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tools/avrusb/circuits/Readme.txt

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+This is the Readme file for the AVR-USB example circuits directory.
+
+
+CIRCUITS IN THIS DIRECTORY
+==========================
+Since USB requires 3.3 V levels on D+ and D- but delivers a power supply of
+ca. 5 V, some kind of level conversion must be performed. There are several
+ways to implement this level conversion, see the example circuits below.
+
+with-vreg.png and with-vreg.sch (EAGLE schematics):
+  This circuit uses a low drop voltage regulator to reduce the USB supply to
+  3.3 V. You MUST use a low drop regulator because standard regulators such
+  as the LM317 require at least ca. 2 V drop. The advantage of this approach
+  is that it comes closest to the voltage levels required by the USB
+  specification and that the circuit is powered from a regulated supply. If
+  no USB cable is used (connector directly soldered on PCB), you can even
+  omit the 68 Ohm series resistors. The disadvantage is that you may want to
+  use other chips in your design which require 5 V. Please check that the AVR
+  used in your design allows the chosen clock rate at 3.3 V.
+
+with-zener.png and with-zener.sch (EAGLE schematics):
+  This circuit enforces lower voltage levels on D+ and D- with zener diodes.
+  The zener diodes MUST be low power / low current types to ensure that the
+  1k5 pull-up resistor on D- generates a voltage of well above 2.5 V (but
+  below 3.6 V). The advantage of this circuit is its simplicity and that the
+  circuit can be powered at 5 V (usually precise enough if the cable drop is
+  neglected). The disadvantage is that some zener diodes have a lower voltage
+  than 3 V when powered through 1k5 and the choice of components becomes
+  relevant. In addition to that, the power consumption during USB data
+  transfer is increased because the current is only limited by the 68 Ohm
+  series resistor. The zeners may even distort the signal waveforms due to
+  their capacity.
+
+with-series-diodes.png and with-series-diodes.sch (EAGLE schematics):
+  This is a simplified low-cost version of the voltage regulator approach.
+  Instead of using a voltage regulator, we reduce the voltage by the forward
+  voltage of two silicon diodes (roughly 1.4 V). This gives ca. 3.6 V which
+  is practically inside the allowed range. The big disadvantage is that the
+  supply is not regulated -- it even depends strongly on the power
+  consumption. This cannot be tolerated for analog circuits.
+
+tiny45-rc.png and tiny45-rc.sch (EAGLE schematics):
+  This is mostly an example for connecting an 8 pin device using the internal
+  RC oscillator for system clock. This example uses series diodes to limit
+  the supply, but you may choose any other method. Please note that you must
+  choose a clock rate of 12.8 or 16.5 MHz because only the receiver modules
+  for these frequencies have a PLL to allow higher clock rate tolerances.
+
+
+GENERAL DESIGN NOTES
+====================
+All examples have D+ on hardware interrupt INT0 because this is the highest
+priority interrupt on AVRs. You may use other hardware interrupts (and
+configure the options at the end of usbconfig.h accordingly) if you make sure
+that no higher priority interrupt is used.
+
+If you use USB_SOF_HOOK or USB_COUNT_SOF in usbconfig.h, you must wire D- to
+the interrupt instead. This way the interrupt is triggered on USB Start Of
+Frame pulses as well.
+
+Most examples have a 1M pull-down resistor at D+. This pull-up ensures that
+in self-powered designs no interrupts occur while USB is not connected. You
+may omit this resistor in bus-powered designs. Older examples had a pull-up
+resistor instead. This is not compatible with the zener diode approach to
+level conversion: 1M pull-up in conjunction with a 3.6 V zener diode give an
+invalid logic level.
+
+All examples with ATMega8/88/168 have D+ at port D bit 2 (because this is
+hardware interrupt 0) and D- on port D bit 4 because it is also a clock input
+for timer/counter 0. This way the firmware can easily check for activity on
+D- (USB frame pulses) by checking the counter value in regular intervals. If
+no activity is found, the firmware should (according to the USB
+specification) put the system into a low power suspend mode.
+
+
+
+----------------------------------------------------------------------------
+(c) 2008 by OBJECTIVE DEVELOPMENT Software GmbH.
+http://www.obdev.at/