Tear Down: Scientific calculator boils design down to two ICs

While it's too big to fit in your pocket, the 35s follows in the tradition of a long series of handheld calculators.

Every once in a while, I come across a product where I say, I've got to take that apart. It's not the most scientific way of choosing products to feature in our popular Tear Down section, but it works for me. The subject if this month's Tear Down was actually sitting on another editor's desk, in his own Tear Down queue. Because of my “history” with this product, I quickly volunteered to write this story.

While not the sexiest consumer product out there, our engineering audience will likely see the appeal that HP's latest scientific calculator had for me. The 35s, which this new model is dubbed, is similar in many ways to the 15c I bought twenty-some-odd years ago when I entered engineering school (note that I paid about $125 for my 15c, while the 35s costs about $60). There's no doubt that the 15c was the premier calculator of its time. Just about everybody in engineering school seemed to have one. The 35s, shown in Figure 1, is the latest in a long line of scientific calculators from HP.

The 35s arrived in time for the 35th anniversary of HP's first scientific calculator. Hence, the “35” moniker.

I bet many of you are smiling to yourselves saying the same thing, “yeah, I bought one of those, too.” But the kicker for me, and I'd bet for many of you, is that my 15c is still going strong. In fact, it still serves as my everyday calculator. And it doesn't get lost like many of the other objects on my desk, such as the tape dispenser, stapler, and scissors, thanks to its use of reverse Polish notation (RPN). My family members take the attitude that it's easier to find another calculator than to learn RPN. And that suits me just fine. The 35s can be used in either RPN or algebraic mode, although it defaults to RPN when you first turn it on.

Another reason I wanted to take the 35s apart was to see how the design had evolved over the years. While I wasn't willing to destroy my 15c for the sake of this article, I did get some good insight from Sam Kim, the director of product development for HP's calculator group. He claimed to be the force behind the 35s and went into the design thinking that he wasn't satisfied with the company's current line of calculators. Kim said, “I really wanted to design something for the old-time calculator user while also attracting new customers.”

Just two key components
On one hand, I was disappointed to see what was actually inside the calculator. That's because it's not a very interesting design. It's just two key components–an 8502 microcontroller and a flash memory chip. Both of those parts are covered by a blob of epoxy, as you can see in Figure 2. The processor is under the bigger blob.

View the full-size image

The 8502 is designed by Sunplus Technology, a Taiwanese company. It's based on the 6502, an 8-bit processor that first appeared on the Commodore 64, which was popular around the same time that I purchased my 15c. The flash memory is a generic part, and no specific vendor is used. It's generally based on whatever is available at the right price at the right time.

The 8502 that's used here also contains the system's power management and all the typical chip-set functions. This includes an LCD diver, a keyboard controller, and so on.

Kim explained that my old 15c was built with a 4-bit processor and had very limited memory. However, it had (and still has) the ability to perform matrix manipulation, a feature that was way before it's time, at least in a handheld calculator. And it was programmable.

Software is the key
With the design down to just the two main ICs, it quickly became clear that the real challenge of this product is in the software. Kim says, “The hardware has become pretty much routine. Most of it is off-the-shelf stuff. We spend the majority of our time on things like: what should the user interface look like; how will the programming occur; what kind of keys should we put on there; and the basic look and feel of the calculator when it's in the user's hand. How does it balance; is it too big or too small, too heavy or too light. So it's the industrial design and the software that are the most challenging parts of the design.”

Unfortunately, it's the industrial design that I take issue with–the calculator is simply too big. It's not a true pocket-size calculator. While the HP folks wouldn't admit it, I got the sense that the next generation will be more to my liking, namely smaller. I also expect to see a larger display, more like a full matrix model. But then a tradeoff would have to be made somewhere, both in terms of the cost and the size of the calculator.

The firmware also resides on the 8502 microcontroller. That leaves almost the entire flash memory available for user programming. The 32-kbyte flash leaves about 30 kbytes available to the end user, enough for about 20,000 lines of code.

The initial specifications for the 35s came from a team within HP. Once that spec was agreed upon, the design was outsourced to a team in Taiwan. But even after it was sent overseas, the HP internal design team continued to work on other aspects of the 35s, in parallel with the external team in Taiwan. The internal HP team actually resided in four separate locales: Boise, Cupertino, San Diego, and Vancouver.

The eventual high-volume manufacturing is being done in China. All meetings for the design occurred by phone, as there were no “show-stopper” design issues to be overcome.

“We were working on the mechanical aspects while they were doing the software and electrical design,” says Kim. “The original industrial design was done jointly by the two teams. It was never a 'throw-it-over-the-wall' design.”

Definitely over-engineered
An interesting aspect of the 35s is that it takes “over-engineering” to a new level. For example, it's held together by 25 screws. Kim explains, “The keyboard is the most important part of a calculator. It must work year after year. Those 25 screws hold it down rock solid. The keyboard will never get loose and this thing will last forever. It costs us more, but it demonstrates the attention to detail we've put in.”

If the 25 screws weren't enough, the engineers decided to add a slew of plastic hold-downs on top of the circuit board. As a result, I had to literally break off each of those hold downs to get the board out.

HP claims that it performs a one-million key press-test on the keyboard. This means that the keyboard will survive at least a million key presses. Putting that in perspective, it's unlikely that a user will ever reach that number of key presses.

HP put the 35s through 11 different abuse tests and 20 common tests. These generally have to do with temperature thermal shocks, drops, etc. Many of the tests highlight common occurrences that a handheld calculator would be subjected to in everyday use, but others are abuses that a users will likely never subject the calculator to. They do environmental tests, using the calculator at 55°C, for four hours. Then it's stored at 65°C, with 95% relative humidity, for 120 hours. There are similar tests for low temperatures. The team isn't satisfied unless they get 100% of the tested units to pass.

Unlike one of the predecessor models (the 41), there's no way for the user to tap in to the 35s' firmware. The programming only occurs at the user level, although HP is tinkering with giving users limited access to the firmware.

Kim offers, “The 35s is pretty much hacker-proof, but the old 41 had a lot of hacking going on because it was more of an open platform. There was something called synthetic programming, and there was a crowd that evolved around it. I was actually a member of that crowd, the diehard HP users.”

Despite all the testing and over-engineering, the key to building a popular and powerful scientific calculator comes down to basics–making it easy to use, programmable, with all the functions a user would want, all accessible via the keyboard using a simple menu structure.

Richard Nass is editor in chief of Embedded Systems Design magazine. He can be reached at .

8 thoughts on “Tear Down: Scientific calculator boils design down to two ICs

  1. I love my HP-34C I bought in 1979. Have you seen what these calculators go for in the resale market? When I looked at a university bookstore to see what kind of scientific calculator was available, they only had the HP business calculator– Sic transit g

    Log in to Reply
  2. “My family members take the attitude that it's easier to find another calculator than to learn RPN.”

    Shame on you! Depriving your family the advantages of RPN is borderline child abuse. My kids were forced to learn RPN at an early age. Once they learned i

    Log in to Reply
  3. I think the effort in the design was an attempt to get close to the old look and feel but at modern crappy-button costs.

    While the 35S is a huge step up from the crappy buttons, it isn't quite as good as the old buttons. I can tell the difference betwee

    Log in to Reply
  4. McSquared: “t, it is difficult to read an LCD”. Have you tried fiddling with the contrast? That often helps.

    I too liked the LEDs but they do have two major limitations: power – the LCD variants will still run for hours once the battery warnings are give

    Log in to Reply
  5. In Engineering, use what works for the price, for the technology and programming. The 8205 obviously has all it needs to get the job done. Unix is just as old and it still works fine!

    Log in to Reply
  6. I'm impressed how much comments does these calculators bring up.
    I remember my calc, it was an HP 28S. It was old already when I used it during my engineering school years (late 90's), for this it draw the attention from my pears. It was the beginning of t

    Log in to Reply
  7. “Old HP calcs are still known for their key feel, and a 31CX got me through two engineering degrees. I'm still nostalgic for the alphanumeric font they used. But It's super sad to hear how much of the design was outsourced on this current model. HP is a ho

    Log in to Reply

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.