Download PIC DESIGN (version 1) 2/11/2009 The decision to design and

PIC DESIGN (version 1)
2/11/2009
The decision to design and construct a custom microcontroller was dictated by both practical
and educational reasons. Although microcontroller development boards are readily available, they are
expensive and would have required significant modification to interface with the required peripherals.
The additional educational value of starting from scratch made it an easy choice.
The design of the microcontroller began by completing the specifications for the devices it
would control. This list of peripherals, shown in table 1, was used to select the proper PIC to meet the
I/O demands.
Table 1: Peripheral Devices
Peripheral
LCD
Keypad
Switch
Solenoid
Motor
Requirements
1 serial transmit pin
8 pins with 4 pull-ups
1 pin with pull up
On/Off 12V and 2oo mA
On/Off 12V and 3oo mA
Number required
1
1
5
8
1
The PIC16f877 was selected because it supplied an ample number of I/O pins without the need
for complications such as multiplexing. This chip has the added advantage of being identical to those
used in E15 laboratories and is therefore well supported by the departments development
infrastructure and knowledge base. With the chip selection complete, the supporting architecture was
developed in Multisim. For the PIC to operate, it must be provided with proper power and ground
connections to the chip as well as the necessary programming connections through the RJ12 connection.
This chip model also requires an external oscillator. Figure 1 shows the PIC with only these supporting
features in place.
Figure 1: Minimum PIC microcontroller circuit.
The LCD display operates on by serial communication and therefore requires only 1 pin and is
relatively easy to program. The draw back is that that Serial Transmit pin must be reserved for it. The
remainder of the pins was assigned in blocks to operate either the input switches of the solenoid and
motor outputs.
The switch connections required the addition of weak pull-up resistors to ensure a stable signal,
shown in figure2. The purpose of these resistors is to maintain a high voltage to the input pins until the
switch is closed and they are grounded. This prevents the pins from floating at intermediate values while
the switch is open. Although the PIC contains several internal pull-ups, there were not a sufficient
number.
Figure 2: pull-up resistor and switch.
The keypad also requires that half of its inputs be connected to pull-ups. It operates on a matrix
encoded system where pressing a key shorts two of the inputs, the schematic and decoding table is
include in figures 3 and 4. Input is sensed by successively setting each column high and scanning the
row outputs, this procedure will be discussed in greater detail in the software discussion.
Figure 3: matrix code
Figure 4: Keypad schematic
The solenoid and motor outputs all require significantly more current than the 25 mA each PIC
pin can supply. These channels were equipped with simple MOSFET amplifier circuits to boost the
available current, shown in figure 5. The diode, acting as a ‘snubber diode,’ prevents the high inductance
loads from damaging the system when they are shut-off.
Figure 5: amplifier schematic.
Once the entire microcontroller circuit was designed in schematic form it had to be transferred
to the Ultiboard CAD tool to develop a design for the printed circuit board (PCB). Components from the
standard Multisim library contain their physical footprint, which is used by Ultiboard to place the
necessary Thru-holes. Custom components and footprints can also be defined. The user then places
each component footprint in the desired location within Ultiboard. The wire traces are generally routed
automatically, although the user can intervene. The trace thickness is calculated with any number of
simple online applets (http://www.circuitcalculator.com/wordpress/2006/01/31/pcb-trace-widthcalculator/). This design required the use of separate digital and analog ground networks because the
high current sent to ground by the solenoids and motors. The completed Ultiboard CAD drawing is
shown in figure 6. The PCB design flies were then sent to a Advanced Circuits for fabrication. Upon
receipt of the board the requisite components were soldered into place.