HP Classic Calculators Emulator

A Bit of History…

I have used HP RPN calculators all my life. I wonder why there aren’t any RPN calculators anymore. They have been phased out in favor of the standard infix calculators.

I always wanted an HP-15C calculator and never got the chance to buy one, so this is my take on building one.

It is designed with only through-hole parts for easy soldering.

The brains of the calculator is the ATMEGA328. It takes care of everything (display, keypad scanning, sound, clock, etc.). I decided to use the ATMEGA328 since it is pretty common in the Arduino community and it is one of the few microcontrollers still produced as a through-hole part. No other semiconductor is used.

Even though the ATMEGA328 is an 8-bit microcontroller and running at 8Mhz, it is still pretty fast. In the end, the emulation is at least 2X faster than the original.

I've added a speaker for audible keyboard feedback, and a 32.768khz crystal for the real time clock.

The emulator can run any of the Voyager series calculators as well as the HP41C (my personal favorite!).

REAL Engineers ONLY use RPN!

Features...

All through-hole parts (easy to handle and solder)
Runs about 4 to 5 times faster than the original
PX-15C has 160 registers, 96 more than the original.
PX-16C have the same memory than the original
PX-41C has 2.5 times more memory than the original (the same as having installed two and a half memory modules)
Continuous memory (memory is not lost when the calculator is turned off)
Auto power off
Runs on a single ATMEGA328 8-bit microcontroller running at 8MHz (same as Arduino Uno)
Speaker for audible keypress feedback
192x64 lcd display
Display backlight
Real time clock (uses a 32.768KHz crystal)
Uses a single CR2032: 3V, 235mAh battery
Power consumption: 1µA off, 350µA idle, 3mA working (this is with speaker and backlight off)
With normal use, it should last several months
In programming mode, the names of the keys are displayed on the header of the display on the PX-15 and PX-16
It's a bit smaller than the original - 4.2" x 2.67" (10.668 x 6.7818 cm).

Emulator Settings

With the calculator turned off, press the '0’ button ('÷' on PX41C) and turn it on. Do not release the '0' ('÷' on PX41C) button yet.
The display will turn on and show the version along with mini-instructions.
When you release the '0' button ('÷' on PX-41C), the setting will be shown.
Press the ‘ON’ button to exit.

Options that can be customized:

BKLT: Backlight intensity (0-9)
BKTM: Backlight timeout in sec (0-255)
CONT: Contrast (0-15)
SLPT: Sleep timer (0-255)
BEEP: Audible feedback
1224: Hour format (12hr or 24hr)
Time and Date: Hours are set in 24hr format

Use the '÷' and '×' ('9' and '6' on 41c) buttons to select an option.
Use the '+' and '-' ('3' and 'R/S' on 41c) buttons to increase or decrease value by 1.
Use the '3' and '∑+' ('2' and '.' on 41c) buttons to increase or decrease value by 10.

Advanced Settings Options:

Pressing the '9' key (‘8’ on PX-41C) will toggle between normal settings and advanced settings.

Options that can be customized:

RUNC: Number of cycles to execute at a time
KEYC: Key cycles to wait before processing it
DBOU: Key debounce time
DISC: Number of cycles to add before redrawing display (41C only)
SSPC: 41C only, number of slow speed cycles before speedup
BATL: Current battery level, can't be modified
BATT: Low battery indicator threshold

Assembly Tips...

Building 101

For assembly instructions click here.

Firmware

Current firmware version v0.89.

Download Firmware v0.89

Fixed/Added:

Fixed broken self tests (PX-15c still shows ERROR 9).
Fixed broken low battery indicator.
Better key debounce.
Changed settings menu activation to 'ON' and '0' Keys, ('ON' ',' key is used to change radix)
On settings menu:

Keys '3' and '∑' increments and decrements value by 10.
Press key '9' to access emulator tweaks.

RUNC: Number of cycles to execute at a time.
KEYC: Key cycles to wait before processing it.
DBOU: key debounce time.
DISC: 41c only, number of cycles to add before redrawing display.
SSPC: 41c only, number of Slow Speed Cycles before speedup.
BATL: Current Battery Level, can't be modified.
BATT: Low Battery Threshold, can't be modified.

Bugs

Firmware version v0.85:
Bugs:

15c: in Prog Mode, key decoder does not decode STO, RCL when /*-+ are used. nor TEST funcion. x mean is shown as X.

Firmware version v0.81:
Bugs:

On rare occasions when you turn on the calculator the display won't turn on. It seems this only happens with a low battery.
So, you must turn it back off and on again.
Performing the self-test (pressing the ON and X keys at power up) will show ERROR 9.
You will need to press the reset key to restore the calculator to working order.

First Release: Firmware version v0.80:
Bugs::

On rare occasions when you turn on the calculator the display won't turn on. It seems this only happens with a low battery.
So, you must turn it back off and on again.
In programming mode, the key decoder shows the rolldown label backwards.
When you have an Overflow condition, the display will show ±9.999999999×10⁹⁹ but it won't blink like in the original.

Known Issues:

On PX-15C: Performing the self-test (pressing the ON and X keys at power up) will show ERROR 9.
You will need to press the reset key to restore the calculator to working order.
The pause (PSE) instruction is shorter since it is running faster than the original.

WARNING!: Pressing the reset key or uploading new firmware will erase all user content (programs and registers)!

Loading New Firmware:

To load new firmware, you need a standard AVR SPI (Serial Peripheral Interface) programmer. Such as the usbasp.
You can even use an Arduino Uno as a ISP programmer.

Also, you will need avrdude or some other software to program the microcontroller.

Most AVR programer have a 6 pin (2x3) or 10 (2x5) pin header. The calculator uses a 6 pin (1x6) header. So you will probably need to buy an adaptor or simply make one.

I simply cut the ribbon cable and solder a 6 pin male header.

CAUTION!!!
Be sure the voltage on the programer is set to 3V, failure to do so will destroy the LCD.
Remove the battery or disable power on the programmer. Use only ONE power source.

Programming Adaptor Image Gallery.

3D Printed Case

You can print your own case.

For the short keys (7mm buttons), the STL files can be downloaded from Thingverse 3D Case.
For the tall keys (11mm buttons), the STL files can be downloaded from Thingverse: Bumper.

Our friend Shigemaru Nishiyama has designed a battery cover for the 3D printed bumper.
You can download it from thingverse.

You can download the pdf keyboard overlay here:

RPN

RPN (Reverse Polish Notation) is a mathematical notation that allows users to solve problems by mimicking how they learned to do math on paper. The operators (+,–,x,÷) are placed after the arguments (for example, 3+4 becomes 3 {ENTER} 4 +) allowing users to stack number sequences and operations, working from the bottom up. RPN eliminates the need for parentheses in complex calculations and reduces keystrokes, making problem solving quicker and more efficient.

Display

The LCD display has 12,288 (192x64) pixels. Enough space to draw all bitmap fonts along with the status bar. By the way, did I mention that is's backlit?

The Brains

The ATMEGA328P microcontroller from ATMEL (now Microchip) is behind the PX-15c. It was chosen because of it's popularity with the Arduino community and it is still one of the few microcontrollers that is produced as a through hole part.

Speaker

There is a standard magnetic speaker that is used as audio feedback on button pressed.

Tactile buttons

There are 39 standard of the shelf 6x6mm tactile switches.

Clock

Did I mention it has a real date and time clock?
You can always see the date and time on the status bar. The time heartbeat is provided by a standard 32.768KHz watch crystal.

Energy

Energy comes from one itty-bitty buton cell, a standard CR2032 3V, 235mah lithium battery that should last a long time with normal use.
To conserve energy, the PX-15c will speed down when ideal and will go to sleep after some time of inactivity. The backlight will also turn off after some time.

Mentions/Reviews

Hackaday Articles:

Reviews:

Hackaday: BUILDING THE DIY HP41C: A FIELD REPORT.
EEVblog YouTube: EEVblog 1463 - Mailbag.
Pocket Griffon Blog: Assemble the PX-15c! (in Japanese).
Fun stuff Blog: PX-15C calculator kit.
Embedded Village Electronic Work Open Lab: Let's make a reverse polish calculator kit. (in Japanese).

Resources:

The Museum of HP Calculators
The Museum of HP Calculators: Forum
hp41.org: HP-41 PROGRAMMABLE CALCULATOR

Articles:

HP41 in Space.

PX-16 in the wild:

Simple exponents on an HP-16c, (IMA PENGUIN)

FAQ's

Yes, all the emulators are programmable.

RPN is considered a more intuitive and efficient way to enter data in a calculator. Once you try it, you won't be able to go back to infix style calculators.

The idea to use only through-hole components is so that anyone with basic soldering skills can have a go at it.

No, this is not a graphing calculator. The original HP-15C did not have graphing capabilities. Since we are emulating it, neither does the PX-15C.

Well, because most 16, 32 or 64bit microcontrollers are not produced in through-hole factor and the ATMEGA328 is very popular within the Arduino community.

All code was written in standard, plain C heavily optimized for space, not speed, and compiled using GNU AVR Toolchain on a Mac. Some functions are written in assembly to speed up execution.

They can be downloaded from:
HP-15C Owner's Handbook.
HP15C Advanced Functions Handbook
HP-15C Quick Reference
HP-16C Owner's Handbookl.
HP-41C/41CV Owners Handbook and Programming Guide

Your can purchase the kits tirectly from tindie:
PX-15c kit.
PX-16c kit.
PX-41c kit.

Many thanks to all the wonderful people from the "The Museum of HP Calculators" Forum.
They are the reason this project exists. I can't begin to name all of the great people who reverse-engineered the ROMs (Eric Smith, etc).

15C, 16C or 41C - Take Your Pick
Build Your Own Classic HP Calculator Emulator

A Bit of History…

REAL Engineers ONLY use RPN!

Image Gallery

Features...

Emulator Settings

Assembly Tips...

Building 101

Firmware

Bugs

Loading New Firmware:

3D Printed Case

RPN

Display

The Brains

Speaker

Tactile buttons

Clock

Energy

Mentions/Reviews

FAQ's

Want more information?
Contact us!

A Bit of History…

REAL Engineers ONLY use RPN!

Image Gallery

Features...

Emulator Settings

Assembly Tips...

Building 101

Firmware

Bugs

Loading New Firmware:

3D Printed Case

RPN

Display

The Brains

Speaker

Tactile buttons

Clock

Energy

Mentions/Reviews

FAQ's

Is it programable?

Why use RPN?

Why do you use through-hole components?

Can it graph?

Why are you using a 8Bit microcontroller?

What is it coded on?

Where can I find the original manuals?

Where can I get one?

Credits:

Want more information?Contact us!

Want more information?
Contact us!