Microprocessor applications often require only a simple microprocessor circuit to control an external board, read a few sensors, control a motor or LED using PWM or even run a servo. Implementation of such a circuit is a hassle: dedicating a complex dev board to the task, maybe using a solderless breadboard or hand-assembling the circuit on perf board. None of these is an ideal solution, costing more than needed, having a less-than-robust solution or taking more time than should be needed.
An extremely low-cost circuit that can accomplish simple tasks without any extra hardware or features, yet can serve as a building block to more complex projects would be an ideal solution. The “Throw-Away PIC” (TAP-28) circuit board is so cheap that it can be dedicated to a project without worrying about getting it back.
The TAP-28 supports many of the popular PIC 18F-series 28 pin parts. It includes 2 general-purpose pushbutton switches and 4 LEDs, connectors for ICSP, UART and I2C/SPI and includes 2 analog input/general purpose and 2 PWM/general purpose I/O connectors on a 2" x 3.25" 2-sided printed circuit board. The board uses through-hole parts for easy assembly. A daughter board may be connected to use additional components with this board.
The TAP-28 is designed to support growth in the PIC development community, and as such, the design is licensed to the community under a Creative Commons License detailed elsewhere in this document.
No claims are made as to the performance or reliability of this board, nor to its suitability for any particular application. No safeguards to protect an inexperienced user from causing damage to the microcontroller have been included on this board – this board is intended as an alternative to a hand-assembled perf board, so some level of knowledge is assumed.
To determine if a particular PIC microprocessor is suitable for use on this board, compare its pinouts to the examples shown. For technical support, please consult your favorite PIC forum.
2. Positive power plane is discontinuous on bottom side of board, leaving an unconnected area on the top left side of the board. See illustration on following page.Workaround: jumper unconnected area.
3. Ground terminal of J10 terminal block connection is not connected.Workaround: If the J10 terminal block position is used with either a terminal block or wire leads for a power connection, jumper to J11 shell terminal for ground connection.
4. Upper right mounting hole is out of position (near USB connector).
The image above displays where and what to do.
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Microprocessor
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18F242, 18F252, 18F2420, 18F2520 or similar 28 pin microprocessors in a narrow DIP package are suitable for use with the TAP-28The 18F2550 and other USB variants are not recommended as some of the functions are mapped to different port pins.
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Power
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Regulated power should be supplied to the board – no voltage regulator is included on the board. A full size USB-B jack is provided for power only (no USB support) or a terminal block or soldered leads may be used for connection of a regulated power supply. A 5 volt charger salvaged from a cell phone works well.
Caution: There is no voltage regulator or reverse polarity protection on the board. Incorrect voltage or reversed polarity will damage the microcontroller.
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LEDs and Switches
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LED1 - A5
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General purpose LEDs connected to the port pins shown. Setting the pin LOW will illuminate the LED. Each LED is connected via a solder jumper and may be isolated from the microprocessor.
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LED2 – B3
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LED3 – C0
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LED4 – B2
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RST SW
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Microcontroller reset. May be required if a boot loader is being used.
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S1 – B5
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General purpose switches tied to the port pins shown. Pressing the switch pulls the pin to ground. Each switch is connected via a solder jumper and may be isolated from the microprocessor.
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S2 – B4
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3 Pin Connectors
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Port A0
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ADC input, servo control or general purpose I/O.
Power and ground are also supplied on this connector.
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Port A1
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Port C1/CCP2
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PWM output, servo control or general purpose I/O.
Power and ground are also supplied on this connector.
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Port C2/CCP1
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ICSP Connector
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ICSP (in-circuit, serial programming) for programming the PIC microprocessor using a PICKIT 2 or other programmer. Wired in accordance with the PICKIT2 standard (which is the standard for 6 pin in-line connectors. PORTB.6 and PORTB.7 are not used elsewhere on the board to prevent problems with ICSP (but they are available on the daughter board connectors).
May also be used as 2 general purpose I/O ports with power and ground available.
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UART Connector
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UART: Connector for UART applications, including I/O using the PICKIT 2 UART tool. Includes TX(PORTC.6) and RX(PORTC.7). Requires level shifting (i.e., a MAX 232 chip) for direct connection to a PC serial port. For serial I/O requiring handshake lines, also includes INT1(PORTB.1) and INT2(PORTB.2).
May also be used for general purpose applications with 4 port pins, power and ground available.
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I2C/SPI Connector
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I2C/SPI: Connector for I2C and SPI applications, connected as per the PICKIT Serial Analyzer. For I2C, includes SDA and SCL with the addition of a port pin for interrupts. For SPI, includes SDI, SDO, SCK and /CS.
May also be used for general purpose applications with 4 port pins, power and ground available.
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For a given application, some components on the TAP-28 board may not be needed. For a board dedicated to a particular project, this will further reduce its cost. The blocks below indicate the function of various components.
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The Microprocessor
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U1
C1, C2
C3
R1
Y1
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28 pin microprocessors with a pinout similar to the PIC18F242 may be used on this board. A socket may be used if desired.
A 20 MHz crystal is the usual choice, with C1 and C2 being 22 pF. C3 is a bypass capacitor of 0.1 µF. R1 pulls /MCLR high.
This is the minimum compliment of parts to use the TAP-28.
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Power
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J10
J11
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The TAP-28 board requires a regulated 5V (or possibly lower) power supply. J10 may be installed to use a USB cable for power. Note: If the underside of J10 is conductive, insulate the area above J11 with electrical tape to prevent shorts. Alternatively, a terminal block may be soldered in the J11 position or leads from a power supply connected to the J11 position.
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ICSP
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J3
S3 (optional)
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ICSP is the simplest way to program the TAP-28. J3 is pinned out for use with the PICkit and many other ICSP programmers.
S3 is optional to reset the microprocessor by pulling /MCLR low.
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The LEDs
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LED1, LED2
LED3, LED4
R3, R4, R5, R6
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LEDs 1 -4 are T1 3mm LEDs. The anode(+) side should be installed toward the edge of the board. The resistors are current limiting resistors for the LEDs.
Shorting JMP1, JMP2, JMP3 and JMP4 with solder connects the LEDs to the circuit.
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The Switches
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S1, S2
R7, R8
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S1 and S2 may be used as desired for program control and input. R7 and R8 are pullup resistors for these switches.
Shorting JMP5 and JMP6 with solder connects the switches to the circuit.
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UART Connector
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J9
R12
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The UART connector may be used for TTL-level serial communications or for general purpose use and it connected to the PIC’s hardware USART. The pinout matches the PICkit 2 UART tool.
R12 is a pullup resistor connected to Port B1 and pin 1 which may be useful for handshaking. Short JMP10 to connect it to the circuit.
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J1 & J2
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J1
J2
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J1 and J2 are normally female headers used to connect a daughter board to the TAP-28. They are also useful to make jumper connections to a solderless breadboard from the TAP-28. All port pins are available on J1 & J2.
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3 Pin Jacks
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J4, J5, J6, J7
C4
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These jacks may be used for analog input (J4, J5), PWM output (J6, J7), servos or general I/O.
C4 is recommended if servos are connected to prevent voltage transients.
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I2C/SPI
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J9
R9
R10
R11
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J9 is connected to the PIC’s Master Synchronous Serial Port (MSSP) module for I2C and SPI communications. R9 and R10 for pullup resistors for the I2C SCL and SDA lines. R11 is a pullup resistor which may be used for an interrupt line. This connector may also be used as 4 general purpose I/O pins.
The pullup resistors are connected to the circuit by the adjacent solder jumpers.
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Part
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Value
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Comments
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C1, C2
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22 pF, 5%, 50V
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Ceramic disk, 0.1" lead spacing
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C3
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0.1 uF, 20%, 50V
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Ceramic disk, 0.1" lead spacing
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C4
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10 uF, 20V
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Tantalum or electrolytic, 0.2" lead spacing
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R1, R7, R8, R11, R12
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10k, 5%, 1/8w
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R3, R4, R5, R6
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301, 5%, 1/8 w
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depends on LED parameters
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R9, R10
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4k7, 5%, 1/8w
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I2C pullup
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J1, J2
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15 pin female header
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0.1" lead spacing
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J3, J8, J9
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Molex 22-23-61 6 pin hdr
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0.1" lead spacing. Standard header pins may be used.
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J4, J5, J6
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3 pin header
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0.1" lead spacing
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J10
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USB B, full size
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pinout appears to be universal
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J11
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Terminal block
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0.2" spacing, optional if J10 not used
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LED1, LED2, LED3, LED4
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LED, T1 3mm
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Color as desired. Max current is 20 mA
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S1, S2, S3
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Tactile switch
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6 mm SPST mom, Omron B3F-10XX series or similar
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U1
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PIC 18F2420 or similar
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28 pin PIC 18F series, 18F242, 18F252, 18F2420 or similar (0.3" wide DIP)
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Y1
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20 MHz
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20.000 MHz HC49/US (low profile)
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Notes:
Click to enlarge
Click to enlarge
By far, the easiest way to load program code into the TAP-28 board is via ICSP programming. There is no need to remove the microcontroller for programming or to install a bootloader.
The PICkit 2 from Microchip offers exceptional value for its price. A six-conductor jumper cable is the best way to connect the PICkit to the TAP-28, although it may be directly connected if a simple 6 pin header is installed instead of the suggested keyed connector. The PICkit can also power the board during development so that a power supply need not be connected.
During program development, it can be handy to use the serial port to monitor various parameters on the PIC. Output to the UART can be made to check operation of a subroutine or to check variable values.
The PICkit 2 is an ideal way to connect to the PIC UART. It can be plugged directly into the UART connector on the TAP-28 board. A more convenient method is to use a short 6 conductor cable. These are supplied with many programmers and are available from SparkFun.
Be sure to set the baud rate in the PIC program and select the corresponding speed in the PICkit 2 UART tool software on the PC.
Servos contain internal electronics to convert a pulse train into a position. The PIC need only supply the pulse train to make the servo rotate; no switching of high currents is necessary.
Servos typically have a pigtail with a 3 pin female connector. The color code is usually one of two patterns:
| Signal | Power | Ground |
| white | red | black |
| yellow | orange | brown |
Any of the 3 pin connectors may be used to control a servo.
The TAP-28 board has 2 analog inputs brought out to connectors. These inputs can read a 0 – 5 volt DC level using the PIC’s power supply as the reference level.
The above illustration shows a potentiometer (variable resistor) connected to one of the inputs. Turning the shaft will vary the voltage at the input from 0 – 5 volts.
Some sensors have internal signal conditioning and provide a 0 – 5 volt output which can be connected directly to the PIC. Pressure sensors are one example. Sensors that do not contain built-in signal conditioning may only have an output voltage of a volt or less. Such sensors will probably need additional signal conditioning to provide good resolution.
Ports A0 and A1 may be used for analog input.
Dallas 1-wire devices actually communicate over 2 wires: I/O and ground. The devices are powered by voltage on the I/O line when it’s idle. A large number of devices may be connected on the same bus, limited by cable length and the capacitance of the cable and attached devices. Consult www.maxim-ic.com for more information.
For a simple bit-bang interface, any of the 3-pin connectors will work with the addition of a single pull-up resistor (determine the value from the data sheet). An even easier solution is to use one of the lines on the I2C/SPI or UART connector that has a pull-up resistor on the board. Enable the pull-up resistor with a solder jumper and connect ground and I/O.
Requirements for the various 1-wire devices vary, and some of them may work better with or even require a power connection. The data sheet is the most reliable place to find exact requirements.
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