2 layers75.7 × 59.4 mm
2 layers40.6 × 50.8 mm
WALTsm
google/walt474A device for measuring latency of physical sensors and outputs on phones and computers
- Size
- 40.6 × 50.8 mm
- Copper layers
- 2
- Thickness
- 1.6 mm
- Parts
- 26
- Nets
- 42
- Pads
- 105
- Tracks, vias
- 117, 3
- Pours
- 6
- Routed
- Yes
- KiCad
- KiCad 4
- Created
- 26 Feb 2016
- Updated
- 7 Nov 2025
Overview
WALTsm is an open-source printed circuit board by google, from the google/walt repository on GitHub, published under the Apache-2.0 licence. It is a 2-layer board of 40.6 × 50.8 mm with 26 parts on 42 nets, routed with 117 tracks and 3 vias.
It is built around the LM321MFX and ADXL335BCP. Its parts include 8 resistors, 4 diodes, 3 ICs, 3 connectors and 2 capacitors. It belongs with the sensors and measurement boards and the NXP (LPC, Kinetis, i.MX RT) boards of this library.
Main components
2README
README.mdWALT Latency Timer
DISCLAIMER: This is not an official Google product.
- Post about WALT on Android Developers Blog
- Instructional videos showing how to use WALT
- Detailed usage instructions
- Mailing list - walt-discuss
- Low-traffic mailing list for major announcements walt-announce
WALT is designed to measure the latency of physical sensors and outputs on phones and computers. It can currently perform the following measurements:
- Tap latency - time from the moment a finger-like probe touches down (or up) on the screen until the kernel timestamps an ACTION_DOWN (or ACTION_UP) event. This physical contact with the screen is timed using an accelerometer mounted on the probe.
- Drag latency (scroll).
- Screen draw latency - using a photodiode that detects whether the screen is black or white.
- Audio output and microphone latencies.
- MIDI input and output latencies
The WALT app for Android can be installed from Google Play or downloaded in the releases section; the iOS app must be built from source.
Notes
- Hardware build instructions can be found in this repository under
hardware/. - Clock synchronization details are described here.
- The Android/iOS device and Teensy clocks have a tendency to diverge due to differing clock frequencies. This means they will go out of sync after several minutes. The workaround is to use the app to re-sync the clocks. Some, but not all tests in the app will sync the clocks when starting a measurement.
- Python code used to communicate with WALT from Linux and ChromeOS can be found here.
Parts
26| Qty | Value | Footprint | References |
|---|---|---|---|
| 3 | 100 | walt_footprints:R_0603_pad07mm_long | R5, R7, R8 |
| 2 | 0.1uF | Capacitors_SMD:C_0805 | C1, C2 |
| 2 | 330 | walt_footprints:R_0603_pad07mm_long | R1, R2 |
| 2 | CONN_01X06 | walt_footprints:Pin_Header_Straight_1x06 | P1, P2 |
| 2 | LED | walt_footprints:R_0603_pad07mm_long | D3, D4 |
| 2 | Photodiode | walt_footprints:BPW34_DIP2 | D1, D2 |
| 1 | 3,3k | walt_footprints:R_0603_pad07mm_long | R4 |
| 1 | 3.3k | walt_footprints:R_0603_pad07mm_long | R6 |
| 1 | 510k | walt_footprints:R_0603_pad07mm_long | R3 |
| 1 | ADXL335 | Housings_DFN_QFN:QFN-16-1EP_4x4mm_Pitch0.65mm | U2 |
| 1 | JACK_TRRS | walt_footprints:TRRS_SJ_43514 | J1 |
| 1 | LM321MFX | TO_SOT_Packages_SMD:SOT-23-5 | U3 |
| 1 | TeensyLC_with_headers | walt_footprints:Teensy_DIP-28_W15.24mm | U1 |
The 20 most common of 26 parts.
Questions
How big is the WALTsm PCB?
It measures 40.6 × 50.8 mm and has 2 copper layers, 1.6 mm thick.
What parts does the WALTsm use?
26 parts, built around the LM321MFX and ADXL335BCP. The full list of its parts is on this page, with links to each part's symbol and footprint where PCBKit's library has them.
Where can I download the WALTsm KiCad files?
From the google/walt repository on GitHub, at the commit this page was made from: the board is hardware/kicad/WALTsm.kicad_pcb. PCBKit opens the board in the browser, with nothing to install.
Can I use the WALTsm design in my own project?
Its repository publishes it under the Apache-2.0 licence: you may reuse it on that licence's terms (keep the author's notice, and for a copyleft licence share your changes alike).
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