AN92584 Datasheet, PDF (28/42 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
4.4.3
Application Design for Low Power
For the typical usage profile, the application design is shown in Figure 15. When the device is in use, it scans the
heart-rate sensor at a constant rate and processes the sensor output to get a heart-rate value. For an HRM, the
sensor scan rate is usually about 20 milliseconds. During the sensor scan, the system is put in the Sleep mode.
The BLE connection events are asynchronous to this operation, and the connection interval is much larger than the
sensor scan rate. The typical connection interval is one second for HRM applications. At every connection event, the
device sends the sensor data to the heart-rate collector device through notifications. The system then goes to the
Deep-Sleep mode when all the BLE and system processing is completed.
When the user no longer needs the heart-rate information, the device disconnects from the peer device and enters an
idle state with a very low current consumption (such as the Hibernate or Stop mode) where all the system
functionality is turned OFF and a low-power wakeup source (such as a GPIO or the WAKEUP pin) is set up to
resume functionality. For example, an external piezo can be used as a wakeup source by driving a GPIO. The piezo
is activated and triggers an interrupt when the user wears the HRM. For this application, the SAR ADC and three
CTBms available on the chip (PSoC 4 BLE only) are used to interface to the external sensor.
Figure 15. HRM Firmware Design
Application Flow
Scan sensor and get raw data
Ultra-low-power mode
All functionality
turned off; wait for
user assertion
Disconnection
GPIO Wakeup
Process raw data to get Heart
Rate value
1 second
elapsed since last
check?
Yes
No
Timer Interrupt to
maintain sensor
scan rate
Push a notification packet to
BLE link layer
Process BLE events
BLE Connectivity
Connection Event
BLE connection
event (Rx/Tx)
handled
asynchronous to
the main
application
Enter Lowest Power Mode
possible (Sleep or Deep Sleep)
depending on BLE activity
4.5
4.5.1
Example Application: Remote Control
An emerging application of BLE connectivity is in the HID market with products such as the smart remote control for
smart TVs and set-top boxes. These applications differ from the wearable device applications mainly in the BLE data
rate and the usage profile that involves start-idle-stop cycles. This example discusses a simple BLE remote control
with a trackpad. It demonstrates an additional level of complexity in the system compared with the HRM and shows
how a low-power design should be done in such a case.
System Architecture
A BLE remote control with a trackpad as a BLE Peripheral device performs the following functions:
 Trackpad sensing: The touchpad of the remote device will have an array of sensor rows and columns. The
number of rows and columns will vary depending on the size and resolution of the touchpad required. The
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Document No. 001-92584 Rev. *A
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