January 2005, Issue 174

Embedded Wi-Fi with TRENDnet

---

Wi-Fi HARDWARE

Like the four Wi-Fi main board subsystems, the Wi-Fi PCB’s design is predicated on the CompactFlash Wi-Fi radio and its form factor. Photo 1 shows the four Wi-Fi design subsystems.

(Click here to enlarge)

Photo 1—There are so few components here because the key to the Wi-Fi board’s operation is in the firmware. The TRENDnet Wi-Fi card can do a lot if you know how to ask.

I used a four-layer PCB to minimize noise and create a cleaner PCB layout. The CompactFlash Wi-Fi radio interfaces to the Wi-Fi design’s host controller by way of an off-the-shelf surface-mount CompactFlash card receptacle. The four doublewide rows of mounting posts surrounding the baseline QFP land pattern are connected to all four of the Wi-Fi design’s universal subsystems in accordance with the Wi-Fi main board circuitry depicted in Figure 1.

(Click here to enlarge)

Figure 1—You can clearly see the four Wi-Fi main board subsystems here. The bulk of the electronics is contained within the TRENDnet Wi-Fi card. Depending on the host controller you select, the microcontroller/microprocessor subsystem can be as simple as a single IC.

If you don’t implement the baseline microcontroller in your design, you can fabricate a separate daughterboard that contains your selected microcontroller or microprocessor and its programming/debugging circuitry and then plug it into the 0.1"-centered double-row positions. The Wi-Fi main board’s daughterboard feature allows any form factor from DIP to QFP to use the Wi-Fi main board’s remaining three universal subsystems. In addition, any microcontroller core voltage, I/O pin voltage level conversion circuitry, or programming/debugging interface can be accommodated using the daughterboard configuration.

I chose a half-sized crystal oscillator instead of the standard crystal/capacitor clock configuration. A socketed crystal oscillator makes it easier to change clock frequencies. Most standard crystal/capacitor oscillator tanks require different types of crystals and different values for the associated capacitors; it depends on the microcontroller or microprocessor you’re using. The socketed crystal oscillator design point eliminates the need to design the PCB to accommodate the divergent microcontroller and microprocessor oscillator configurations.

Most programming/debugging interfaces are either JTAG-based or proprietary to the particular microcontroller in use. I’ve found that a 10-pin programming/debugging interface is normally sufficient. However, I have used 15-pin programming/debugging interfaces. Rather than try to provide all of the possible programming/debugging interface configurations on the main Wi-Fi PCB, I decided to include a 10-pin interface to satisfy the needs of the baseline microcontroller and leave the micro-specific programming/debugging interface to be implemented on the daughterboard.

I always beat myself up when I neglect to include power supply points in a PCB design, so this Wi-Fi design has a pair of four-pin power supply points. This makes it easy to hook up a logic probe or any other 3.3-V device you need to power when you’re working with the Wi-Fi board. As you can see in Photo 1, I’ve also included some uncommitted 0.1"-centered holes for whatever else you need to mount on the Wi-Fi board. I added a universal Reset switch circuit to the Wi-Fi board to handle those moments of operational uncertainty.

The Wi-Fi main board doesn’t require a lot of components, so assembly is quick. Based on the fine-pitch, surface-mount components and CompactFlash connector, you’d think that a stencil and some specialized soldering tools would be the only way to assemble this board. That’s mostly true. A stencil would be necessary to produce the Wi-Fi board in quantity; however, I discovered a solder dispensing tool that saves me the expense of procuring a stencil for prototype purposes.

My EFD Ultra 1400 fluid dispensing system allows me to lie down precise amounts of solder paste or flux with just a tap of a foot pedal (see Photo 2). It uses shop air and a computerized dispensing controller to pump consistent quantities of solder paste from a specially designed syringe system. I can put down 0.25- to 1.55-mm drops or lines of solder paste depending on the size of the tip attached to the Ultra 1400’s air-driven syringe. After you mount your desired syringe tip, you can teach the Ultra 1400 fluid dispensing system controller to put down a specific amount of fluid every time you tap the foot pedal. On the other hand, you can put the system in Free Flow mode, which allows solder paste to flow as long as the foot pedal is depressed.

(Click here to enlarge)

Photo 2—No more cramped thumbs from pushing syringe plungers. This gadget saves hours of assembly time.

I used the Ultra 1400 to assist with the mounting and soldering of all of the SMT components including the CompactFlash receptacle. After I applied the solder and mounted the components, I completed the assembly process using a small batch oven.

As a rule I use Metcal soldering stations because they heat up quickly and can be fitted with a number of special-purpose SMT soldering tips. You can use different equipment to solder the delicate SMT devices. I was pleasantly surprised to find that WAHL battery-powered ISO-TIP irons work remarkably well in the SMT environment. I used one to touch up and change components on the main board.