EnerChip Solar Energy Harvesting Demo Kit
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 CBC5300 energy harvester 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.
Circuit 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. Careful selection of the message protocol for the RF link can have a significant impact on the
overall power budget. In many cases, using higher power analog circuits that can be turned on, settle quickly,
and be turned off can decrease the overall energy consumed. Microcontroller clock frequency can also have
a significant impact on the power budget. 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. Avoid
using circuits that bias microcontroller digital inputs to mid-level voltages; this can cause significant amounts
of parasitic currents to flow. Use 10MΩ to 22MΩ 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.
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. The CHARGE line on the CBC5300 has a 10MΩ 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.
The CBC5300 energy harvester module has a feature for disabling the on-board EnerChip thin film storage
devices. A handshake line BATOFF is provided for use of this feature. A high level will disable the EnerChips.
This is useful in very low ambient energy conditions to steer all of the available energy into the load. EnerChip
devices have very low self-discharge rates (typically 2.5% per year) so it is not necessary to continuously charge
them.
DS-72-08 Rev18
©2009 Cymbet Corporation • Tel: +1-763-633-1780 • www.cymbet.com
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