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