I'm just going to dump all my discoveries here, and hopefully they would be useful to the Nintendo Switch community.
If you think something is wrong, have some observations that I might have missed, stuff you want to contribute, or just general questions please feel free to post here.
If you want to use the information here somewhere else, feel free to do so, but do please credit me (dekuNukem).
The "JC" is for the 10-pin Joycon connector, see below for details.
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Joycon runs at 1.8V
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There is no silkscreen marking component and test point numbers, maybe Nintendo is trying to discourage people from doing funky things to the Joycon?
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Also, in a bizarre move, Nintendo didn't use the traditional "one side pulled-up and other side to ground" way of reading buttons, instead they used a keypad configuration where buttons are arranged to rows and columns. They used the keypad scanner built-in inside the BCM20734 with 128KHz clock for reading the buttons. That means it would be extremely hard to spoof button presses for TAS and twitch-plays. Maybe the Pro controller is different, need to buy one though.
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The only button that's not part of the keypad is the joystick button, which is still activated by pulling it down to ground.
There are 2 SPI devices on the bus, one 4Mb MX25U4033E flash memory and one LSM6DS3 6-axis MEMS accelerometer and gyroscope.
Here is a capture of SPI lines when the Joycon battery is connected, and then attached to the console.
It looks like SCK runs at 12.5MHz when accessing the flash memory, but switches to 6.25MHz when accessing the MEMS chip.
Upon connection the microcontroller initializes a software reset of the MEMS chip, then set up the accelerometer and gyroscope as follows:
| Accelerometer | Gyroscope |
|---|---|
| ODR 1.66KHz, full-scale ±8g, AA filter bandwidth 100Hz, low-pass filter enabled, slope filter enabled with cut-off frequency at 416Hz. | ODR 208Hz, full-scale 2000dps |
The Joycon then polls LSM6DS3 every 1.35ms(740Hz) for both accelerometer and gyroscope data in all axises, totaling 12 bytes(6 axises, each axis 2 bytes).
Since the Joycon polls MEMS data every 1.35ms but only send out controller update every 15ms, there might be some internal averaging to smooth out the data, needs to go through the numbers to find out.
Well there's a capture of the SPI lines when the Joycon is powered up (battery connected), which contains all the address and data Joycon reads from the flash memory. I don't have time to go through it right now but of course you can if you want.
When attached to the console, the Joycon talks to it through a physical connection instead of Bluetooth. There are 10 pins on the connector, I'm just going to arbitrarily name it like this:
Looking at the pins on the left Joycon, the left most one is Pin 1, and the right most one is Pin 10. I simply removed the rumble motor, burned a hole on the back cover, and routed all the wires out through that.
And here is a capture of the docking of the left Joycon.
| Logic analyzer channel | Joycon Connector Pin | Function | Remarks |
|---|---|---|---|
| - | 1 | GND | - |
| - | 2 | GND | - |
| 0 | 3 | BT status? | Only high when connected to console via bluetooth, low when unpaired, sleeping, or attached to the console directly |
| 1 | 4 | 5V | Joycon power and charging |
| 2 | 5 | Serial data, console to Joycon | Inverted level (idle at GND) |
| 3 | 6 | Attach status? | GND only when directly attached to console and not sleeping, 1.8V otherwise. |
| - | 7 | GND | - |
| 4 | 8 | Serial data, Joycon to console | Standard level (idle at 1.8V) |
| 5 | 9 | ? | Always at GND |
| 6 | 10 | Flow control | Looks like RTS line, Joycon will only send data when this line is high |
- When first connected the baud rate is at 1000000bps(!), after the initial handshake the speed is then switched to 3125000bps(!!). The handshake probably exchanges information about the side of the Joycon, the color, and bluetooth address etc.
I took apart 2 left Joycons, one grey one red. Below you can see the difference in the response between two Joycons.
| Console to Joycon | GREY Joycon response | RED Joycon response | Different? | Remarks |
|---|---|---|---|---|
A1 A2 A3 A4 19 01 03 07 00 A5 02 01 7E 00 00 00 |
19 81 03 07 00 A5 02 02 7D 00 00 64 |
19 81 03 07 00 A5 02 02 7D 00 00 64 |
Same | Handshake start; 1000000bps |
19 01 03 07 00 91 01 00 00 00 00 24 |
19 81 03 0F 00 94 01 08 00 00 FA E8 01 31 67 9C 8A BB 7C 00 |
19 81 03 0F 00 94 01 08 00 00 8F 87 01 E6 4C 5F B9 E6 98 00 |
Different | Joycon info; possibly color; side; battery level; BT info; etc |
19 01 03 0F 00 91 20 08 00 00 BD B1 C0 C6 2D 00 00 00 00 00 |
19 81 03 07 00 94 20 00 00 00 00 A8 |
19 81 03 07 00 94 20 00 00 00 00 A8 |
Same | Command to switch to 3125000bps |
19 01 03 07 00 91 11 00 00 00 00 0E |
19 81 03 07 00 94 11 00 00 0F 00 33 |
19 81 03 07 00 94 11 00 00 0F 00 33 |
Same | ?; 3125000 bps from now on |
19 01 03 07 00 91 10 00 00 00 00 3D |
19 81 03 07 00 94 10 00 00 00 00 D6 |
19 81 03 07 00 94 10 00 00 00 00 D6 |
Same | ? |
19 01 03 0B 00 91 12 04 00 00 12 A6 0F 00 00 00 |
19 81 03 07 00 94 12 00 00 00 00 B0 |
19 81 03 07 00 94 12 00 00 00 00 B0 |
Same | ? |
19 01 03 08 00 92 00 01 00 00 69 2D 1F |
61B Controller status | 61B Controller status | Different | Handshake done. Console sends this controller status request command every 15ms from now on. |
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Pin 5 (Serial data, console to Joycon) is normally pulled high on the console side when nothing is connected. Since this line is inverted on the Joycon side, it will be pulled down when a Joycon is attached to the console, thus initializing a handshake.
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It seems Pin 5 needs to be pulled down for a while for the handshake to take place, 500ms works for me.
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Handshake starts at 1000000bps, and the console will send a 4-byte start sequence of
A1 A2 A3 A4, around 46us later followed by 12 byte command of19 01 03 07 00 A5 02 01 7E 00 00 00. It will send those commands repeatedly every 100ms (10Hz) for 3 seconds. Joycon respond with19 81 03 07 00 A5 02 02 7D 00 00 64. If no response is received it gives up and wait for another event on the line. -
The console then sends
19 01 03 07 00 91 01 00 00 00 00 24, to which Joycon respond with a 20-byte response that's different on each Joycon. That response definitely contains the color information of the Joycon, and also possibly contains the serial number, BT info, battery level, etc. After the response is received the little Joycon insertion animation starts on the screen. -
They console sends
19 01 03 0F 00 91 20 08 00 00 BD B1 C0 C6 2D 00 00 00 00 00, a command that switches baud rate from 1000000 to 3125000. Joycon respond with19 81 03 07 00 94 20 00 00 00 00 A8. Note that the faster baud rate takes effect from the next command. -
Now serial comm is at 3125000bps. Console sends
19 01 03 07 00 91 11 00 00 00 00 0E, Joycon responds with19 81 03 07 00 94 11 00 00 0F 00 33. -
Console sends
19 01 03 07 00 91 10 00 00 00 00 3D, Joycon responds with19 81 03 07 00 94 10 00 00 00 00 D6. -
Now the pairing is seemingly done, the console will now send
19 01 03 08 00 92 00 01 00 00 69 2D 1Fevery 15ms to ask for a controller status update. See "Protocol" section below for details.
It turns out the last byte of each command seems to be a checksum of some sort, and without figuring it out it would be rather difficult testing what each command does because the console will not accept commands with the wrong checksum.
Luckily here are some examples of the checksum, seeing it changes drastically with the difference of a single bit it's probably not some simple xor or modular checksum. If you can figure it out it would be really helpful.
19 01 03 07 00 91 10 00 00 00 00 3D
19 01 03 07 00 91 01 00 00 00 00 24
19 01 03 07 00 91 11 00 00 00 00 0E
19 81 03 07 00 94 10 00 00 00 00 D6
19 81 03 07 00 94 11 00 00 0F 00 33
The first 4 bytes are a header, with the 4th byte being the length of the remaining packet (not counting the checksum). The next 7 bytes are some type of data, with the 8th byte being the CRC of that data. The CRC used is CRC-8 with a polynomial of 0x8D and an initial value of 0x00.
There's some example code for calculating this CRC using a lookup table in packet_parse/joycon_crc.py.
In normal operation the console asks Joycon for an update every 15ms (66.6fps), the command for requesting update is:
19 01 03 08 00 92 00 01 00 00 69 2d 1f
Around 4ms later, Joycon respond with a 61 bytes long answer.
One sample:
19 81 03 38
00 92 00 31
00 00 e9 2e
30 7f 40 00
00 00 65 f7
81 00 00 00
c0 23 01 e2
ff 3e 10 0a
00 d6 ff d0
ff 23 01 e1
ff 37 10 0a
00 d6 ff cf
ff 29 01 dd
ff 34 10 0a
00 d7 ff ce
ff
The first 8 byte is always 19 81 03 38 00 92 00 31
The 16th and 17th byte (on line 5, before 65 f7) are the button status, when a button is pressed the corresponding bit is set to 1.
Byte 19 and 20 (f7 81 between 5th and 6th line) are the Joystick values, most likely the raw 8-bit ADC data. Byte 19 is X while byte 20 is Y. Again, bizarrely, the X value is reversed, as in the f7 should actually be 7f (127 at neutral position). The Y value is correct though (0x81 is 129).
Still working on decoding those... It has to contain battery level, button status, joystick position, accelerometer and gyroscope data, and maybe more.
Right now I only took apart left Joycons and yet to tear into the console itself, because I want to finish the Zelda first. But so far it looks like Nintendo make some really weird design decisions both in software and in hardware, probably to make what I'm doing more difficult. Anyway, I'll update this from time to time when I have new discoveries. Please share if you find this useful.