Test: Bluetooth Low Energy (advertising, GATT and the coin cell)

Work in progress

This is a standalone test page for a demonstration under development. It is not yet part of a chapter. It continues the greenhouse from the MQTT and CoAP labs.

Wireless & IoT

Bluetooth Low Energy: Advertising, GATT and the Coin Cell

derekmolloy.ie

1. Advertising2. The packet3. Connections4. GATT5. Throughput6. Power7. BLE vs othersSandbox

Advertising: shouting into three channels

Next: The packet

Node 1 is now a XIAO nRF52840 on a coin cell: no Wi-Fi, no broker, just a 2.4 GHz radio that is off almost all the time. To be found, it advertises: every advertising interval it wakes, sends a short packet on channel 37, then 38, then 39, and goes back to sleep. Those three channels sit in the gaps between Wi-Fi channels 1, 6 and 11. A random 0 to 10 ms is added to each interval so two devices cannot stay in step. A phone scans by listening on one channel for a scan window, every scan interval, moving to the next channel each time. Discovery happens when a packet lands inside a window on the right channel. The waterfall shows frequency against time; the trace below is node 1’s current.

Challenge: Find settings where the phone takes more than 5 s on average to find node 1.

The advertiser

Every advertising interval, plus 0 to 10 ms at random: one packet on 37, then 38, then 39, then sleep.

interval 100 ms

The scanner

Listens on one advertising channel for a window, once per scan interval, moving 37 → 38 → 39.

100 ms in every 100 ms

Discovery

Happens when a packet lands inside a window on the same channel. Both duty cycles decide how long that takes.

-

Node 1 · XIAO nRF52840 · F4:12:FA:3B:9C:21

advertising every 100 ms · 0 dBm

Phone · central

not scanning

node 1 transmitsphone or Pi transmitsscan window✕ lostbusy Wi-Fi channel· real radio time, shown slowed down; packets are drawn at least 2 px wide

Start the phone scanning← Start here

Scan againReset

Pause

×0.1×¼×1

Show

500 ms5 s20 s

t = 0.00 s

Node 1 advertises every

20 ms100 ms1 s2 s

The phone scans

ContinuousBalanced (1 s in 4)Low power (0.5 s in 5)Off

Wi-Fi trafficch 1ch 6ch 11

Average to find (200 tries)

0.05 s

9 in 10 within

0.09 s

Worst

0.10 s

The three averages come from 200 simulated attempts with the scanner starting at a random moment. Phones choose their own scan settings: a foreground app gets near-continuous scanning, a background one much less, so a device a user is looking at in an app is found far faster. The advertising channels sit beside Wi-Fi channels 1, 6 and 11 rather than on top of them, so Wi-Fi traffic barely touches advertising (channel 37 is closest to Wi-Fi 1).

Radio log, newest first

Nothing on the wire yet.