Speed and Power Management
The Propeller can be clocked using either an internal, on-chip oscillator (providing a lower total parts count, but sacrificing some accuracy and thermal stability) or an external crystal or resonator (providing higher maximum speed with greater accuracy at an increased total cost). Only the external oscillator may be run through an on-chip PLL clock multiplier, which may be set at 1x, 2x, 4x, 8x, or 16x.
Both the on-board oscillator frequency (if used) and the PLL multiplier value may be changed at run-time. If used correctly, this can improve power efficiency; for example, the PLL multiplier can be decreased before a long "no operation" wait required for timing purposes, then increased afterwards, causing the processor to use less power. However, the utility of this technique is limited to situations where no other cog is executing timing-dependent code (or is carefully designed to cope with the change), since the effective clock rate is common to all cogs.
The effective clock rate ranges from 32 kHz up to 80 MHz (with the exact values available for dynamic control dependent on the configuration used, as described above). When running at 80 MHz, the proprietary interpreted Spin programming language executes approximately 80,000 instruction-tokens per second on each core, giving 8 times 80,000 for 640,000 high level instructions per second. Most machine-language instructions take 4 clock-cycles to execute, resulting in 20 MIPS per cog, or 160 MIPS in total for an 8-cog Propeller.
In addition to lowering the clock rate to that actually required, power consumption can be reduced by turning off cogs (which then use very little power), and by reconfiguring I/O pins which are not needed, or can be safely placed in a high-impedance state ("tristated"), as inputs. Pins can be reconfigured dynamically, but again, the change applies to all cogs, so synchronization is important for certain designs. (Some protection is available for situations where one core attempts to use a pin as an output while another attempts to use it as an input; this is explained in Parallax's technical reference manual.)
Read more about this topic: Parallax Propeller
Famous quotes containing the words speed and, speed, power and/or management:
“The greatest felony in the news business today is to be behind, or to miss a big story. So speed and quantity substitute for thoroughness and quality, for accuracy and context. The pressure to compete, the fear somebody else will make the splash first, creates a frenzied environment in which a blizzard of information is presented and serious questions may not be raised.”
—Carl Bernstein (b. 1944)
“The correct rate of speed in innovating changes in long-standing social customs has not yet been determined by even the most expert of the experts. Personally I am beginning to think there is more danger in lagging than in speeding up cultural change to keep pace with mechanical change.”
—Mary Barnett Gilson (1877–?)
“I consider, then, the power to annul a law of the United States, assumed by one state, incompatible with the existence of the Union, contradicted expressly by the letter of the Constitution, unauthorized by its spirit, inconsistent with every principle on which it was founded, and destructive of the great object for which it was formed.”
—Andrew Jackson (1767–1845)
“This we take it is the grand characteristic of our age. By our skill in Mechanism, it has come to pass, that in the management of external things we excel all other ages; while in whatever respects the pure moral nature, in true dignity of soul and character, we are perhaps inferior to most civilised ages.”
—Thomas Carlyle (1795–1881)