Nuts and Volts - November 2008

design will receive 2.8 VDC on the Vdd power rail.

USB Power Detect

As described in the previous section, the Vdd power rail of the design can be either 4. 8 or 2.8 VDC. While the operation of the processor will not be affected (the PIC16F685 has a 2.0-5.5 VDC operating range), the LED array is a different matter. If the SW in the tree were left unaware, the difference in the Vdd power rails would appear as a very noticeable brightness difference in the LEDs. To prevent that, the design includes a trivial USB power detection circuit.

If a powered USB cable is connected, the USB.DETECT+ signal will be connected to the USB 5 VDC power pin which will result in the PIC16F685 I/O pin RA0 being seen as a logic 1 to software. If a powered USB cable is not connected, the USB.DETECT+ signal will be left floating and the 10K ohm, R23 resistor will pull the USB.DETECT+ signal to ground, which will result in the PIC16F685 I/O pin RA0 being seen as a logic 0 to software.

Note that while Tree 1.1 uses USB power, it will not appear as any device on your computer. It is only — what

we like to call — a power sucking alien. The peak current of 50 mA and the average current of 5 mA make Tree 1.1’s power consumption far too low to be noticed by the USB power management functionality in your computer. (The power management gets involved when a USB device wants more than 100 mA continuously.)

Pushbutton

There is one momentary on pushbutton on the design. This pushbutton is combined with the 10K pull-up resistor R22. The resulting signal PATT.CHG is connected to the MCLR- (Master CLeaR-) pin of the PIC16F685. When this button is pressed, the PATT.CHG signal will go low and reset the PIC16F685 through the MCLR- pin. Through a bit of clever programming, this single button functions as both the pattern change button when the tree is active and as the “wake up” button to reactivate a tree that has been turned off. See the software section for more details.

PCB Layout

■ FIGURE 2

The layout of this design (shown in Figure 2) was dictated almost entirely by its visual appearance. A typical design would use the electrical properties of the embedded components and/or their functional requirements as the driver for the layout.

Component Placement

The placing of the components was almost entirely driven by “What will the components look like on the board?” The LEDs, obviously, become the lights of the Christmas tree. But what of the other components; what is their role? Well, besides lights, decorations are the other typical item to put on a Christmas tree. Thus, the transistors, resistors, and diodes are all the decorations of the design.

If your house is anything like ours, the excited little ones around the Christmas tree during the decorating process don’t always get things just perfect. In that spirit, the decorations of our tree design are not “just perfect.” The decorations of this tree are clumped together here and spread out there, all to make the overall effect more realistic.

Routing

The last visual attribute that we worked on was the routing. In this case, the routing on the component side of the tree is the vertical routing layer. We deliberately chose vertical routing on the front of the tree to mimic tinsel on a real tree.