Measuring wired split keyboard input latency

How to use a logic analyzer to measure input latency

A logic analyzer is like an oscilloscope except it can only measure digital voltage levels. Since it only needs to detect two voltage levels, the circuitry is simpler and hence cheaper to manufacture than an analog oscilloscope. The one I bought was about $10 and has a 24MHz sample rate, 8 channels and connects to your computer via USB. I used sigrok an open source program for working with logic analyzers and oscilloscopes to record data from my computer.

A simple USB logic analyzer.
A simple USB logic analyzer.

To measure input latency of a keyboard, we want to use the logic analyzer to record 3 kind different kinds of events

  1. When a key is pressed or released.
  2. When the firmware detects that the key has changed.
  3. When the firmware passes information over USB.

Measuring 1 is easy as we can just connect a probe to a column of the key matrix and set all the rows of the matrix low when we are not scanning the matrix. Then when any key in that column is pressed, the probe will see the signal pulled low. We can measuring 2 and 3 with a logic analyzer by connecting probes to any of the spare IO pins, and then modify the firmware to toggle those pins whenever the events occur.

Also, keep in mind that when measuring input latency, that the input latency is limited by the poll interval of a USB endpoint. For full speed USB2.0 devices the minimum poll interval is 1ms. So reducing the latency below 0.5ms won’t have much of an effect because our reports will on average always be ready before the USB endpoint is.

Measuring input latency of tmk/qmk’s Let’s Split keyboard

The Let’s Split is a split keyboard that use two pro micro’s, one in each half. One controller connects to the host via USB, while the other scans its matrix and transmits it to the host side over either a serial or an i2c connection.

Let’s Split serial

Let’s Split firmware uses a simple bi-directional, one-wire serial protocol at about 40k baud implemented in software. The master polls the slave for it’s key matrix in between scans of its own matrix. Here’s what a typical serial transaction looks like:

Let's Split serial transaction
Let's Split serial transaction

Let’s Split serial input latency

The setup I used was two pro micro’s on a breadboard each connected to an external matrix. Here’s the results from recording a typical key press on the master and slave sides:

Let's Split master key press using serial
Let's Split master key press using serial
Let's Split slave key press using serial
Let's Split slave key press using serial

As we can see the latency is about 15ms when the key is pressed from the master side, and 30ms when the key comes from the slave side. The latency is this bad primarily because the master polls the whole matrix state of the slave every iteration of the main loop.

We can also see that the slave key presses take twice as long to register as the master. This is because the master’s contiuous polling of the slave brings its scanning algorithm to a complete crawl.

An easy fix for the Let’s Split serial code would be to make it no longer rely on the master polling the slave, but instead have the “slave” be the one sending the messages whenever its matrix state changes. This way once the matrix change is detected, it would only about 2ms before the master is informed of the change.

Let’s split I²C

Let’s split code also supports an optional I²C mode. The I²C mode operates at 400k baud. However, because I was using a stale version of the repository, the version I tested was only operating at 100k baud (I only realized this after I had made the measurements).

Here’s what a typical I²C transaction looks like:

Let's Split i2c transaction
Let's Split i2c transaction

Let’s Split I²C input latency

Using the same test setup as before, here’s the results of the I²C mode at 100k baud.

Let's Split master key press with i2c
Let's Split master key press with i2c
Let's Split slave key press with i2c
Let's Split slave key press with i2c

It’s a bit strange this time as the slave key presses actually tend to register faster (7ms) than master key presses (~10ms). Again the reason for is because of the interplay between the debounce algorithm and the master continuously polling the slave. But most of the latency here is due to the debouncing algorithm used. And if I had used I²C at 400k baud, the debouncing algorithm would be the primary source of input latency.

Measuring input latency of my controller in wired mode

The controller I’m working on (I really need to come up with a name for it), also supports split wired keyboards via I²C. It uses hardware I²C of the atxmega32a4u, at 400k baud. It uses an interrupt based approach and only sends the minimal amount of information needed to convey changes in matrix state. Here’s what the latency looks like for my controller with the setup I used:

My controller local key press.
My controller local key press.
My controller foreign key press.
My controller foreign key press.

The debouncing algorithm that I used reasons that, if we know a switch is not bouncing, then detecting a change of state indicates the key has been pressed/released. Further changes are then not accepted for that key are then not accepted until the debouncing period is over.

Conclusion

It goes to show that it’s hard to know exactly how well you code performs unless you have a concrete way of profiling it. Also, the debouncing algorithm used in a keyboard can have a big effect on input latency.

Next time I’ll look at the latency of my controller in wireless split mode.

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