Propeller Manual

Introducing the Propeller Chip
Shared Resources
There are two types of shared resources in the Propeller: 1) common, and 2) mutually
exclusive. Common resources can be accessed at any time by any number of cogs. Mutually
exclusive resources can also be accessed by all cogs, but only by one cog at a time. The
common resources are the I/O pins and the System Counter. All other shared resources are
mutually exclusive by nature and access to them is controlled by the Hub. See the Hub
section on page 24.
System Clock
The System Clock (shown as “CLOCK” in Figure 1-2) is the central clock source for nearly
every component of the Propeller chip. The System Clock’s signal comes from one of three
possible sources: 1) the Internal RC Oscillator, 2) the Clock Phase-Locked Loop (PLL), or
3) the Crystal Oscillator (an internal circuit that is fed by an external crystal or
crystal/oscillator pack). The source is determined by the CLK register’s settings, which is
selectable at compile time or at run time. The only components that don’t use the System
Clock directly are the Hub and Bus; they divide the System Clock by two (2).
Cogs (processors)
The Propeller contains eight (8) processors, called cogs, numbered 0 to 7. Each cog contains
the same components (see Figure 1-2): a Processor block, local 2 KB RAM configured as 512
longs (512 x 32 bits), two Counter Modules with PLLs, a Video Generator, I/O Output
Register, I/O Direction Register, and other registers not shown in the diagram. See Table 1-3
for a complete list of cog registers. Each cog is designed exactly the same and can run tasks
independently from the others.
All eight cogs are driven from the same clock source, the System Clock, so they each
maintain the same time reference and all active cogs execute instructions simultaneously.
See System Clock, above. They also all have access to the same shared resources, like I/O
pins, Main RAM, and the System Counter. See Shared Resources, above.
Cogs can be started and stopped at run time and can be programmed to perform tasks
simultaneously, either independently or with coordination from other cogs through Main
RAM. Regardless of the nature of their use, the Propeller application designer has full
control over how and when each cog is employed; there is no compiler-driven or operating
system-driven splitting of tasks between multiple cogs. This empowers the developer to
deliver absolutely deterministic timing, power consumption, and response to the embedded
application.
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