AVR Butterfly Carrier - Assembly Tips
The ECROS Technology Butterfly Carrier has been designed so that you can probably assemble it with parts in your spares draw. If you have to buy parts, the ones chosen for the design are inexpensive and available from many sources.
You will need some 0.1 inch pitch headers and matching sockets to put on the carrier and the Butterfly itself. You will also need headers for the 6-pin serial ISP and JTAG-ICE connections.
You can use any convenient 0.1 inch pitch header/socket system to connect the Butterfly and the Carrier. An obvious choice is the ubiquitous 0.025 inch square pin header system. You can also choose whether to put the pins on the Butterfly and the sockets on the Carrier of the other way around. However, if you want to take the Butterfly off the carrier and wear it using its pin, you should think this through before mounting the pins on it.
ISP and JTAG-ICE Headers
Your choice of header types for the ISP and JTAG-ICE connections is restricted by the need to mate with those devices. In most cases, you will need headers with 0.025 inch square pins.
The Butterfly Carrier has a simple power supply circuit which you can use to power the Butterfly in place of its 3 volt battery. There are diodes on the Butterfly itself that make it safe to supply external power without removing the battery. Power will be drawn from whichever source has the higher voltage. Refer to the Butterfly data sheet for the maximum voltage. ECROS Technology recommends 3.3 volts. The actual supply voltage on the Butterfly's ATmega169 MCU and other circuitry will be a few hundred millivolts less than the applied voltage due to the diode drop.
Power Jack and Protection Diode
A footprint is provided for a center-positive power jack of the type used by most wall-wart power supplies. If there is a possibility that a center-negative or AC wall-wart might be mistakenly connected, it is recommended that a protection diode be installed in the space provided. If this will not happen, you can put a wire link in place of the diode. A 6 to 9 volt DC wall-wart is recommended. Higher voltages are possible but care must be taken with power dissipation in the regulator. Lower voltages are possible with a low-dropout regulator. It may be possible to use an AC wall-wart, if that is all that's available, by installing the protection diode as a half-wave rectifier. A large-valued electrolytic capacitor must be installed for smoothing. This is not recommended and should be a last resort.
Battery Power Source
Test points are provided for the connection of wire leads to a battery power source. This bypasses the regulator, so you should use a voltage suitable for the Butterfly. The positive lead must be connected to the test point marked BAT+. If you connect the negative lead to the test point marked BAT-, it will be automatically disconnected when you connect a wall-wart to the power jack. If you do not install a power jack, or if it does not have the switch, connect the negative lead to the middle test point marked GND.
Two footprints are provided for the power regulator. One is for a TO-92 part and the other for TO-220. Use a regulator of the first type if its power dissipation will be low. Use a regulator of the second type for higher power dissipation due to current demand from circuitry you plan to add or a high supply input voltage. The position of the input, ground and output pins is fairly well standardized, but it is best to check before making your selection. The ST Microelectronics L78L33ACP, Mouser stock number 511-L78L33ACZ, is a good choice for up to 100 mA current and less than half a watt power. The middle lead of the device must be kinked towards the round side of the package to go into its PCB hole.
Most regulators require capacitors to be installed close to their input and output pins for stability. Consult your regulator data sheet for recommendations. The Butterfly Carrier provides footprints for two capacitors on each side of the regulator. Closest to the regulator are spaces for ceramic capacitors with leads spaced at 0.2 inches. Slightly further away are spaces for electrolytic capacitors with axial leads at a spacing of 0.1, 0.15 or 0.2 inches.
Be sure to install electrolytic capacitors with the correct polarity. The positive terminal is clearly marked on the silkscreen. Most axial electrolytic capacitors have a negative lead which is slightly shorter than the positive lead and the negative side is marked with a black bar on the body of the device.
Power Indicator LED
The anode of LED1 should go on the side towards the Butterfly where the top-side trace connects it to R1. The cathode should go towards the edge of the board. On most LEDs, the cathode lead is slightly shorter than the anode lead.
If you would like to be able to reset the Butterfly without cycling power, the Butterfly Carrier has a space to install a reset button. The PCB footprint matches a type of "6 mm" tactile-feedback switch made by many manufacturers. An example is Mountain Switch, Mouser stock number 101-TS6111T1602. If you don't need a reset button, there is no harm at all in simply leaving it off.
To make connecting to the AVR Butterfly serial interface more convenient, the Butterfly Carrier has a space to install a DB9 male or female connector. Suitable parts are made by many manufacturers. An example is the Kobiconn female receptacle, Mouser stock number 152-3409, which is included in the ECROS Technology Parts Kit.
All pins of the DB9 connector are brought to test points, with the exception of pin 5, which is connected directly to ground. The RXD and TXD pins of the Butterfly serial interface are brought to two more test points near the DB9 test points. All are clearly marked onthe PCB silkscreen, or you can refer to the board layout. You can wire RXD and TXD to the DB9 however you wish, depending on whether you use a straight-through or crossover cable and whether you have a DTE (computer) or DCE (modem) at the other end. If you use a straight-through cable to a COM port on a PC, wire RXD to pin 3 and TXD to pin 2.
Please note that Atmel's design of the serial interface driver circuitry of the AVR Butterfly is ... well, let's say it's very economical. The logic level signals are not accessible, so the Butterfly Carrier cannot improve on this situation. Don't expect to get perfect RS232 signal levels and be conservative in your choice of bit rate. ECROS Technology has tested Butterfly Carriers assembled with the Parts Kit at a bit rate of 19.2 kbit/s using TeraTerm Pro and a Dell Inspiron 4150 notebook computer.