Wilfa Svart scale BLE Protocol

WilfaSvart_Scale_BLE_Protocol.md

Wilfa Svart BLE Scale Protocol Documentation

This document outlines the Bluetooth Low Energy (BLE) protocol for communicating with Wilfa Svart Uni Scale devices, specifically the Gen 1 and Gen 2 models, the former being the model powered by AA batteries, and the latter being rechargeable with a USB-C cable. The implementation is reverse-engineered from official firmware behavior and the Android application.

Overview

The Wilfa Svart scale exposes BLE services and characteristics for:

• Receiving weight data
• Monitoring unit changes and other state updates
• Sending commands (e.g., tare, change unit)

The protocol adapts dynamically depending on whether the connected scale is Gen 1 or Gen 2. Detection is performed via the presence of model-specific services during peripheral discovery.

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Key Differences Between Gen 1 and Gen 2

Feature	Gen 1	Gen 2
Unit support	Grams, Ounces, Pounds	Grams, Ounces
Weight characteristic	FFB2 (read + notify)	FFB1 (read + notify + write)
Notifications characteristic	FFB1 (read + notify)	FFB1 (read + notify + write)
Write characteristic	FEC7 (write-only)	FFB1 (read + notify + write)
Command separation	Commands sent to FEC7	Commands sent to FFB1
Identification service	FEE7	2600

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BLE Services and Characteristics

Common Scale Service (FFB0)

Present on both models and used for transmitting primary data.

Gen 1 Exclusive Write Service (FEE7)

Presence of this service signals Gen 1. Write characteristic located here.

Gen 2 Exclusive Identification Service (2600)

Presence of this service signals Gen 2. It is unused functionally but used to identify the model.

BLE Topology

Model	Service UUID	Characteristic UUID(s)
Gen 1	FFB0	FFB1 (notify), FFB2 (notify)
Gen 1	FEE7	FEC7 (write)
Gen 2	FFB0	FFB1 (read/notify/write)
Gen 2	2600	— (used only for identification)

This table summarizes the BLE topology for both models, linking each characteristic to its parent service UUID.

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Characteristics per Model

Gen 1

Requires references to three different characteristics.

• Weight Characteristic (FFB2) — Emits weight readings exclusively (notify only)
• Notifications Characteristic (FFB1) — Emits notifications like unit change, low battery, etc. (notify only)
• Write Characteristic (FEC7) — Command interface (write only)

Gen 2

Only requires a reference to one multipurpose characteristic.

• Unified Characteristic (FFB1) — Used for multiple purposes (read/notify/write):
  • Notifications like weight readings, unit changes, low battery warnings, etc.
  • Command interface

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Packet Format

Each BLE notification or response packet begins with a 2-byte header (Big Endian), followed by packet-specific data.

Packet Headers

Header	Description	Gen 1 Characteristic	Gen 2 Characteristic
0xFA01	Weight reading	FFB2	FFB1
0xFA03	Disconnect event	FFB1	FFB1
0xFA04	Unit changed	FFB1	FFB1
0xFA05	Low battery warning	FFB1	FFB1
0xFA06	Unstable scale	FFB1	FFB1
0xFA07	Scale overloaded	FFB1	FFB1

All non-weight-related notifications use the FFB1 characteristic on both generations. Only the weight reading differs between Gen 1 (FFB2) and Gen 2 (FFB1).

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Command Format

All commands are written as raw byte arrays to the write characteristic (either FEC7 on Gen 1 or FFB1 on Gen 2).

Tare Command

F5 10

Change Unit Command

F5 11 <unit_code>

Where <unit_code> is:

Unit	Code
Grams	00
Ounces	01
Pounds¹	02

¹ Only supported on Gen 1.

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Interpreting Unit Change Packets (0xFA04)

When the scale sends a packet with the header 0xFA04, it signals that the unit of measurement has changed. This is typically a result of a manual unit change on the scale (e.g., via button press), or in response to a successful command issued by the host (gen 2 only).

Packet Structure

Byte Index	Description
0–1	Packet header 0xFA04
2	Unit code (1 byte)

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Interpreting Weight Reading Packets (0xFA01)

Weight values are encoded as a 16-bit signed integer (Big Endian) at byte offset 2.

Packet Structure

Byte Index	Description
0–1	Packet header 0xFA01
2-3	Weight (2 bytes)

The raw value represents grams x 10, so to obtain the actual weight in grams:

weight_in_grams = (Int16 at offset 2) / 10

The value is always emitted in grams, regardless of the selected unit on the scale. This value must be converted on the fly to the selected display unit (grams, ounces, or pounds) using standard conversion factors, based on the unit that was last broadcasted with the FA04 packet.

Weight reading frequency

Weight reading notifications are issued roughly every 120 ms (i.e., ~8 times per second.)

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Handling Disconnections

The scale may proactively emit a 0xFA03 packet to indicate disconnection. On receipt, the service shall initiate a teardown of state and peripheral references.

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BLE Discovery Logic

To determine the model:

1. After service discovery:
   • If FEE7 is present → Gen 1
   • Else if 2600 is present → Gen 2
2. Configure characteristic subscriptions and writing based on the identified model.

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Summary

• Gen 2 simplifies hardware and protocol complexity by unifying characteristics.
• Gen 1 separates concerns across multiple characteristics.
• Both use a shared packet format and standard BLE idioms for data flow and command handling.

The latter three packet headers are documented here in Wilfa's own manual:

Header	Description
0xFA05	Low battery warning
0xFA06	Unstable scale
0xFA07	Scale overloaded