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SpiffChorder hardware

The image below shows the schematics for the SpiffChorder hardware. The interface to the USB connection is handled with a few passive components. Since the USB communication is handled by bit-banging in the software, an accurate clock source is needed. A 12.000 MHz crystal provides this. Since the beginning of the SpiffChorder project, the AVR USB stack has been expanded to also work at 16 MHz and 16.5 MHz, but this has not been tested with the SpiffChorder.

SpiffChorder hardware schematics (click for larger version)

Bill of Materials

Below is a list of the components needed to build the SpiffChorder.

C1, C222pF ceramic capacitor.
C3, C5100nF ceramic capacitor (decoupling).
C410uF electrolytic capacitor, min. 10V.
CON12×3 pin header, e.g. shrouded header from Harting.
CON2USB connector, alternatively cable with type-A plug.
CON3Connection to switches. Can be wired directly.
CON4Connection to LEDs. Optional.
D1, D23.6V zener-diode.
IC1Atmel ATmega8.
R12.2 kOhm resistor.
R24.7 kOhm resistor.
R3, R482 Ohm resistor.
R5-R7LED current limiting resistors. About 1 kOhm, depending on selected LEDs.
R8-R14Optional pull-up resistors. 10 kOhm or 4.7 kOhm.
X112.000 MHz crystal.


  • C1 and C2 should ideally match the load capacitance Cl of the crystal X1. If Cl is not known, 22pF for C1 and C2 seems to work.
  • CON1 is for in-system programming. If a socket is used for the ATmega8, this is not needed. Also, current implementations of the SpiffChorder allows the use of a boot loader working over the existing USB connection. This allows the firmware to be updated without special programming hardware. But the boot loader needs to be installed in the ATmega8 before it is soldered in place (unless the CON1 connector is mounted).
  • CON2 is a USB connector. Use either a type-B receptacle, in combination with a USB type-A to type-B cable. Or find a USB cable with a type-A plug in one end, and cut off the other end. The schematics show the standard colors for the USB connections.
  • CON3 if for connecting the switches. A connector is not needed, as the switches can be wired directly. Each switch should connect one of the inputs to ground when pressed. In other words a normally open (NO) type switch should be used.
  • CON4 provides the connection to 3 status LEDs. These show the current mode of the SpiffChorder, and are therefore optional. Again the LEDs can be wired directly, no connector is needed. If the LEDs are not mounted, the current limiting resistors R5-R7 can be omitted as well.
  • IC1 is the ATmega8. The schematics show the pin-numbering for the P version (DIL package). If using the SMD version, use the pin-names instead. The ATmega8L should not be used, as it does not support running at 12MHz. I have used ATmega8-16PU, but this is an old non-ROHS compatible part. Any of the -16PC, -16PI or -16PJ will also work.
  • R5-R7 are current limiting resistors for the (optional) LEDs. The value of these depend on how bright the LEDs should be, and the voltage drop across the LED. For newer LEDs, a value of 1 kOhm seems to give plenty of light, but you can go as low as 220 Ohm, resulting in about 20 mA current through each LED. Note that this is a considerable amount of current compared to the rest of the SpiffChorder.
  • R8-R14 are optional pull-up resistors. These should only be needed if the wires between the microcontroller and the switches are excessively long. A value of 10 kOhm or 4.7 kOhm should work, but remember that the lower the value, the greater the current consumption when the switch is pressed.
spiffchorder/hardware.1199648428.txt.gz · Last modified: 2008/01/06 14:40 by spiff
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