Peripheral

Peripheral

The peripheral is a BLE device that advertises its presence and accepts connections from a central device. Previously referred to as "Slave" in the Bluetooth specification, the term was updated to "Peripheral" in Core Specification 5.3. Peripherals are typically battery-powered sensors, beacons, wearables, or accessories that prioritize low power consumption.

How a Peripheral Operates

A peripheral begins in the advertising state, broadcasting advertising packets on channels 37, 38, and 39 at a configurable advertising interval. These packets contain information about the device's capabilities, supported services, and local name. A central device scanning for peripherals can read these packets and decide whether to connect.

Once a connection is established, the peripheral responds to the central's transmissions during each connection event. Between events, the peripheral's radio is off and the MCU can enter deep sleep. The peripheral can further reduce power by leveraging slave latency to skip connection events when it has no new data to transmit.

GATT Server Role

In most BLE architectures, the peripheral acts as the GATT server. It hosts a database of services and characteristics that describe the data it can provide. A heart rate monitor peripheral, for example, exposes the Heart Rate Service (ATT">UUID 0x180D) containing the Heart Rate Measurement characteristic, which the central reads or subscribes to via notifications.

Power Optimization

Peripheral devices are designed for minimal energy consumption. Key power optimization techniques include using long connection intervals (500 ms--4 s), maximizing slave latency, enabling the on-chip DCDC converter, and choosing an SoC with low sleep current. Chips like the nRF52832 achieve 1.7 uA in system-on sleep with RAM retention, while the nRF54L15 pushes this below 1 uA, enabling multi-year operation on a CR2032 coin cell.

Multi-Role Capability

Modern BLE SoCs allow a device to simultaneously act as both a peripheral and a central, or as a peripheral advertising to multiple centrals. A BLE relay device, for example, might connect as a peripheral to a phone while acting as a central to downstream sensors, creating a two-hop topology that extends network reach.