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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.
Bill of Materials
Below is a list of the components needed to build the SpiffChorder.
| reference | description |
|---|---|
| C1, C2 | 22pF ceramic capacitor. |
| C3, C5 | 100nF ceramic capacitor (decoupling). |
| C4 | 10uF electrolytic capacitor, min. 10V. |
| CON1 | 2×3 pin header, e.g. shrouded header from Harting. |
| CON2 | USB connector, alternatively cable with type-A plug. |
| CON3 | Connection to switches. Can be wired directly. |
| CON4 | Connection to LEDs. Optional. |
| D1, D2 | 3.6V zener-diode. |
| IC1 | Atmel ATmega168. |
| R1 | 2.2 kOhm resistor. |
| R2 | 4.7 kOhm resistor. |
| R3, R4 | 82 Ohm resistor. |
| R5-R7 | LED current limiting resistors. About 1 kOhm, depending on selected LEDs. |
| R8-R18 | Optional pull-up resistors. 10 kOhm or 4.7 kOhm. |
| X1 | 12.000 MHz crystal. |
Notes
- 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 is 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.