μblog: engineering from the trenches

It is important to know when to enough is enough while working on an engineering design. It is possible to keep delaying a project delivery because more and more features are added. Some may argue that adding features generally makes a product better, however, the extra features start to take away from the products over-all usability at a certain point. This is the problem of feature creep. See this nice review of the issue.

The authors of the above review make an excellent point when they point out that adding new features to an existing design often simply means putting more buttons on the front panel. They demonstrated this with a Samsung printer, however, we can see this with other ubiquitous technologies such as mobile phones. I really enjoy my Motorola Q, but my father would probably not use 95% of the features on the phone as they offer limited utility to him and require a high level of technical proficiency.

It could be a matter of adding another button on the front panel or another menu option, the result is that many products are becoming overwhelming for non-specialists. This post is not a rant, it is merely a reminder to myself that sometimes more is less. Sometimes it is better to have less features in a product and to avoid building a Christmas tree (unusable mess of wires) in the process. I guess that it could also be a reminder for the people who are trying to design the next Sony Walkman, Nokia Phone, iPod or any other gadget that will sell millions, It might be better to consolidate the user interface and offer less options rather than tacking on an another bullet point to the features list.

The image above (and below) could qualify as a Christmas tree type of circuit, but it is an ugly looking prototype where the second-generation features have been clobbered onto the first generation hardware to make sure that everything works properly.

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This is in part a follow-up to the CC1110 IC Friday post from a few weeks back. TI has released a USB-dongle based development kit featuring their CC1111 system-on-chip radio communications IC. The board features an msp430 and the CC1111 on a self-contained USB stick and is about 50USD, a very reasonable price. For those that want something cheaper, the samples for the family of low-power RF chips are already available and shipping. Finally, the board reference design can be found here and is also mirrored below.

( swrr049.zip )

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This is the TSB41AB1 from TI. A standard IEEE1394 (2000 rev) transceiver IC with a substantial amount of copper on the top.

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After reading reviews of some technical documents which I have written, I could not help but feel as though my undergraduate curriculum in computer engineering had been a bit light on skills related to technical writing. I don’t think that I am alone here given that many of what are considered top engineering textbooks are so poorly written. The purpose of technical writing is to convey information in a clear, accurate and concise manner, however, basic errors still tend to make it into final book revisions. If we look at the following two sentences, the difference is just a single comma, but the meaning is totally different.

The data register is sent to the cross-bar before it is cleared. (Cross-bar is cleared.)

The data register is sent to the cross bar, before it is cleared. (Data register is cleared.)

I have started to remedy this situation by looking at a couple of books on grammar and English usage and trying to apply all of my mental utility to grasp the concepts. (As usual, here are a few PDFs to help with grammar and usage.) Hopefully we can all improve our writing a small amount and try to avoid mistakes such as the one that follows.

[Professor] Nelson admitted to smoking marijuana cigarettes soon after police arrived there, the complaint says. Police reported that they confiscated a number of items, including bags with marijuana and rolling papers. (reference)

( sp7084.pdf ) ( breviary_of_english_usage.pdf )

The image above is from the Early Office Museum.

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Today we have the INA2332 from Texas Instruments.  This is a dual instrumentation amplifier based on the two-op amp approach.

ina2332-10x-stitched.jpg

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Somehow I agreed to lead a review session for the Fundamentals of Engineering (FE) exam, electrical engineering specialization. The lesson I learned from that experience is teaching is not as easy as it seems. This is an examination that is administered by the National Council of Examiners for Engineering and Surveying (NCEES). The purpose of the FE exam is to be the first step on track to a Professioanl Engineer (PE) certification, also from the same agency. The PE certification then allows the holder to approve engineering designs that may require safe operation. Here is some basic study material for those who are trying to cram it in the last few weeks remaining before the October test date, or those who are looking to prepare for later. Looking over the benefits and my desire to get into the biomedical field, I am now thinking about taking the FE exam in the Spring, so hopefully I can study better than I can teach.

Picture is from Theodor Horydczak’s collection of Washington D.C. in between 1923 and 1959.

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When trying to do parameter estimation given a set of data, there are typically two approaches: least squares estimation and maximum likelihood estimation. In both cases, a model must be constructed where the first case tries to fit the data to the model by minimizing residual errors while the second method tries to estimate the probability density function associated with the collected data and thereby determine the parameters. While trying to make sense of things, I found this tutorial on maximum likelihood estimation by In Jae Myung of Ohio State University to be very helpful as it provided a description as well as MATLAB code examples. (He also also publishes a list of books currently on his bookshelf!)

Update: Here is a link to a paper outlining R.A. Fishers arrival at the concept of maximum likelihood. An interesting thing to note here is that given a likelihood function P, log(P) is often maximized, yielding maximum likelihood because the function would have to be differentiated and given that the probability distribution of many naturally occurring events is Gaussian, differentiating the logarithm of such a probability density function just makes more sense.

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Today’s IC Friday is imaging the secured sub-1GHz radio system from TI. This chip features both the RF subsystem as well as a microcontroller and 32kB flash. Thanks to reader Matthias Wenzel for this suggestion.

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As a small follow-up to my post about package footprints, here is a  Packaging Databook from Intel. About half of the book is on the physical specifications of various packaged employed by Intel, such as physical constants of package materials. There are also some good sections on electrostatic discharge considerations and reflow soldering methods.

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What to do when you need to mount a ball-grid array (BGA) package on a circuit board without sophisticated equipment? One popular option is to create something called a “reflow oven” which is able to control your circuit boards temperature with respect to time. The idea behind reflow soldering is that we may want to apply a thin layer of solder paste (solder with flux) over the exposed pads on a printed circuit board, then place all of the surface-mount components on that side, and then heat the board so the solder melts and the components become electrically attached. This is pretty much the only method for attaching components whose pads are completely on the underside making them inaccessible to soldering irons. The temperature profile is fairly standardized (here, here and here) and consists of first removing any excess moisture from the packages, then ramping up to the temperature required to melt the solder, then to cool off in a safe manner that prevents component or joint damage. It should be noted that these temperature profiles aim to limit the time components spend at elevated temperatures (>250C) to minimize the risk of damage due to heat.

What I am proposing is something much simpler: lets use a hot  plate to heat the PCB and achieve the same sort of reflow process. The main disadvantage is that the process is much less controlled and the dimensions of the board must be small enough to fit on the hotplate. The primary benefits are its simplicity. I am fortunate enough to have a hotplate which has a thermocouple to the surface and can measure the surface temperature with some degree of proficiency, so an alternate method will be required for other types. Some kind of infra-red measurement method would probably work well.

The idea is that we first apply solder paste to the board, when necessary. In this example, I am mounting a MICROSMD8 package where there is ample solder on the board and the chip to achieve connection. It is often a good idea to put some clean-free flux on the board in any case. Everything is first pre-heated for ten minutes at 50-80C to get rid of some of the moisture. The assembly is then heated to about 230C. At this point, the chips should already be aligned over the target pads. The reason for this temperature is that unlike the oven, the top surface of the PCB is exposed to air and thereby creates a thermal gradient. We need to control the heat on the top surface so that the solder just barely melts. This can be noted when watching the PCB under a microscope or with a magnifying glass as the solder will become very shiny when it melts.  As the solder melts on the chips and PCB, the surface tension will pull the chip into alignment. The whole assembly can then be slowly cooled and tested electrically. When populating larger projects, it is best to put on the larger chips first and then place something to act as a heat-sink on top. I have had success with larger DSP chips where I placed inverted bolts on top to radiate away some of their heat while adjusting other components. Finally, don’t forget that a cold PCB looks the same as a hot one, so be sure to avoid burning yourself.

( an081.pdf ) ( an353.pdf ) ( xapp427.pdf )

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