AN92584 Datasheet, PDF (27/42 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
Use Slave Latency
Slave latency is a BLE feature that allows a Peripheral to listen to the Central device on connection events at a
reduced rate. This is useful if the Peripheral device is inactive for some time and has to wait for some activity to occur
to send data to the Central device. If no activity is detected for some time, the Peripheral can enter slave latency by
sending a connection update request to the Central. The Peripheral can extend the interval between the connection
events at which it listens to the Central. This allows the BLESS and the system to be put into the Deep-Sleep mode
for longer time. The key advantage of using slave latency is that when the Peripheral application detects an activity, it
can switch the BLESS back ON to listen to the Central at the original connection interval until the data transfer is
completed. This avoids the latency in sending the data upon the first activity.
For example, maintaining an idle connection with a 100-ms interval consumes 148 µA. If the same connection is
updated with a slave latency value of 9, the current consumption drops to 17 µA.
These points are illustrated with two example BLE use cases: a fitness application and an HID application.
4.4
4.4.1
Example Application: Heart-Rate Monitor
A common usage segment of BLE connectivity is the fitness and wearable segment. Many BLE devices track
distance cycled, number of steps climbed, heart rate, number of hours slept, and so on. A heart-rate monitor (HRM) is
one of the most common fitness applications that use BLE today to monitor fitness levels.
System Architecture
An HRM performs the following high-level activities (see Figure 14):
 Sensing: The HRM has an analog front end to capture signals from the human body. In general, it consists of
one or more of the following activities:
o Sensor: The sensor captures the input signal. Optical sensors (LED-photodiode pairs) and electrodes are
commonly used.
o Filtering and amplification: The input signal is filtered and amplified for accurate detection. Usually,
operational amplifiers and hardware filters are used for this purpose.
o Analog-to-digital conversion: This stage converts the analog signal to a digital signal, which is sent to the
application firmware for further processing.
o Firmware processing: The application implements further filtering and amplification, followed by an algorithm
to convert the signal into a heart-rate value by using a timing procedure.
 BLE connectivity: The HRM maintains a BLE connection when in use and transmits the converted heart-rate
value to a heart-rate collector device, usually with a refresh rate of once per second.
Figure 14. HRM System Block Diagram
Heart Rate Monitor
Sensing
Input Signal
Sensor and
Transducer
Signal
Conditioning
ADC
CPU- Firmware
Processing
BLE Connectivity
4.4.2 HRM Usage Profile
An HRM is typically used in one of these ways.
1. Using the HRM during a training or exercise routine, generally for 1-2 hours per day.
2. Wearing the HRM all day in an ultra-low-power mode and turning it active multiple times during the day for short
durations. The average active time is approximately one hour per day.
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Document No. 001-92584 Rev. *A
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