AN92584 Datasheet, PDF (26/42 Pages) Ramtron International Corporation – Designing for Low Power and Estimating Battery Life for BLE Applications

Designing for Low Power and Estimating Battery Life for BLE Applications
If no sensor use or activity is detected for some time, the device can reduce the rate at which it scans for the activity.
In addition, it can increase the connection interval of the BLE link and enter low-power modes to reduce the system
current. While this may introduce latency in reacting when an activity occurs, the overall system power can be
significantly reduced. For example, in trackpad designs, scanning alternate sensors instead of all the sensors to
detect a movement can save power. The sampling rate for data can also be minimized where and when possible.
Lower Data Sampling Rates
The sampling rate of the SAR ADC in the device should be reduced to the minimum frequency required to effectively
reproduce the analog signal being sampled. Based on the sampling frequency, the SAR ADC configurations (such as
the ICONT_LV register fields) that allow low-power operation should be used. In addition, the channel resolution can
also be reduced to decrease the conversion time and thus save power.
Use Asynchronous Wakeup
PSoC 4 BLE provides a low-power comparator that continues to operate while the system is in the Deep-Sleep mode.
If any sensor activity is detected through the comparator, it can wake the entire system from the Deep-Sleep mode.
This avoids keeping the CPU active for periodic scanning for sensor activity. In addition, the GPIOs are available as
asynchronous wakeup sources, which can be used to wake up the system from the Deep-Sleep mode based on the
detection of an external activity.
Reduce WCO Crystal ppm
The WCO crystal accuracy (measured in ppm) affects the listening window at the connection events in a Peripheral
device. Due to the drift in timing caused by an inaccurate crystal, the Peripheral will have to listen for a larger window
to “locate” the packet from the Central device. The higher the inaccuracy, the larger is the listening window and the
higher is the current consumption. Refer to AN95089 – PSoC 4/PRoC BLE Crystal Oscillator Selection and Tuning
Techniques for details on WCO crystal selection and tuning.
Figure 13 shows the improvement in average current for a one-second connection interval in PSoC 4 BLE as the ppm
is reduced. A 500-ppm crystal is considered for the chart.
Figure 13. ppm Versus Current Reduction
By using a more accurate crystal, the active time for which the radio is listening can be reduced. For example, in the
PSoC 4/ PRoC BLE device, reducing the WCO inaccuracy by 50 ppm reduces the average current by approximately
2 µA, thus increasing the battery life.
Increase Connection Interval
The connection interval for a BLE link is set by the Central device when a connection is created with the Peripheral
device. It is preferable that the Peripheral have a connection interval that matches the rate at which the sensor data
from the Peripheral is sent to the Central device. If the initial connection interval set by the Central is lower than
necessary, then the Peripheral should request the Central to increase the connection interval to reduce current
consumption.
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Document No. 001-92584 Rev. *A
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