Going against the low-power grain to resurrect and improve a 31-year-old HP calculator

Monte J. Dalrymple is a man with a mission: take one of HP’s most celebrated calculators, the HP 41C, and bring it into the 21st century. He’s done this by reverse engineering the 30-year-old CMOS “Nut” processor designed into the HP 41C—which ran at a heart-stopping 360 KHz—and installing that processor, plus an MMU, 100 plug-in ROM module images, and a few more features into an Actel FPGA on a replacement CPU board than can operate as much as 50 times faster than the original. (Processor speed is programmable.) All to upgrade a calculator that hasn’t been manufactured for more than 20 years. Such is the stuff of obsession. (Dalrymple says his day job is working as a processor designer.)


The original Nut processor with its third-of-a-MHz processor consumed 10 microamps of standby power. The new FPGA-based processor, now dubbed “NEWT,” consumes 11 times that amount, a whole 110 microamps. So standby battery power reduces from more than a year to perhaps a month or two. (The HP 41C runs on four N batteries.) The new processor draws 7.9 mA while running.


Here’s a photo showing the original and the new CPU boards (shown left and right respectively).


HP 41CL processor board


Note that the old CPU board exclusively uses through-hole IC packaging technology, which forced the ROMs (in 8-pin DIPs) to be piggybacked due to the lack of circuit board real estate. The NEWT CPU board uses modern surface-mount technology, developed back in the 1980s after the HP 41C was developed.


In case you too are obsessed with this pinnacle of HP calculator development…

You’ll find the manual for the NEWT processor here.


You’ll find the manual for the HP 41CL replacement processor board here.


And you’ll find an article about the project here.

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2 Responses to Going against the low-power grain to resurrect and improve a 31-year-old HP calculator

  1. I would note that although the new design draws more current in the off state, while running it actually does better than the original, even when running at 50x speed.

    The project started as a way to demonstrate that moving a design to a new process didn’t need to be expensive. I kept hearing that “it’s too expensive to design custom chips for calculators”. Being in the business I know that this isn’t necessarily true.

    Anyway, thanks for the mention. I’ve kept track of my progress over the last year at http://www.systemyde.com/hp41 for those interested.

    Monte Dalrymple

  2. Geoff says:

    This is the greatest HP 41 advance in 30 years. First the 41C then Synthetics for the 41, then the 41CV, 41CX and now to top them all and to bring a calculator into the 21st century:

    the HP 41CL by Monte.

    Beta testing the unit now and it is fantastic!

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