DS-72-40 Datasheet, PDF (15/17 Pages) Cymbet Corporation – EnerChip™ CC Energy Harvester Evaluation Kit

CBC-EVAL-10B EnerChip CC EH Evaluation Kit
System Design Recommendations to Save Power
In most system power budgets, the peak power required is not as critical as the length of time the power is
required.
1. Careful selection of the message protocol for the RF link can have a significant impact on the overall power
budget.
2. In many cases, using higher power analog circuits that can be turned on, settle quickly, and immediately
turned off can decrease the overall energy consumed.
3. Microcontroller clock frequency can also have a significant impact on the power budget.
4. In some applications it might be advantageous to use a higher microcontroller clock frequency to reduce
the time the microcontroller and peripheral circuits are active.
5. Avoid using circuits that bias microcontroller digital inputs to mid-level voltages; this can cause significant
amounts of parasitic currents to flow.
6. Use 22MΩ (or larger) pull-up/down resistors where possible. However, be aware that high circuit
impedances coupled with parasitic capacitance can make for a slow rise/fall time that can place the
voltage on the microcontroller inputs at mid-levels, resulting in parasitic current flow. One solution to
the problem is to enable the internal pull-up/down resistor of the microcontroller input to force the input
to a known state, then disable the resistor when it’s time to check the state of the line. If using the
microcontroller’s internal pull-up/down resistors on the inputs to bias push-button switches in a polled
system, leave the pull-up/down resistor disabled and enable the resistor only while checking the state of
the input port. Alternatively, in an interrupt-driven system, disable the pull-up/down resistor within the first
few instructions in the interrupt service routine. Enable the pull-up/down resistor only after checking that
the switch has been opened.
7. Microcontroller pull-up/down resistors are typically less than 100kΩ and will be a huge load on the system
if left on continuously while a button is being pressed or if held for any significant length of time. For
even greater reduction in power, use external pull-up/down resistors in the 10MΩ to 22MΩ range. Bias
the external resistor not with the power rail but with a microcontroller port. The same algorithm used for
internal pull-up/down resistors can then be used to save power.
8. The CHARGE/ line on the CBC-EVAL-10B has a 1.0MΩ pull-up resistor with a very slow rise time. Use an
internal microcontroller pull-down resistor to force the CHARGE/ line low all of the time and then disable
the pull-down resistor to check the state of the line. This will keep the CHARGE/ line from biasing the input
at mid level for long periods of time, which could case large parasitic currents to flow.
9. The CBC-EVAL-10B module has a feature for disabling the CBC3150 charge pump. A handshake line
BATOFF is provided for use of this feature. A high level will disable the charge pump. This is useful in very
low ambient energy conditions to steer all of the available energy into the load. EnerChip batteries have
very low self-discharge rates (typically 2.5% per year) so it is not necessary to continuously charge them.
10. While it is relatively straightforward to calculate a power budget and design a system to work within the
constraints of the power and energy available, it is easy to overlook the power required to initialize the
system to a known state and to complete the radio link with the host system or peer nodes in a mesh
network. The initialization phase can sometimes take two to three times the power needed for steady state
operation. Ideally, the hardware should be in a low power state when the system power-on reset is in its
active state. If this is not possible, the microcontroller should place the hardware in a low power state as
soon as possible. After this is done, the microcontroller should be put into a sleep state long enough for the
energy harvester to replenish the energy storage device. If the power budget is not exceeded during this
phase, the system can continue with its initialization. Next, the main initialization of the system, radio links,
analog circuits, and so forth, can begin. Care should be taken to ensure that the time the system is on
during this phase does not exceed the power budget. Several sleep cycles might be needed to ‘stairstep’
the system up to its main operational state. The Cymbet CBC-EVAL-10B module has a handshake line
CHARGE/ to indicate to the microcontroller when energy is available. Another way to know whether energy
is available is to have the microcontroller monitor the voltage on its power bus using one its internal A/D
converters.
©2014 Cymbet Corporation • Tel: [phone] • www.cymbet.com
DS-72-40 Rev C

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