Tuesday, August 31, 2010

Reed Switches

Read an FAQ on "reed switches" here and the wikipedia entry here.
Here is what a pack of 1000 pieces of reed switches looks like.

Friday, August 27, 2010

Printed Circuit Boards - Construction Materials

There is a lot of confusion amongst students and fresh graduates with regards to the PCB fabrication process. I have accumulated some knowledge from around the web (References: Link1 | Link2) and added some of my own here to serve as a ready reckoner of a few important notes regarding PCBs.

PCB Process in Brief:

Copper laminate -> Drill board -> Deposit Copper -> Photolithography -> Tin lead plate or finishing -> Etching -> Hot air level -> Solder mask -> Electrical Testing -> Routing and V-scoring -> Product inspection -> Final cleanning -> Packaging.

PCB Terminology

  • Base Laminate - That sheet of insulating material (a.k.a. dielectric) to which a thin copper foil is affixed on either or both sides. This material is constructed by repeatedly uniting two or more layers of material together. The process of creating a laminate is lamination, which in common parlance refers to the placing of something between layers of plastic and glueing them with heat and/or pressure, usually with an adhesive. Refer wikipedia entry for Laminates. Base Laminates are of many types (depending on the materials used to make them) - FR2, FR4, CEM1 etc.
  • Pre-preg - stands for "pre-impregnated". The Base Laminate is made of insulating layers of a particular dielectric typically laminated together with epoxy resin prepreg. A pre-preg is a sheet of material that has been impregnated with a resin cured to an intermediate stage. Read "How's FR4 Made" for a clearer explanation.
  • Epoxy - Used to bind the various dielectric plies together during the process of lamination. Eead the Wikipedia article for a clearer picture.
  • PTH or Plated Through Hole - A hole with the plated copper on its sides to provide electrical connections between conductive patterns present on two or more conducting layers of a PCB. There are two types of PTH - one is for mounting leaded components and may or may not create electrical connections between patterns of the two or more conducting layers through which it passes. The other one (called a via) is not used for mounting components and is used only for the purpose of creating electrical connections between patterns (i.e., traces) on two or more conducting copper layers.
  • Blind via - A via hole that does not pass completely through the printed circuit board. A blind via starts from one side or another.
  • Buried via - A via connects two or more inner layers but no outer layer, and cannot be seen from either side of the board.
  • Pad - The portion of the conductive pattern on printed circuits designated for the mounting or attachment of components.
  • Bare Board - A finished printed circuit board (PCB) that has no components mounted yet.
Base Laminates Types (taken from here with some additions)
  • FR2 (a.k.a. Paper Phenolic)
    The base is composed of multiple plies of paper impregnated with flame retarding phenolic resin. The major advantage of the material is the low cost, good electrical and punching performance. In India, you will find most cheap single sided PCBs made of this material. It is brown in colour. Wikipedia article on FR-2

  • FR3 (a.k.a. Paper Epoxy)
    The same as FR2 but the paper is impregnated with an epoxy resin providing better electrical and mechanical properties. The material is used in TV, computer and telecommunication applications.

  • CEM1
    The material is a composite with paper as the core and woven glass cloth on the outer surfaces - each impregnated with epoxy resin. The material can provide the punching characteristics of the previous materials and many of the electrical and mechanical advantages of FR4 laminate. The surface of this material is white in colour because of the woven glass but brown on the sides because the core is made of paper.

  • CEM3
    This material again has woven glass cloth on both outer surfaces but with non-woven glass in the core. It has epoxy resin and is more suitable for through plated holes than CM1.

  • FR4 (a.k.a. Glass Epoxy)
    By far the most often specified product produced from multiple layers of woven glass cloth impregnated with epoxy resin. It has superior properties to the other laminate sheets with a higher price tag. The glass transition (Tg) temperature of the material is between 125-140°C. This laminate is mainly used for plated through and multilayer functions. It is used in all types of electronic products including military space and advanced computer systems. Wikipedia rticle on FR-4

  • FR5
    Basically the material is the same as FR4 but with a modified epoxy filling which improves the Tg to between 150-165°C. This improves the thermal stability of the product. Generally used where a product is required to work at higher temperature.
Also have a look at the Materials section of Wikipedia entry on PCB.

Copper Thickness
The copper in a PCB is rated in ounces, and represents the thickness of 1 ounce of copper rolled out to an area of 1 square foot. For example a PCB that uses 1 ounce copper has a thickness of 1.4 mils. mil is a unit of measurement of length. 1 mil = 1/1000th of an inch. 1 inch is 2.54 cm. PCBs are available with varying copper thicknesses:

1/2 ounce per square feet > 0.7 mils or 18 microns thick1 ounce per square feet > 1.4 mils or 36 microns thick2 ounce per square feet > 2.8 mils or 72 microns thick

Here is a photo of 3 PCBs of the same design fabricated using three different base laminates. All PCBs are 1.6mm thick and have tracks etched only on a single side. A green colour solder maske is applied to the side on which tracks are etched. The copper thickness is 36 microns but after tinning, the thickness rises to about 70 microns. Tinning is the process of applying a thin layer of Tin to the copper pads whhich stay exposed even after the solder mask is applied. Tinning protects the pads which would have been otherwise oxidised and would prevent solder from sticking to the copper. (Copper gets oxidized when it comes in contact with air, Tin doesn't get oxidised) The dimensions of the PCBs are 160 mm by 103 mm

Top Side (Component Side)

Bottom Side (Solder Side)

Side View

The PCBs featured in the photos are of the same design : "89C51/89C2051 General Purpose PCB" The PCBs and the components in a kit form are available from:
Visha Electronics Corporation
Address: 1st Floor, Kalpana Building, 349 Lamington Road, Opp. Police Station, Mumbai 400 007, INDIA
Phone: +91 22 23862650
+91 22 23862622
Fax: +91 22 23884401
Email: info@vishaworld.com

Prices of the Bare PCB as of August 2010:
FR2 Version - 50 Indian Rupees
CEM1 Version - 70 Indian Rupees
FR4 Version - 90 Indian Rupees

A page describing the assembly and use of this PCB for developing the projects around the 8051 microcontroller is located here.

Links to 3D images of PCB cross-sections found on the web:
Image1 | Image2 | Image3 | Image4 | Image5 | Image6

Also checkout the course website for "Concepts of PCB Design" @ University of Bolton

Friday, August 20, 2010

The Insides!

What better way to teach students about architecture of embedded systems than to actually open up a few and show it to them?
We opened up a digital camera, an iPod, a GSM Cell phone, a TV game and a few computer accessories. We then stuck their innards between two thick pieces of A4 sized transparent plexiglass each and bolted them together to form exhibits. For the digital camera, we even printed out a pair of transparencies to outline the various sections on the main board. During the workshops, they proved to be a great practical teaching aid for exposing students to some real life design. After a few workshops, the glue gave way and the various sections inside the exhibits started falling apart. Since the sides were open, the dust made matters worse. And because we used to pack all of these 5 exhibits together, the bolts on each of them ended up scratching the surfaces of the plexiglass.
In the second version of these exhibits, we ought to make the transparent box sturdier and dust proof.

The Insides - of Fuji FinePix A345 Digital Camera

The Insides - of Fuji iPod Photo 20GB

The Insides - LG G1800 GSM Cell Phone

The Insides - TV Game (NES Clone)

The Insides - Assorted Computer Accessories

EDN Magazine carries a regular feature called "Prying Eyes" in which they open up a gadget and decribe its internals. The Prying Eyes articles are all available online here.

Thursday, August 12, 2010

Restoring P89V51RD2 's Bootloader

If you have used P89V51RD2 a lot, you would have realised that many times it happens that the microcontroller refuses to be recognized by FlashMagic and then you have to resort to replacing the microcontroller with a new one. Assuming that your hardware circuitry and computer are free from any problems, there are only two things that could have gone wrong - your microcontroller has suffered an internal hadware failure (maybe because of overvoltage or ESD) or the bootloader residing inside the microcontroller has got corrupted. Microcontrollers with spoilt bootloaders can easily be fixed by re-programming the bootloader into their flash memories. So before dunking those seemingly dead micros into the waste baskets, try to resuscitate them by reprogramming their bootloaders. Heres how you can do this using a Universal Programmer:

Step 1:
Acquire a "Universal Programmer". UC Micro System's UNIPro 110u is one such programmer. The photos below explains the process of restoring the bootloader using this programmer. The procedure for doing the same using a different brand of universal programmer would be similar.

Step 2:
Download and install the universal programmer's PC frontend software. In case of UNIPro 110u you can get the latest version from here (as of the day of posting this blog, the latest version was 4.20 and it did not yet work on Windows 7).
Step 3:
  1. Visit the "P89V51RB2_RC2_RD2 Product Information Page" at NXP here.
  2. At the bottom of this page, you will find a link: "P89V/LV51RD2 Boot Loader update V5" (Direct link to ZIP file)
  3. Download this ZIP archive (p89v_lv51rd2_bl_upd.zip). It contains the bootloader HEX file.
  4. Within this ZIP archive, you will find two HEX files. We are interested only in P89V51RD2_BL_V05.hex. (The other HEX file is for updating the bootloader via flashmagic in cases where the original bootloader is still intact inside the microcontroller)

Step 4:
Select the microcontroller in the programmer frontend software.

Step 5:
Select the HEX file: P89V51RD2_BL_V05.hex

Step 6
Press the program button! (In case of UNIPro 110u, ignore the "Need AC Adaptor message" - USB power is enough to program P89V51RD2)

If the microcontroller gets programmed successfully, then the bootloader would now reside in the flash memory of the microcontroller. The microcontroller will now work again with FlashMagic.

NOTE regarding updated bootloader:
An updated version for the bootloader for P89V51RD is available from FlashMagic's website. Direct link here. Read the enclosed PDF file in the ZIP archive to know how to use it.

NOTE for microcontrollers manufactured by SST:
This process will also work with
SST89E516RD (manufactured by SST) whose bootloaders are corrupted.
EasyIAP (which is equivalent in functionality to FlashMagic) is used to communicate with SST89E516RD and download program into it.
While operating the Universal Programmer, you will need to select the microcontroller as "SST89E516RD". The bootloader for this microcontroller is available here. Download the ZIP archive named "IntMode_x5xx.zip" (direct link here). The bootloader is in form of a BIN file (instead of a HEX file) named "F51MBLL5.BIN".
The "IntMode_x5xx.zip" archive also contains the setup file for EasyIAP which is similar in functionality as FlashMagic but is instead used for programming SST's microcontrollers.