How to set supervision timeout?

Must satisfy: timeout > (1 + slave_latency) * CI * 2. Apple requires timeout >= CI * (slave_latency + 1) * 3 for stability. Typical values: 2-6 seconds for wearables, 10-20 seconds for slow sensors. Too short causes false disconnects in noisy environments; too long delays detecting a lost device.

What happens when it expires?

Both devices declare the connection lost and return to idle/advertising state. No explicit disconnect packet is exchanged. The application receives a disconnection event with reason 'connection timeout'. Reconnection requires new advertising and scanning, typically taking 1-5 seconds.

Why do connections drop unexpectedly?

Common causes: supervision timeout too short for the CI/latency combination, RF interference causing consecutive packet losses exceeding the timeout, device moved out of range, peripheral entered deep sleep and missed the timeout, or iOS/Android background restrictions throttling BLE scanning.

Wireless Protocols

BLE Supervision Timeout

BLE Supervision Timeout is the maximum duration a device will wait without receiving a valid packet from its connection peer before declaring the connection lost. Configurable from 100 ms to 32 seconds in 10 ms steps. It acts as a connection watchdog: if interference, range loss, or device failure prevents packet exchange for longer than this timeout, both devices independently terminate the connection and return to idle state.

Category: Wireless Protocols

Range: 100 ms - 32 s

Understanding Supervision Timeout

The timeout must be large enough to accommodate the connection interval and slave latency: the peripheral may legitimately not respond for (1+SL)×CI. The Bluetooth spec requires timeout > (1+SL)×CI×2. Apple recommends a 3× multiplier for robustness.

Too short a timeout causes false disconnects in noisy environments (RF interference, crowded 2.4 GHz). Too long delays detecting that a device has moved out of range or powered off, leaving the central scanning a dead connection.

Timeout Constraint

Bluetooth spec minimum:
T_sup > (1 + SL) × CI × 2

Apple recommendation:
T_sup ≥ (1 + SL) × CI × 3

Example: CI=100 ms, SL=4:
Min: (1+4) × 100 × 2 = 1000 ms
Recommended: 1500 ms (1.5 s)

Timeout Recommendations

Application	CI	SL	Timeout
Gaming HID	7.5 ms	0	500 ms
Wearable	30 ms	9	2 s
Sensor	500 ms	4	10 s
Asset tracker	4 s	0	20 s

Common Questions

Frequently Asked Questions

How to set it?

Must be > (1+SL)×CI×2. Apple requires ≥3× for stability. Typical: 2-6 s for wearables, 10-20 s for slow sensors.

What happens on expiry?

Both devices declare connection lost. No disconnect packet sent. App gets "connection timeout" event. Reconnection takes 1-5 s.

Unexpected drops?

Timeout too short, RF interference causing consecutive losses, out of range, deep sleep miss, or mobile OS background throttling.

Related Terms

← Slave Latency BLE Throughput →

BLE Reliability

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