pitfalls.txt

利用AVR单片机实现的慢速USB设备协议

If something does not work as it should, please check the following list.
It is compiled from the feedback we received so far.


FIRMWARE:
- If you use an ATMega or other device with internal RC oscillator, make sure
  that you flash the fuse for the external crystal oscillator.
- Make sure that all USB inputs (D+, D- and the interrupt) are configured as
  inputs with no internal pullup resistor. You must write a "0" to the port
  output bit and to the data direction bit of each input line.
- Make sure that you have updated the defines in usbdrv.h to reflect your
  pin assignments. D- must be bit 0 of a port.
- Make usbdrv.o the first object in linking. The USB receive buffer must not
  cross a 256 byte boundary of the RAM address space. Alternatively, you can
  put the buffer in a separate segment and instruct the linker to order this
  segment first. To verify the addresses assigned to the USB buffers use
      avr-nm -n main.bin | grep usbRxBuf
  and ignore the 0x800000 offset.
- If you need a very long cable, try the define "USB_SAMPLE_EXACT". You may
  be able to extend the cable range. The driver should work with the maximum
  USB cable length without this define, though.


HOST DRIVER:
- On Linux you must be root in order to use libusb (unless you have configured
  hotplug appropriately). The command line utility must therefore be executed
  by root.
- Our host software can be compiled on Windows using MinGW and libusb-win32.
- libusb-win32 version 0.1.10.1 (others have not been tested) has a bug with
  suspend mode. Once the PC was in suspend mode, the device is "lost".


HARDWARE:
- EMI: Our reference implementation is susceptible to EMI problems. We found
  out that the chip performs a reset although the external reset pin does
  not change level (checked with a DSO connected to reset and triggered by
  USB reset). This may be a consequence of over-clocking (we had the AT90S2313
  in stock and it just worked). We recommend EMI precautions in any case:
  (1) Use the watchdog timer of the device, (2) generate an USB reset state
  for an extended period of time after device reset (this is outside the USB
  spec, but it works: the hub or host interprets it as disconnect). Otherwise
  the host's and device's notion of the device-ID may get out of sync.
- If you use the EEPROM, it is very likely that you need some kind of brownout
  protection. A capacitor of several 100 microfarads in the supply is enough
  to reliably cause brownout problems. See Atmel's Application Note AN180
  for more information. Your local supplier's price table may be a better
  selection guide for brownout protection chips, though...
- Operating Voltage: PowerSwitch has been designed to operate at 3.5 V in
  order to meet the data signal voltage specification. The circuit does
  usually work without a regulator (directly powered by the 5 V from USB).
  USB 1.1 even demands that all devices withstand exposure to the bus supply
  voltage for an extended period of time. However, since these logic levels
  violate the specification, they exceed the common mode range of some USB
  I/O driver chips found in some hosts and hubs. We therefore recommend that
  you either reduce the CPU's supply voltage or add zener diodes to USB D+ and
  D- to clamp the voltage at 3.6 V.


AVR DEVICES CONFIRMED TO WORK:
Although the driver should work with all AVR microcontrollers which meet the
criteria, it may be good to know that somebody has actually tried a particular
chip. If you have built an AVR-USB project with a chip not listed here,
please drop us a note. The following chips have been tested:

  AT90S2313   works, although it is overclocked
  ATTiny2313
  ATMega8
  ATMega88
  ATMega8535
  ATMega32