Nuts and Volts - November 2013

isn’t much else that would be useful in terms of basic troubleshooting.

The rest of the components are supporting hardware in order to make the PIC happy. Y1 is our ceramic resonator clock source. JP1 is a pin header used for in-circuit programming. Resistor R1 (33K ohms-39K ohms) is the MCLR (programming voltage) pull-up resistor. C2 is a decoupling capacitor with a value of 1 µF to 5 µF. C4 is a ceramic capacitor used by the PIC18LF2550’s internal regulator. There is a range of values that C4 can have, but I’ve found 0.47 µF works the best in most situations.

D3 and D4 are 3. 9 Vdc zener diodes used as transient and over-voltage protection for the USB data lines. There are many ways to perform this task. I had a bunch of zener diodes, so I used them. D1 is a 1N4148 type used to isolate the PCB voltage from the USB attached to the PC. If we are using the power from the USB connection, leave D1 connected as shown.

If, for some reason, you choose to use a separate power supply in addition to the PC, remove D1 from the circuit. This will disconnect the PC power from the circuit and allow our own board voltage sources of IC3 or (directly) X1 to power our device. The design used here has the USB as the power source, so as not to damage the PCB or any attached USB host when using optional power connections.

Figure 3 shows our EAGLE produced PCB layout.

Notice there is a row of pin headers on the top and a double row on the bottom. The top row is connected only to the junction of the switch pull-up resistor and the switch.

These are used in the case where your switches use a shared ground. The bottom double row is used for two wire connected momentary switches as shown in Figure 4.

Additional power options are on the right of the PCB.

Firmware

The final piece of the puzzle is that the PIC/PCB needs to act like a HID keyboard device. Luckily, we don’t have to start from scratch because my PIC compiler of choice — CCS’ PIC-C — has been good enough to provide some example USB HID files that we can quickly modify in order to get our device up and running. These files begin with “usb_kbmouseXXX.c” and are within the PCW USB directory of the compiler.

If you do not have this compiler, no big deal. The concept is always the same: Tell the attached PC what kind of HID device we are via the device descriptor, and send messages to (in other cases, receive from) the PC that are accepted as HID type messages. Beyond that, we probably want to tell our PIC what messages (key/scan codes) to send, depending on which key closures are shorted to ground. The provided firmware sends these key/scan codes: 0-9, a-d, <return>, and <backspace>.

Figure 5 is a screenshot of the main function of our PIC18LF2550’s firmware code. We’ll go over the main function first and its generalities, followed by a couple of smaller sections of code that aid in understanding what is actually happening.

Lines 264-282 get the PIC up and running with configuration and initialization code related to setting port pins as inputs or outputs, registers, and setting up the USB device descriptor.