MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 33

• Manufacturer Part Number: MCP6271R
• Description: 170 ?a, 2 Mhz Rail-to-rail Op Amp
• Manufacturer: Microchip Technology Inc.

The is a ~50,000X improvement from the 4 MHz crystal
oscillator. From this data, one might summarize the appropriate
clock for this type of application is an internal clock. The power
consumption of an internal clock is nearly equivalent to the
power consumption of a crystal oscillator. This strategy works,
as long as your microcontroller is not required to run time-critical
operations, such as USART communications or timing a precision
pulse.
A third clock source that you may be evaluating is the resonator.
Figure 5 illustrates the start-up time of a resonator.
Figure 5: The start-up time of a 4 MHz resonator is faster than a
4 MHz crystal and slower than a 4 MHz internal clock.
There is a clocking system that you can use that is better than
any of these three clocking options. The best of all worlds is
to quickly determine if your circuit needs a precision clock. If
the microcontroller needs a precision clock, the oscillator or
resonator is turned on. If not, the microcontroller will shut down.
This determination is made quickly, after the microcontroller
leaves its sleep mode. If you combine the internal clock with an
external resonator or crystal oscillator, this type of decision can
quickly be made. You can find significant improvements in power
consumption when you use two clock sources (instead on one).
This technique is called the “Two-Clock Start-Up Strategy”. In
this hardware/firmware configuration, the microcontroller uses
two clocks. Both clocks are off during the sleep mode of the
application. At the time of wake-up, the internal clock is turned
on to quickly determine if the crystal oscillator is required. If it
is required, the clock continues to execute code until the crystal
oscillator is up and running. At this time, the microcontroller
switches over to the crystal oscillator and turns off the internal
clock.
Working the Digital Angle with Sleep Modes
The central focus of a successful low-power design is a
microcontroller that has a variety of sleep modes and clock
modes. You can conserve system power with the idle modes
and sleep modes of the microcontroller. The idle modes of the
microcontroller power down the CPU while allowing functions
such as the 10-bit ADC to continue to operate. The sleep mode
implements a complete shutdown of the microcontroller.
Miscellaneous Articles
When the clock of your microcontroller switches states the
various logic gates in your microcontroller pull current from
the power source. When looking at the current consumption in
your microcontroller, the first stop is to look at the clock power
consumption. If you examine the types of clocks available to you,
the internal oscillator will run with less power than the frequency
equivalent crystals, oscillators or resonators.
Some microcontrollers have three fundamental modes of
operation. The first is the full-bore run mode, where everything
is up and running. An intermediate mode is the idle or wait
mode, where the peripherals are usually running but not the
microcontroller. The third and most important mode for lower-
power, battery operation is the sleep or stop mode. In this
sleep mode, the device stops consuming power completely. The
sleep mode generally disables the system’s clocks, but power
conservation is more effective if you also disable the external
clock sources.
Here are some additional suggestions to complete your low-
power strategy. Drive any unused I/O pins into a high or low
state. Use the internal oscillators for clock sources, where
possible. They generally are the lower power choice. Shut down
all peripherals not in use, like the Pulse Width Modulator (PWM),
ADC, USART, etc. Use as many look-up tables as possible in your
code, instead of using the CPU to compute the results. Check
the power consumption of all external components. For instance,
measure the voltage drop of all external resistors in the circuit.
Lower the I/O pins that are used to power external peripherals
such as serial EEPROMs or external analog devices. Another
surprise can be your LEDs that are turned on. A single LED can
wipe out your power-savings efforts. In general, look for current-
consumption gremlins.
Conclusion
Device power savings in battery-powered applications are
extremely important. You can achieve true value by using the
microcontroller’s programmability. You can do this by changing
the power-supply voltage at the output of a regulated charge
pump. A second area would be to power down non-critical
peripherals when not in use. Another option is to control the
clocking strategy in order to optimize power versus functionality.
Integrated circuit manufacturers are continuing to improve
the dynamic performance of their peripheral devices while
reducing the quiescent-current and supply-voltage requirements.
Microcontroller manufacturers are adding modes, such idle
and sleep, that save average power over long periods of time.
The combination of lower-power peripherals and microcontroller
modes enhances your chances of having a low-power,
battery-powered solution.
Got your checklist? Now take all of these variables and put
on your low-power “state of mind” hat. You, as the perceptive
programmer/hardware expert, need to evaluate each one of your
applications and every situation inside those applications, to look
for the power-consumption gremlins. Good luck!
This article is excerpted from Bonnie Baker’s book, “A BAKER’S
DOZEN: Real World Solutions to Real World Analog Design
Problems,” published Spring 2005 by Elsevier.
Analog and Interface Guide – Volume 2
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