Usbflattiny
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
- ATtiny25/45/85 datasheet (PDF)
- Application Note AVR042: AVR Hardware Design Considerations (PDF)
- Application Note AVR053: Calibration of the internal RC oscillator (PDF)
- Application Note AVR911: AVR Open-source Programmer (PDF
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). |
gEDA/PCB 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
