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131 lines
5.1 KiB
Markdown
131 lines
5.1 KiB
Markdown
# USB4VC Technical Notes
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[Get USB4VC](https://www.kickstarter.com/projects/dekunukem/usb4vc-usb-inputs-on-retro-computers) | [Official Discord](https://discord.gg/HAuuh3pAmB) | [Getting Started](getting_started.md) | [Table of Contents](#table-of-contents)
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-----
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This document contains technical information for USB4VC, which should be helpful for tinkering, contributing, and making your own Protocol Cards.
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Still mostly under construction, aiming to added more details before public release in April. In the meantime, if you have any particular technical questions or comments, l[et me know!](#questions-or-comments)
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## Linux Input Event Codes
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Raspberry Pi runs Linux, which uses **Input Event Codes**. Here's a very simplified description:
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* Connected input devices show up in `/dev/input` as files:
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```
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$ ls /dev/input/
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event0 event1 event2 mice mouse0 mouse1
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```
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* Reading from one of those files returns input events. Try `cat /dev/input/mice`!
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* On RPi, **16 Bytes** are returned each time there is an event. They are arranged as follows:
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| Byte | Name |
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|-------|-------------|
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| 0-7 | Timestamp |
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| 8-9 | Event Type |
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| 10-11 | Event Code |
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| 12-15 | Event Value |
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* A list of all Event Type, Code and Values [can be found here](https://github.com/torvalds/linux/blob/master/include/uapi/linux/input-event-codes.h)
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* Keyboard keys and mouse/gamepad buttons are EV_KEY type, mouse moments are EV_REL (relative) type, and joystick moments are EV_ABS (absolute) type.
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* Once you know the Event Type, you can look up Event Code to see which key/button/axes is being updated.
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* Event Value will be 1 (pressed) or 0 (released) for buttons, and a signed value for relative or absolute axes.
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* [Here's a good article](https://thehackerdiary.wordpress.com/2017/04/21/exploring-devinput-1/) that goes into a bit more details.
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* You can install `evtest` and see what your device is returning in real time.
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* USB4VC reads those events from all input devices, processes them, and send them out to the Protocol Card.
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## Hardware Pinout
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The Protocol Card connector is the same as the Raspberry Pi Header, although the pins are flipped around.
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Most of the pins are already in use:
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## SPI Communication Protocol
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Raspberri Pi communicates with Protocol Card through SPI. [Here's a quick introduction](https://www.circuitbasics.com/basics-of-the-spi-communication-protocol/) if you're unfamiliar.
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RPi is master, P-card is slave. Mode 0 is used (CPOL and CPHA both 0), SCLK is 2MHz.
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[include a sample capture.]
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RPi and P-Card communicates via fixed-length 32-byte packets. Detailed description of messaging protocol [can be found here](https://docs.google.com/spreadsheets/d/e/2PACX-1vTDylIwis3GZrhakGK0uXJGc_SAZ_QwySmlMfZXpSdFDH6zoIXs1kHX7-4wUTeShZth_n6tJH8l3dJ3/pubhtml#
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).
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## Latency Information
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Input latency can be introduced during all stages of input chain. Using keyboards as example, we have:
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* Step 1: Switch physically depressed
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* Step 2: Keyboard sends out event on USB/Bluetooth
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* Step 3: Raspberry Pi processes the event and informs the Protocol Card
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* Step 4: Protocol Card sends out the keystroke to retro computer
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* Step 5: Retro computer responds to the keystroke.
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The part we're interested in is **Step 3 and 4**, as any time spent here is the additional delay from USB4VC.
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To accurately measure the delay, I used a logic analyzer connecting to keyboard USB lines and PS/2 output on the IBM PC Protocol Card. Here is the delay in action:
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From the keystroke appearing on USB, to keystroke appearing on PS/2, it took 487 *microseconds*.
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Zoomed out capture with multiple keypresses:
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I tested out different generations of Raspberry Pis, and here is the result:
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| RPi Generation | Avg. Latency |
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|----------------|---------|
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| 2 | 1ms |
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| 3 | 0.63ms |
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| 4 | 0.5ms |
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You can find the [capture files here](captures/latency), open with [saleae app](https://www.saleae.com/downloads/), search `PID ACK` for USB input events, more info [in this video](https://www.youtube.com/watch?v=wdgULBpRoXk)
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## Developing your own Protocol Card
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USB4VC will send out keyboard and mouse event on SPI regardless of whether a protocol card is inserted or not. Although it will display appropriate options for switching protocols if the protocol card ID is recognized.
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SPI Format, AVR based arduino probably wont work, suggested to use STM32, include link.
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RPi Header pinout, explain what each pin does.
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Current limits etc.
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## Questions or Comments?
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Feel free to ask in official [Discord Chatroom](https://discord.gg/HAuuh3pAmB), raise a [Github issue](https://github.com/dekuNukem/USB4VC/issues), [DM on Twitter](https://twitter.com/dekuNukem_), or email `dekunukem` `gmail.com`!
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## Table of Contents
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[Main page](README.md)
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[(Youtube Video) USB4VC in Action](https://www.youtube.com/watch?v=H8XEUh1vhFY)
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[Getting Started Guide](getting_started.md)
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[Kit Assembly Guide](kit_assembly.md)
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[Technical Notes](technical_notes.md)
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[Launch Plan / Kickstarter Info](kickstarter_info.md)
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