Usbflattiny

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usbflattiny 0.1 schematics
usbflattiny 0.1 schematics
usbflattiny 0.1 top
usbflattiny 0.1 top
usbflattiny 0.1 bottom
usbflattiny 0.1 bottom

usbflattiny is a small Free Software and Free Hardware USB dongle for various purposes.

The microcontroller firmware is GPL (version 2 or later) and uses V-USB (GPL version 2), the schematics and PCB layouts are licensed under the CC-BY-SA 3.0 license.

Downloads

The latest schematics, PCB layouts, and the microcontroller firmware are available from gitorious: 

$ git clone git://gitorious.org/usbflattiny/usbflattiny.git

The Gerber files, schematics, and some PNG images of the 0.1 version of the board are available from: 

$ wget http://randomprojects.org/wiki/images/f/f3/Usbflattiny-0.1.zip

Design

  • The usbflattiny hardware is a tiny (20mm x 12.25mm), bus-powerered USB 1.1 device with an Atmel ATtiny45-20SU microcontroller.
  • It can behave as a HID device, or alternatively other USB classes.
  • The device has four LEDs so you can create blinking stuff with it.
    • During normal operation (usbflattiny is connected to USB), four LEDs can be controlled by the ATtiny45 via the pins PB1, PB3, PB4, and PB5.
    • In addition, it is possible to control the LEDs from the USB host (using a small libusb-based user-space tool).
  • The PCB/design (2 layers) is SMD-based, the ATtiny45 is an SO8 chip, all resistors, capacitors etc. are 0603-sized SMD components.

Free Software tools used

  • The hardware is designed using the Free Software EDA tools gEDA/gschem and PCB.
  • The firmware is written in C, using avr-gcc, avr-libc, and the GPL'd software-USB implementation V-USB (previously known as AVR-USB).
  • Any host USB software (if needed) will be implemented using libusb.
  • For programming the ATtiny45 any AVR ISP programmer can be used, e.g. the Free Software and Open Hardware usbprog.
    • The host-software for the ISP programmer is likely the Free Software avrdude in most cases.

ISP programming

  • The ATtiny45 is flashed via any AVR ISP programmer (e.g. usbprog) connected to the MISO, MOSI, SCK, RESET, VCC, and GND pins.
    • There is no standard 6-pin or 10-pin ISP header on the PCB (in order to reduce the PCB size).
    • Instead, flashing is done by attaching tiny probes directly to the ATtiny45, or temporarily soldering some wires to the ATtiny45 pins.
  • GND on the ISP programmer must be connected to GND on the ATtiny45 (i.e., they must have a common ground).
  • The ISP programmer powers the ATtiny45 while it's being programmed. During this time usbflattiny must not be connected to USB.

USB

  • We use the GPL'd software-USB implementation V-USB (previously known as AVR-USB).
  • Planned config: 
#define USB_CFG_IOPORTNAME      B
#define USB_CFG_DMINUS_BIT      0
#define USB_CFG_DPLUS_BIT       2

Datasheets

Parts list

Quantity Device Footprint Value Refdes CSD Price Comments
1 Atmel ATtiny45-20SU SO8 — U1 1.45 Euro —
1 Capacitor 0603 100nF (0.1uF) C1 0.04 Euro —
2 Z-diode SOD80 3V6 D5, D6 0.05 Euro —
4 LED 0603 — L1, L2, L3, L4 0.08 Euro —
1 Resistor 0603 2k2 R7 0.05 Euro —
4 Resistor 0603 390R R1, R2, R3, R4 0.05 Euro —
2 Resistor 0603 68R R5, R6 0.05 Euro —

PCB

PCB production settings

I plan to have pcbcart.com produce the usbflattiny PCBs.

Setting Value Comments
Material FR4 —
Layers 2 layer —
Material Details Standard Tg 140C —
Part number usbflattiny —
Board type single unit —
Board Size (width) 20mm There will be a popup saying that boards this small may cost a bit more. Just click OK and ignore it, the price difference seems to be neglectable.
Board Size (height) 12.25mm There will be a popup saying that boards this small may cost a bit more. Just click OK and ignore it, the price difference seems to be neglectable.
Quantity ... Choose how many boards you want (try out different numbers, price varies a lot with quantity).
Thickness (Finished Board) 1.6mm —
Surface Finish ENIG - Electroless Nickle / Immersion Gold - RoHS You should use ENIG as finish — not "Hot air solder leveling" (HASL) — because the solderstop tends to peel off when using HASL.
Copper Weight (Finished) 35um —
Min. Tracing/Spacing 0.2mm (8mil) —
Min. Annular Ring 0.3mm (11.8mil) —
Smallest Holes 0.4mm (15.8mil) —
Holes Numbers Under 300 —
Surface Mount 2 sides —
Soldermask Both Sides —
Peelable Soldermask None —
Soldermask Color Green Choose whichever you like, but price may change.
Matt Color None Only possible for green and black; price difference is neglectable.
Silkscreen Legend 2 sides Removing silkscreen does not make the PCBs much cheaper.
Silkscreen Legend Color White —
Gold Fingers No —
Gold Fingers Number   Leave this field empty.
Gold Fingers Chamfer None —
Slots in Board No Slot in Board —
Slots quantity in board   Leave this field empty.
Testing Yes You really, really want this (so-called E-Test in German)! At pcbcart.com it's always performed, at other manufacturers this may not be the case.
UL Marking No —
Date Code Marking No —
Lead Time in 12 days The setting in 8 days will be slightly more expensive (but not much).

DRC settings

I used the following Design Rule Check (DRC) settings in the PCB software (File -> Preferences -> Sizes):

Setting mil mm
Minimum copper spacing 8.0mil 0.203mm
Minimum copper width 8.0mil 0.203mm
Minimum touching copper overlap 8.0mil 0.203mm
Minimum silk width 8.0mil 0.203mm
Minimum drill diameter 15.8mil 0.400mm
Minimum annular ring 11.8mil 0.300mm

I used the following route style:

Setting mil mm
Line width: 8.0mil 0.203mm
Via hole: 15.8mil 0.400mm
Via size: 39.4mil 1.000mm1
Clearance: 8.0mil 0.203mm

1 Via size = via hole + 2 * min.annular ring = 0.4mm + 0.3mm + 0.3mm = 1.0mm.

Finished PCBs


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