Samsung Galaxy S-Pen Waveform Capture

I was quite intrigued by the S-Pen that came with my Samsung Galaxy Note 4. According to the specifications listed on Samsung’s site, the S-Pen supplied with Samsung Galaxy Note 4 he S-Pen supports 11 bits (2048 levels) of pressure levels and can be detected at a 15 mm hovering distance from the phone surface.

Kristofer at Android Authority had done a teardown of the S-Pen a couple of years ago (it was an older model, but the principal should be largely the same). And as we know the S-Pen works similar to an RFID which is passively powered from the electromagnetic field generated by the phone. I thought it would be interesting to dig a bit deeper to see what the communication patterns look like.

Because the S-Pen is powered by the RF field via inductance coupling, we can use a pickup coil to detect the signals passed between the pen and the pone without having to take apart either. To do this, I dissembled a small speaker and used the voice coil as the pickup device (see picture below).

Coil

When the tip of the S-Pen is placed inside the coil near the screen the communication waveform can be picked up. Alternatively, you could wrap some magnetic wire around the pen-tip and it would work equally well.

SPen1

The communication waveform patterns can be then captured via an oscilloscope. The data wave form looks power optimized and each frame is roughly 15 ms in duration. The oscilloscope capture below shows the waveform within one and a half frame.

waveform

The RF carrier frequency appears to be in the 550 kHz to 560 kHz range (not a standard RFID frequency).

frequency

Here is what the baseline waveform looks like when the pen is not in the vicinity of the screen. Some small bursts of RF signals can be seen, which is presumably the “polling” signal used to detect the presence of the S-Pen.

NoPen

Unfortunately I do not have the equipment to further decode the signal, I did however captured some waveform data with a few different usage modes (e.g. pen hovering, pen pressing, etc.) using my Rigol DS1052E in long memory mode and the data along with the descriptions are included towards the end of the post in case anyone is interested in trying to decode the protocol.

There are a few things we can deduce from nevertheless. Because the carrier frequency is relatively low, this is strictly near field communication. This means that the screen can sense the location of the pen by detecting the dip in the RF fields strength. This makes sense as there is no simple way for the pen to be actively monitoring and transmitting its location information.

The only information that is transferred between the pen and phone is the 11 bit pressure sensitivity along with the 1 bit value that denotes whether the S-Pen button on/off. The remaining encoded information should contain some value that identifies the presence of the S-Pen.

Captured waveform data

waveformdata.tar.gz

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12 Comments

  1. pelrun says:

    The S-Pens all use Wacom’s Penabled technology, which is also used by every Wacom-compatible tablet PC (unlike wacom’s dedicated tablets, which use a variety of other proprietary protocols.) So any Wacom PC pen will work on the Note and vice versa. That includes pens with an ‘eraser’ end!

  2. […] A few years ago, Wacom, the company behind all those cool graphics tablets, teamed up with Samsung to create the S Pen, a rebirth of that weird pen computing thing that happened in the 90s and a very interesting peripheral if only someone would write some software for it. [Kerry D. Wong] was wondering how the S Pen worked and wired up some hardware to take a look at how the pen communicates with the phone. […]

  3. […] A few years ago, Wacom, the company behind all those cool graphics tablets, teamed up with Samsung to create the S Pen, a rebirth of that weird pen computing thing that happened in the 90s and a very interesting peripheral if only someone would write some software for it. [Kerry D. Wong] was wondering how the S Pen worked and wired up some hardware to take a look at how the pen communicates with the phone. […]

  4. […] A few years ago, Wacom, the company behind all those cool graphics tablets, teamed up with Samsung to create the S Pen, a rebirth of that weird pen computing thing that happened in the 90s and a very interesting peripheral if only someone would write some software for it. [Kerry D. Wong] was wondering how the S Pen worked and wired up some hardware to take a look at how the pen communicates with the phone. […]

  5. RIP brave little speaker.
    You gave your life so that we may gain information on the Samsung S-Pen™.

  6. […] A few years ago, Wacom, the company behind all those cool graphics tablets, teamed up with Samsung to create the S Pen, a rebirth of that weird pen computing thing that happened in the 90s and a very interesting peripheral if only someone would write some software for it. [Kerry D. Wong] was wondering how the S Pen worked and wired up some hardware to take a look at how the pen communicates with the phone. […]

  7. […] A few years ago, Wacom, the company behind all those cool graphics tablets, teamed up with Samsung to create the S Pen, a rebirth of that weird pen computing thing that happened in the 90s and a very interesting peripheral if only someone would write some software for it. [Kerry D. Wong] was wondering how the S Pen worked and wired up some hardware to take a look at how the pen communicates with the phone. […]

  8. circuitsense says:

    i tried the samething a few weeks ago, similar results….if anything from here helps…
    http://rishifranklin.blogspot.in/2015/02/samsung-s-pen-internals.html

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