AN92584 Datasheet, PDF (30/42 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
Figure 17. Remote Control Firmware Flowchart
Scan Trackpad
Yes
Process Gestures
Prepare HID report
Touch
Detected?
No
Wakeup
Timer
Interrupt
Exchange BLE
Packets
System in Deep-
Sleep Mode
The trackpad scan is aligned with the BLE connection events so the system can be in the Deep-Sleep mode for the
maximum possible time. Scanning of sensors and processing of gestures take a finite amount of time every scan
cycle. When these activities are completed at the connection event, the system is put into the Deep-Sleep mode to
save power. The connection interval must be chosen appropriately. If the connection interval is higher, the HID report
rate for the device may be lower than required, which will degrade the user experience due to latency in transmitting
the data. If the connection interval is lower, then the current consumption increases. To satisfy the conflicting needs
of a good user experience and a power-efficient system, the application needs to switch adaptively between the
required connection intervals.
A similar challenge is faced with respect to the trackpad scan rate. The trackpad must be scanned at a higher rate to
recognize gestures, but it should be scanned at a lower rate when there is no movement for a long time. The lower
rate must be just enough to detect user activity. To meet these requirements, the application implements a state
machine, as shown in Figure 18.
Figure 18. Remote Control State Machine
Device Init State
Initialization Complete
User Activity
Active State
No user activity for short duration
Idle State
No user activity for long duration
Low-Power State
Low Battery Voltage
Shutdown State
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Document No. 001-92584 Rev. *A
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