CSCI 255 Lab 10 -- PIC System Startup


In addition to the power supply that you created on your breadboard in Lab 3, you will need the following equipment and components to complete your PIC prototype board:

Part 0: Background

In this lab, you will build a prototype PIC system on your breadboard and test it. You'll begin by modifying the 3.3V power supply that you built on your breadboard in Lab 3.

Part 1: Modifying your 3.3V Power Supply

The voltage supply circuit that you built in Lab 3 is depicted below. If you circuit does not look like this one, return to Lab 3 and fix the problem. Some of you may have “lost” your fuse and have replaced it with a wire. It's important replace the fuse, and any missing components, for the final PIC system you are creating today.
start up config        start up schematic

Once your starting configuration matches that shown above, it’s time to augment it. Throughout this lab, try to use black wires to connect to the ground rail and red wires to connect to the power rail. Thus will make the lab easier for you and your instructor.

The power connector and wall transformer

Locate the power connector and wall transformer shown below.
power jack
If you are lucky, you will find a power connector with wires soldered to its leads. In that case you can skip the next step.

If you aren't lucky, plug the power connector into the wall transformer and use your multimeter to determine which two pins on the power connector provide +6V and ground as shown below. Each power connector has three leads, but only two are used.
Solder wires to the power connector. Connect a black wire to the ground pin and a red or orange wire to the +6V pin. The ground wire is then connected to the ground rail and the +6V lead connects to the resettable fuse.
power connector and breadboard

Use the multimeter to verify that you are receiving 6 Volts of power at the connections to the breadboard before proceeding.

The input power switch

The input power switch allows you to isolate your system from the wall transformer without disconnecting wires. Our switch is a single-pole double-throw (SPDT) slide switch. In an SPDT switch, a pole is always connected to one of two throws. If you have an overhead light at home that can be turned on and off from two locations, you have a couple of SPDT switches.

Connect the input power from the power connector to one of the outer pins of the switch and connect the fuse to the pole.
power switch and breadboard
Pay attention to the wiring of the switch. The power switch can be a little confusing. It appears to spread across five breadboard columns; however, it only has three pins. The center pin is the pole, and it must be connected to the resetable fuse. The other two pins are the throws. In the picture above the power supply is connected to the leftmost throw.

The 6-pin header

As our final modification to the power supply, we'll connect a 6-pin header to the board and wire it to the power switch and to ground as shown below. Note that the red wire is connected to the other throw of the switch.
6 pin header      power connection schematic

You should also go ahead and temporarily connect the FTDI TTL-232R-3.3V USB-to-serial cable to the 6-pin header.
power connections
Make sure that the connector is turned so that the black wire of the USB cable is connected to ground on your voltage regulator. If you are using black wires for ground and red wries for power, the cable will match your wiring.

Now you can verify that your power supply is working by moving the switch between your two power supplies, the power connector and the USB cable. You will need to use the multimeter to determine that power is being provided from both sources. (Alternatively, you can add an LED and resistor to your circuit for a minute of two.)

When you pull the cable off your circuit, the 6-pin header may stay attached to the cable. This isn't a problem if you always plug in the cable so that the ground wires match.


Show your breadboard power supply to your instructor.  

Part 2: The PIC microcontroller

The PIC24HJ32GP202 processor is shown below. Note that each pin has a function and that the pins are numbered counter-clockwise with Pin 1 being to the left of the notch on one end of the chip. Pins are also given names. Some names are simple: VDD is power, and VSS is ground. However, most pin have multiple functions and are known by complicated names, such as PGED2/TDI/RP10/CN16/RB10. Fortunately, these pins usually go by shorter names, such as RP10, that correspond to their use with a particular application. 260 pages of detailed information about the PIC processor can be found in its product datasheet.

Our next objective is to place the PIC processor on our breadboard and create the connections described pictorially below. Follow the itemized steps below to create this configuration on your breadboard.

First, identify the top of your PIC microcontroller. With the top of the PIC orientated towards the power supply, place it across the trough of your breadboard. Leave eight to ten breadboard columns between the voltage regular and the PIC processor.

Second, find your pushbutton switch and use a multimeter to determine the pins that connect when the pushbutton is pressed. The other two pairs of pins are permanently connected together.

Third, place the pushbutton switch on your breadboard so that it spans the trough between connected rows. It should be positioned between the power supply and the PIC microcontroller, perhaps a couple of breadboard colums away from the PIC. This switch will be the reset button for your computer.

Fourth, make a connection between the PIC MCLR pin (Pin 1) to the reset switch. Also, connect MCLR to the power rail with a 2.2 kΩ resistor. Finally connect the other side of the reset button to the ground rail. MCLR is grounded when the button is pushed. Otherwise, it is 3.3V.
reset button

Fifth, to smooth out voltage fluctuations on the breadboard, a decoupling capactor is placed between the VDD and VSS pins. Connect your 1 µF capacitor between VSS (Pin 8) and VDD (Pin 13). This capacitor is polarized. The longer leg of the capacitor, which is also marked with a + sign, must be connected to the VDD pin (Pin 13). The shorter leg is connected to VSS (Pin 8).
power cap

Sixth, the PIC processor needs an additional capacitor to stabilize its own internal voltage regulator. This is a 10 µF capacitor placed between VCAP/VDDCORE (Pin 20) and the other VSS (Pin 19). As before, the capacitor leg without the + sign is connected to VSS (Pin 19). Connect VCAP/VDDCORE (Pin 20) only to the +-signed leg. Do not connect VCAP/VDDCORE to the power rail!

Seventh, there is one more decoupling 1 µF capacitor. This one is for power to the analog modules within the PIC processor. This capacitor goes between AVDD (Pin 28) and AVSS (Pin 27). Once again, place the leg with the + sign to AVDD. Then connect AVSS to the ground rail and AVDD to the power rail. Now that your capacitors are connected, it would be a good time to make sure your wiring resembles the following picture.
capacitors connected


Show your board to your instructor.  

Part 3: Connections for programming the chip

Eighth, connect the 6-pin header for the USB cable connection to the chip. When the cable is properly attached, the first pin of the header is GND and the third is VCC. The cable’s VCC is 5 volts and the PIC processor’s VDD is 3.3 volts. The two should never be directly connected! The sixth pin (furthest from ground) of the header is RTS# and should be connected to MCLR (Pin 1) of the PIC. It’s a little easier to run the wire between the sixth pin and the half of the reset button that is already connected to MCLR. The fifth pin of the header is RXD and should be connected to RP11 (Pin 22) of the PIC. Finally, the fourth pin of the header is TXD and should be connected to RP10 (Pin 21) of the PIC. It really doesn't look as bad as it sounds. It's only three parallel wires.
connection to USB cable

Ninth and last, we’re going to place an LED on RB15 (Pin 26) of the PIC. The software provided by the textbook authors should flash this pin if your program is downloaded correctly. Make this connection by placing a 910 Ω (or 1 kΩ) resistor between RB15 (Pin 26) and the power rail. Then place a LED between RB15 (Pin 26) and the ground rail. Be sure the LED is turned in the right direction. It should look something like the following picture.
led in place

Here's a picture and schematic for our setup.
final picture      final schematic


Show your board to your instructor.