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TMCNet:  Scientific calculator boils down to two ICs

[June 23, 2008]

Scientific calculator boils down to two ICs

(Electronic Engineering Times Via Acquire Media NewsEdge) Every once in a while I come across a product that makes me say, I've got to take that apart. That was the case with Hewlett-Packard Co.'s 35s scientific calculator. I spotted it sitting on another editor's desk, in his own teardown queue. But because of my history with this product's predecessors, I volunteered to write the story.

True, this is not the sexiest consumer product out there. Still, the engineering audience will likely see the appeal that the latest in a long line of HP scientific calculators held for me. The product arrived in time for the 35th anniversary of HP's first scientific calculator. Hence, the "35" moniker.

The 35s is similar in many ways to the 15c I bought 20-odd years ago when I entered engineering school (note that I paid about $125 for it, while the 35s sells for 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.

I bet many of you are smiling to yourselves and saying, "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 borrowed, 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 takes 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.

One big 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, who claimed to be the force behind the 35s.

Kim said he wasn't satisfied with HP's existing line of calculators. "I really wanted to design something for the old-time calculator user while also attracting new customers," he said.

Two key components

In a sense, 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 covered by a blob of epoxy. The processor is under the bigger blob.

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 computer, 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, including an LCD driver, 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 has the ability to perform matrix manipulation, a feature that was way before its time, at least in a handheld calculator. And it was programmable.

With the design down to just the two main ICs, it quickly became clear that the real challenge of this product was in the software.

"The hardware has become pretty much routine," Kim said. "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 35s 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 trade-off would have to be made somewhere, in terms of both the cost and the size.

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, Idaho; Cupertino and San Diego, Calif.; and Vancouver, Wash.

The eventual high-volume manufacturing is being done in China. All meetings for the design occurred by phone, since there were no show-stopper design issues to overcome. "We were working on the mechanical aspects while they were doing the software and electrical design," said Kim. "The original industrial design was done jointly by the two teams. It was never a 'throw it over the wall' design."

An interesting aspect of the 35s is that it takes "overengineering" to a new level. For example, it's held together by 25 screws. "The keyboard is the most important part of a calculator. It must work year after year," Kim said. "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 says the calculator performs a 1 million-key press-test on the keyboard, meaning the keyboard will survive at least a million keystrokes-far more than a user will ever subject it to.

HP put the 35s through 11 different abuse tests and 20 common tests. These generally have to do with temperature thermal shocks, drops and so on. Many of the tests highlight common occurrences that a handheld calculator would be subjected to in everyday use, but others are in the category of unlikely assaults. For example, the testers did environmental tests, using the calculator at 55 degrees C for four hours, then storing it at 65 degrees C, with 95 percent relative humidity, for 120 hours. There are similar tests for low temperatures. The team isn't satisfied unless 100 percent of the tested units pass.

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

"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," Kim said. "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 the testing and overengineering, 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.


In Brief

The Hewlett-Packard 35s is a handheld scientific calculator. It's the latest in a long line of such devices, which are extremely popular with the engineering community. I was looking forward to this particular teardown because I used an earlier member of this family (probably the great, great, great grandfather) when I was in engineering school. The functionality has changed much over the years, and the form factor has actually grown somewhat. One of the most notable characteristics was the price: The 35s costs about $60, while I paid about $125 for my 15c 20+ years ago.


Component focus

The Hewlett-Packard 35s is comprised mostly of two key components-an 8502 microcontroller and a flash memory chip. Both parts are covered by a blob of epoxy. 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 about 20 years ago. 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. 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. After the program code is stored in the flash memory, there is about 30 kbytes available to the end user, enough for about 20,000 lines of code.


VIDEO: For more insight, view the related TeardownTV episode at:

Copyright 2008 United Business Media US, LLC. All rights reserved.

Copyright ? 2008 CMP Media LLC

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