal oscillator because it provides higher

69