Category: usb connection

I received my CP2102 usb to serial TTL converter bought on ebay from China for 2.8€ (about 4$). The delivery from China to my home (France) usually takes about 3-4 weeks, but this time I was lucky as it arrived in less than 2 weeks 🙂

As showed in the following picture, I connected it to my Teensy++:

Blue outlined, the CP2102 board, and red outlined, the Teensy++.
First tests were not so hopeful, as it doesn’t work as-is with the latest sixemu package (v0.12).

I had to deal with 2 problems:

1. Rx and Tx pins are for the target device.
2. The max speed reachable by the Linux driver is 921600 bit/s and not 1 Mbit/s as written in the datasheet.

I inverted two wires, and changed both the emitter (CP2102) and receiver (Teensy++) speed to 500Kbit/s.

My benchmark program shows that we can reach about 45bytes/ms only writing and about 40bytes/ms writing and reading at the same time. This is good enough for the application. Running at 250Hz, I measured a 6.7ms worst latency, and a 5ms average latency.

Conclusion: this cheap CP2102 converter from ebay seems usable! I will test it further playing over a long time to see if it’s reliable.

Warning: this hardware solution doesn’t work with sixemu v0.22 because the serial connection doesn’t work at the right speed (next release will solve this).

Moved: link.

sudo dfu-programmer at90usb162 erase ; sudo dfu-programmer at90usb162 flash at90usb162.hex

Done:

• write a clean joystick interface: 4 axes with 16-bit precision, 12 buttons (no PS3 button yet), and a hat (d-pad). This uses the excellent LUFA framework from Dean Camera.
• configure an interrupt for the serial reception (great tutorial from Dean Camera)
• write a RS232-capable emu process (docs).
• write a new Sixemugui

Sixemugui layout:

Results:

• 2ms update period = 500Hz update frequency (same as my G500 mouse) – not sure the PS3 can drive a usb device at that speed – not sure running as fast is relevant considering the Linux latency
• measured worst latency between a usb packet received by the PC from a peripheral (like a mouse) and the usb packet received by the device the joystick is connected to (for easiness, the PC – should be the PS3) is about 4.7ms, average latency is about 3.6ms

Notes:

• It’s probably better to avoid connecting both the mouse and the USB to serial device to the same usb bus.
• A kernel with realtime capabilities could probably lower the worst latency.
• It’s not good to work at a frequency higher than the mouse frequency (no mouse report in the last period => stick is set to the center => jerky movements).
• The dead zone doesn’t seem to be is proportional to the max absolute axis value.
• The report period is probably not constant without a realtime kernel (2ms is probably too low considering the Linux latency).
• For performance reasons, emu & emuclient could be merged in a single process.

Next tasks:

• build a package

Some time ago I announced my intention to work on a usb connection that can replace the bluetooth connection (it’s well known that the ps3 is compatible with some joysticks that comply with the USB HID specification).

Why do I want a usb connection?

• better precision for sticks (better than 8-bit precision)
• higher update frequency (better than 100Hz)
• good solution for a native windows controller
• challenge 😉

What hardware can be used?

This usb connection requires a system with 2 usb ports, one for the PC, and the other for the PS3. My first though was to use a VNC2, which is a programmable device with 2 usb ports. But the development tools are windows only 🙁

Widely distributed, available, and cheap devices with usb capabilities are the at90usb* boards (Teensy++, bumble-b, AvrUsbKey…). These only have a single usb port, which is not enough. But it’s possible to talk to these boards with other means like a UART (RS232, AKA serial connection).

USB to RS232 TTL devices are very easy to find and are very cheap (a few $/€). The theoretical maximum baud rate reachable with these devices is 1 Mbit/s = 125 Kbytes/s = 125 bytes/ms, which seems fast enough. As a comparison, the sixaxis sends about 64 bytes to the PS3 each 10ms.

Another good point for this at90usb + RS232 solution is that I already have all the hardware parts, as you can see in the following picture:

• Red outlined, the USB to RS232 TTL chip (FT232RL from FTDI). It’s on an arduino board, which is not required at all.
• Blue outlined, the bumble-b (at90usb162).
• Green outlined, the push-buttons required by the bumble-b to enter into dfu mode (other usb boards generally have on board push-buttons).

The RS232 connection requires as low as 3 wires (Gnd, Rx and Tx). Rx and Tx wires are crossed.

The Rx and Tx connectors of the arduino are for the AVR chip (which is absent on my board), and are crossed with the FT232RL. That’s why the arduino Rx pin is the FT232RL Tx, and the arduino Tx pin is the FT232RL Rx. I obviously fall into the trap 🙂

First experiments:

• The ps3 seems to handle 16-bit axes.
• The at90usb162 can report its joystick state each 1ms (but not sure the ps3 can work as fast).
• The practical max baud rate of the RS232 connection is about 85 bytes/ms bidirectional (instead of the 125 bytes/ms theoretical max baud rate).
• There seems to be no transmission error for the RS232 connection (I won’t handle errors in a first time).

Next tasks:

• write a clean joystick interface
• write a RS232-capable emu process