Printed Circuit Board

1) Introduction

1.1) Purpose

Various schematic symbols and diagrams are familiarized and a schematic is a collection of electronic symbols connected together with virtual “wires”. When fabricating a printed circuit board (PCB), a schematic is needed to provide input (a netlist) to the layout and routing tool (Varteresian, 2002). In other words, the capture of a schematic diagram is a convention to PCB, subsequent construction and testing. The design of PCBs is very widely used in electronic equipment and systems, for example, good EMC (Electromagnetic Compatibility) practices. These PCB-level EMC practices usually help achieve the required EMC performance at a much lower cost than alternative EMC measures at higher levels of integration, such as whole-product shielding. They also improve signal integrity in both analogue and digital circuits (Armstrong, Aug 1999). Power decoupling techniques can improve the power supply of a PCB and transmission lines techniques are a part of the PCB layout (Armstrong, Oct 1999). The MM232R is a miniature development module which uses FTDI’s FT232RQ. In this assignment, MM232R needed to be designed according to the datasheet.

Besides, switches of four pins also needed to be designed, but only two pins were used in diagram. PIC18F1320, a microcontroller, which features six enhanced power-managed “software controlled” modes, power consumption as low as 0.1 microamps in standby mode and a wide operating voltage range from 2 to 5.5 volts which makes this device ideal for battery managed applications (Unknown, 2007). It has some advantages, such as namely and high computational performance at an economical price, with the addition of high endurance Enhanced Flash program memory.

There are a range of features that can significantly reduce power consumption during operation, including Alternate Run Modes, Multiple Idle Modes, On-the-fly Mode Switching and Lower Consumption in Key Modules (Unknown, 2004). A pre-programmed PIC18F1320 was made available for testing purpose. This assignment is a simple two LED display, 3 push button switches and USB interface PIC18F1320 based PCB. The Pickit connector enables this device to be programmed with the user functions required. The LEDs keep flashing when the PCB was connected with the power supply or USB cable. Pushing the reset button (W1), both LEDs go out for a moment then both come on for a moment and then start flashing again. The aim of this assignment is to make the flashing decrease (increase) in frequency by pushing and holding down W2 (W3). This assignment consists of three parts: PCB design, PCB construction and PCB testing.

1.2) Background

PCB means printed circuit board, also known as a printed wiring board. It is used in electronics to build electronics devices. A PCB serves two purposes in the construction of an electronic device; it is a place to mount the components and it provides the means of electrical connection between the components (Unknown, 2003). It is a thin board made of fiberglass or a similar material. Electrical wires are “printed” onto the board, connecting the central processor to other components on the board. Some examples of PCBs include motherboards, RAM chips, and network interface cards. Sometimes PCBs are abbreviated as “PC boards”, which is fitting, since the boards are commonly used in personal computers. However, PCBs are also found in other types of electronic devices, such as radios, televisions, and computer monitors. Because PCBs are relatively flat, they can also be used in thin devices such as laptops and portable music players (Unknown, 2012).

1.3) Block Diagram

The basic building blocks of a boost converter circuit are shown in Fig 1.1.

Figure 1.1 Block Diagram
2）Requirements analysis

PCB Design: To begin with, MM232R and three switches of four pins are needed to design in ARES, and then the MM232R is needed to draw in ISIS. Next packaging the MM232R and three switches in ISIS. After the circuit diagram is completed, it should be PCB layout. During the PCB layout, all connectors should be near the edge of the PCB for easy access. The PCB will be powered from the MM232R USB interface but when the MM232R is not inserted power connection can be made to J1 and J2 Veropins so make sure these two pins are far enough apart to avoid shorting power connections. There should be room at each of the four corners of the PCB for a rubber foot, which will require a 5mm hole. The rubber foot itself is 13mm in diameter and must be allowed for on the PCB layout.

PCB Construction: First of all, all the holes in the PCB have to be drilled. Most of them are 0.8mm but a few such as switches and veropins are 1.0mm and rubber feet are 5.0mm. There are six resistors (R1=100R, R2=22k, R3=R4=470R, R5=R6=10k), one capacitor (C1=100n) and three pickit connectors. Then soldering the PCB, the LEDs should stand 3mm away from the PCB surface. Next testing the PCB, check that if it is shorted anywhere on the power rail. Also check that the correct pins on the PIC have 0V and +5V values and the MM232R connector has the correct voltage power values. The following shows the test schedule:

a) simulating the circuit diagram by using the oscilloscope; b) testing PCB without PIC or MM232R inserted;
c) testing PCB after PIC was inserted and add the correct power connections; d) testing PCB after MM232R was also inserted and plugging in a USB cable to a PC/laptop. 3）Possible solutions

MM232R Design: According to the MM232R datasheet, the MM232R can be designed in ARES and then package it so that it is can be found in ISIS.

Simulation in ISIS: Adding the MTIPickit.HEX to the PIC18F1320 and oscilloscope which C connects D1 and D connects D2 in ISIS, then set the MM232R to be excluded from the simulation (edit the properties for this device) and the simulation diagram would show by clicking the start/stop icon. Drilling: The 0.8mm holes can be drilled in the ground floor drilling room and 1.0mm and 5.0mm holes can be drilled in the first floor drilling room in Hawke. Testing: When the PCB had been completed constructed then performed a power on test without PIC or MM232R inserted by connecting +5V to the veropins. Check that the PCB was not shorted anywhere on the power rail. Looked at the PIC datasheet to determine the correct pins and check that the pins have 0V and +5V values. Then insert the PIC in the correct way around but without the power connected. Add the correct power connections and the LEDs start flashing alternately. Next insert the MM232R and plug the USB cable which connected with computer, the computer would recognize that a USB device has been inserted and load. 4）Implementation

4.1) PCB Design

First of all, the MM232R was designed according to its datasheet in ARES and then it was packaged so that it can be found in ISIS. Next three switches (four pins) were designed and packaged by using the same methods, however, only two pins were used in the circuit diagram. It needs to convert the schematic into a single side PCB layout in ARES after the circuit diagram was completed. The PCB was required 2×2 inches in size and the pads used for the DIL components were oval rather than the smaller circle that is the default in the ARES library in order to provide enough copper pad when soldering. All connectors were placed near the edge of the PCB for easy access. The LEDS stood 3mm away from the PCB surface. In addition, J1 and J2 Veropins were placed far enough apart to avoid shorting power connections. Finally, rubber feet (5mm hole) were required in the four corners of the PCB, the rubber foot itself is 13mm in diameter. The PIC Circuit Diagram sees Figure 4.1.

Figure 4.1 PIC Circuits Diagram

The designed PCB sees Figure 4.2:

Figure 4.2 Designed PCB
The 3D vision of the top and bottom layers sees Figure 4.3:

(a) top layer (b) bottom layer Figure 4.3 The 3D vision of 2-Phase PIC circuit board

4.2) PCB Construction

The hardware prototype was built on a PCB for reliability using both the top and bottom layers, see Figure 4.4.

(a) top layer (b) bottom layer Figure 4.4 2-Phase PIC circuit board showing its components

4.3) PCB Testing

Inserted the PIC and used the power supply to provide the power (J1 connects +5V and J2 connects 0V). Then connected the PIC to oscilloscope (pin8 connects channel one and pin17 connects channel two), see Figure 4.5.

Figure 4.5 PIC connects with an oscilloscope
Inserted the MM232R and made a USB cable plug in to a computer. Since this PCB is designed to be powered from the USB connection, it can not be connected with an external power supply, see Figure 4.6. [pic]

Figure 4.6 MM232R connects with a computer by using a USB cable 5）Results

The following waveforms were obtained by simulating the PIC circuit diagram in ISIS. They show the reset, slow and fast respectively, see Figure 5.1.

a) reset (W1)

b)

(b) slow (W2)

(c) fast (W3)

Figure 5.1 The waveforms of LEDs by simulating in ISIS

The following waveforms were got when the PIC connected an oscilloscope, the top one is about the waveform of D1 with channel one and the bottom one is about the waveform of D2 with channel two. They show the reset, slow and fast respectively, see Figure 5.2.

a) reset (W1)

b) slow (W2)

(c) fast (W3)

Figure 5.2 The waveforms of LEDs by using an oscilloscope

Insert the MM232R and a USB cable plugged in to a computer, and then detected the device and found out the COM port number is 11. Next set to the correct COM port number 19200 Baud, with Flow control was set to none and a text message appeared on the PUTTY screen and the LEDs started flashing. After that pressed any key on the keyboard and the sign on message appeared again, the LEDs continued flashing. The text messages on the PUTTY see Figure 5.3.

Figure 5.3 Text messages on the PUTTY
6）Discussion of results

The waveforms from the experiment and simulation are similar, but the simulation is more accurate than the experiment, this is because the experiment may produce some errors, including system error and human errors (such as misreading, miscalculation and parallax). In addition, the data can be recorded directly no matter which data we want to read. In this experiment, we can calculate the frequency and Vp-p easily by the data has shown in the previous chapter. When the frequency reached the minimum in the simulation, f1=2.759Hz, Vp-p1=4.85V, f2=2.797Hz and Vp-p2=4.75V, but it only shows the frequency is less than 20Hz in the experiment. However, when the frequency reached the maximum in the simulation, f1=23.3Hz, Vp-p1=4.90V, f2=24.4Hz and Vp-p2=4.90V, it shows the frequency is 24.8880Hz in the experiment, both of them are closed. It was supplied 5V power in the experiment and it is close to 5V in the simulation.

7）Conclusion

Having completed the assignment, the following conclusion can be deduced: the PIC is an easy to use device because of it has been programmed, but it needs to the MM232R is an easy to use device due to it can connect with the computer directly. The experiment has demonstrated that the PCB can be supplied power by using a power supply or the USB connection. The results show that the waveforms from the experiment and simulation have some errors, such as the frequency and the voltage, but they are accepted. The capture of a schematic diagram, it is conversion to a PCB and subsequent construction and testing is a complex process requiring a wide range of different practical skill. The Pickit connector enables the PIC18F1320 to be programmed with the user functions required.

References

Armstrong, M. K., August, 1999. PCB design techniques for. ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL, pp. 185-194.

Armstrong, M. K., October 1999. PCB design techniques for. ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL, pp. 218-226.

Unknown, 2003. What is a PCB. [Online]
Available at: http://www.wisegeek.com/what-is-a-pcb.htm
[Accessed 5 12 2012].

Unknown, 2004. Microchip. [Online]
Available at: http://nnp.ucsd.edu/phy120b/pic_manuals_pdf/pic18F_1320.pdf [Accessed 3 12 2012].

Unknown, 2007. Microchip. [Online]
Available at: http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en010265 [Accessed 3 12 2012].

Unknown, 2012. PCB. [Online]
Available at: http://www.techterms.com/definition/pcb
[Accessed 5 12 2012].

Varteresian, J., 2002. Fabricating Printed Circuit Boards. United States: Elsevier Science & Technology.