A Voyage to the HP Spice Islands

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The Hewlett-Packard HP33 is a Spice series calculator. The model I have is a 1979 HP33E scientific programmable version with 10-1/2 digits in a red LED display (Figures 1-2).

Figure 1. Original HP33E, both sides of case.

Figure 2. Original internal circuit.

It has now been upgraded with the Teenix Spice board to an ‘unlimited’ HP33C; the C denoting ‘continuous memory’ which means my program isn’t lost when the calculator is switched off.

This was a big deal in the late ‘70s with the introduction of the new (but expensive) CMOS memory (in place of NMOS or even PMOS) that drew a small enough current to be hard-wired into a +2.6V battery.

There are many features available on the Teenix board to bring my trusty old calculator into the 21st century. So, let’s sail away to Malaku and Spice up my calculations!

HP produced the Spice series from 1978-1983; they were, in fact, the last of the red LED display calculators. Their code names during development were all spices: the HP33 was Sage; the 34 was Basil; and the financial 38 programmable was Chive.

My HP33E has 49 merged steps for programming, GOTOs, and GOSUBs, plus Conditional Branching. In other words, all the bells and whistles for an advanced late seventies programmable calculator.

As a young grad student in 1975, I had even used the earlier Woodstock HP25 calculator to write the programs for calculating Hertzian stresses for wheel/rail contact as part of my Master’s thesis. Those LED HP programmables were very capable machines.

The Spice series also represented a major improvement in simplified internal circuitry compared to the earlier Woodstock series, which still had a lot of interconnected chips. For comparison, the Spice has just one large 40-pin CPU chip and three eight-pin ROM/RAMs. The onboard switcher power supply (Figure 3) recharges the +2.6V twin NiCad AA batteries and provides the +6.5V for the NMOS circuitry.

Figure 3. Internal switcher power supply board.

That supply is a large board with many discrete components and a transformer (Figure 4).

Figure 4. The complete calculator, dismantled.

The simple external 10V AC transformer adapter for this switcher may damage the calculator electronics if used without the NiCad battery pack installed.

Early Spice calculator versions were solderless (mine was not), using a novel method of pressing the four chips against the circuit board with sockets for the LED display and conventional power supply board connections. Later production was conventionally soldered.

A neat trick was recently suggested by a member of the HP Museum Forum, allowing one to tell which version is fitted without dismantling. Simply weigh any Spice calculator on a postal scale with batteries in. The solderless type weighs 240 gm; the soldered type is (surprisingly) lighter at 180 gm. My Teenix Spice HP33 weighs in at 158 gm — the lightest of the lot since the heavy NiCads, plastic armature, and power board are gone.

In Spring 2023, Tony Nixon (https://teenix.org/) released a sophisticated design of a new board for all Spice models. It’s literally a solderless replacement. All the old boards on a Spice — including the keypad dome switches on the back of the original circuit board, the power supply sub-board, plastic armature to hold the main board in place, and the battery contacts sub-board — are discarded.

The 10-1/2 digit LED display and the case with the switches, keys, and damping pad are all that have been reused. If the earlier solderless Spice is present, just pull the LED display out of its socket. Otherwise, the display needs careful desoldering (the old LEDs don’t like heat).

The LED display is then slowly wiggled into the socket on the new board. This ‘new board’ is actually two boards spaced apart on soldered pins to neatly snap into the calculator case. The second board has a complete set of new tactile dome switches to replace the original HP dome switches (refer to Figures 5-6).

Figure 5. Original and new tactile dome keyboards.

Figure 6. Original and new circuit boards.

For the first time in a legacy LED display calculator (or even modern graphics calculators), Bluetooth is used for easy cable-free communication to a PC (it can be 20’ away or more). The calculator Bluetooth module ID is TEENIXC3x, as seen on the PC Devices and Printers via the Start Menu. There’s also a socket on the board for a USB/serial FTDI module (since radio-based Bluetooth might lose a critical bit or two while re-Flashing the operating program) to plug in when re-Flashing the calculator OS (currently at V8 for Spice). Both interfaces use the identical CalCom.exe PC program, written in Delphi (the visual evolution of Pascal). One simply enters the Bluetooth or USB serial COM ports after starting CalCom.

Everything is ‘on board:’ beeper; real-time clock with its own coin cell; and a foil printed circuit drive to the new power circuit board/charge socket. This latter is configured for a lightweight 800 mAh LiPo flat battery to fit in the empty battery compartment (formerly housing two NiCad AA cells). The battery is rechargeable in the calculator from any 5V USB port with a simple power cable.

The charging sub-board in the calculator is smart enough to shut off charging when the LiPo battery is full (using a constant current followed by a constant voltage algorithm). At that point, the LiPo controller extinguishes the blue charging light in the calculator power socket which is a signal to the user to unplug. It uses a linear controller IC intended for small portable applications using lithium polymer rechargeable batteries. The linear option is a big improvement over a switching power supply for reducing electrical noise, etc.

Speaking of batteries, HP had quite a sophisticated solution in 1978 for detecting a low battery by turning on a ‘dot’ above the LED display minus sign as an indicator. A bipolar voltage comparator IC (LLD for Low-Level Detector) is built into the display, sourcing 1 mA current to the dot whenever the original NiCad voltage fell below 2.2V.

Teenix adapted this for the new lithium battery, using a voltage divider to present 3.2V (a discharged LiPo) as 2.2V to the original LLD IC input display pin. (No need to re-invent the wheel.)

Measuring currents on my upgraded HP33 was interesting. Compared to the 140 mA for the original, only 14 mA is now required with the new board (‘0.0000’ displayed). This solves at one fell swoop the big problem with old LED calculators: power consumption. There’s a jump of 11 mA when Bluetooth is seeking a PC or transferring files.

Figure 7. Side view of old (top) and new (bottom) boards.

Bluetooth has come a long way in the last two decades for low power and modest radio data transfer, compared to power-hungry Wi-Fi with its torrent of information. That small 11 mA extra for Bluetooth allows the calculator to talk to the PC on a different floor in my stone cottage with ease.

On those rare occasions when the calculator displays a continuous ‘-8888888888’ (all segments lit) in the LED display, the current only rises to 26 mA. Clearly, the energy hog back in 1979 was not the red LED display as one might have thought. Rather, it was the NMOS CPU chip and its three ROM/RAM chips.

Figure 8. Inserting the desoldered LED display into the new board.

This meant a fully charged HP33E drawing 140 mA in 1979 could be flat after just 3-4 hours of use, stretching the definition of a ‘portable’ handheld calculator. Now that same calculator (upgraded to modern time, but still using the original LED display) can run for 40 hours between charges — a fortnight rather than a day of use.

A separate USB charger cable outputs 5V through a 2.5 mm power jack plug to the new power supply board. This board uses a Microchip MCP73831-2 LiPo linear controller in a five-pin package to charge the battery at 150 mA constant current (the choice being set by a 6.8K external resistor on the sub-board). The blue LED on the calculator power socket illuminates while charging. When 4.20V is reached at the battery, the controller switches to constant voltage charging, dropping out after the charge current falls off sufficiently and extinguishing the blue LED.

Figure 9. Success first time on power-up!

The modest charge rate means five hours to recharge a battery completely every couple of weeks typically. This low rate is a good thing since, together with the well-chosen linear controller, it obviates any thermal runaway problems with the small lithium battery.

The six-pin EXCOM socket on the new calculator board requires the case to be dismantled (not an easy task with the Spice design) to plug in the re-Flashing USB/serial cable via the FTDI module and USB cable whenever a new operating version is available from Teenix. To avoid needlessly distressing the 44 year old plastic case for new software upgrades, I experimented and found only three of the six pins are actually used by CalCom (the PC interface). These are Rx, Tx, and GND, being pins 2, 3, and 6 respectively on the EXCOM socket.

Figure 10. CalCom program running on PC to HP33 via Bluetooth.

This finding allows a small 2.5 mm stereo jack to be mounted on the lower case LH front to carry the three serial lines instead. This is actually an old dodge used by Casio in the early 1980s for communicating calculator to calculator.

The case can then be permanently closed. The only other missing signal at that point would be the flashing or steady blue LED on the board’s Bluetooth module to show coms status. I drilled a 2 mm hole in the lower case above the LED to make it still visible when closed, protecting the calculator from dust ingress through the hole with a purple bezel cover, glued unobtrusively on the inside.

Figure 11. Bluetooth module/antenna added to main board.

The operating PIC program on the Teenix board to simulate any Spice calculator (HP31, 32, 33, 34, 37, 38) uses the authentic HP operating instructions for each model from the 1970s which are read from old ROMs. These instructions for every key sequence and model were painstakingly re-written in Microchip PIC Assembler. In fact, one can choose which Spice calculator to be used from the calculator keyboard! However, without the front case overlay and correctly stamped keys, swapping versions makes little sense.

The PIC chosen is the advanced 18LF47K40 eight-bit RISC microcontroller in a 40-PDIP package. It has 128K bytes of Flash, 3.7K bytes of SRAM, and 1K of EEPROM. A 31-level stack is featured. About 9K of the Flash can be used for the PIC program itself. Also on the main circuit board is a 128K byte serial I2C EEPROM memory from SGS Thompson (the 24M01).

Figure 12. One megabit EEPROM (bottom) and real-time clock (upper).

This EEPROM allows four million write cycles — eight times that of a typical Flash memory. It holds its data without power for decades. Storage will never be a problem with the new board. One must have sympathy for the original HP designers working with 1% of this amount. The PIC itself runs at 12 MHz from an internal oscillator and only draws 1.4 mA. There’s also a 10-bit ADC (analog-to-digital converter) on the PIC which is not used.

A rather ingenious real-time clock from Maxim is fitted, complete with a DS3231 back-up coin cell. This communicates to the PIC using the ubiquitous two-wire I2C serial bus invented by Philips. It has a 32 kHz crystal inside the IC substrate and can be temperature corrected by a sensor also in the die. This is important for those low-frequency watch crystals as they have a pronounced variation with temperature, unlike their higher frequency cousins. This is why digital watches have not greatly improved over quality mechanical watches for timekeeping.

Figure 13. Jumpers and a miniature stereo jack added for serial coms.

The temperature compensation reduces the time error to a tenth of a normal quartz watch. The same temperature sensor is used by the operating system to show ambient temperature on the LED display, on demand. Given the tiny size of the eight-pin DS3231 clock package in free air, it should be fairly accurate. The HP33 calculator in my study recorded 21°C when the wall thermometer showed 21°C.

The backup coin cell means the clock is reliable no matter what happens to the LiPo battery. Other chips include two low dropout 3.3V regulators and a single-rail op-amp — all from Microchip. There’s a serial I/O coms chip from Texas Instruments near the FTDI socket.

The Bluetooth radio module mounted on the main board is from Cambridge Silicon Radio (UK) with its own 26 MHz crystal. The 2.4 GHz antenna is printed on its sub-board. This radio easily reaches two floors up in my limestone cottage for calculator-to-PC communications.

Figure 14. A 2 mm hole added to back of case to view Bluetooth status.

The module is fitted with a blue LED that flashes while seeking its Bluetooth master (the PC), then goes to a steady blue when CalCom is launched through the Bluetooth COM port and connects. It consumes a modest extra 11 mA from the HP33 in active use.

Figure 15. PIC microcontroller in 40-pin PDIP package.

This Bluetooth link is a first for a 45 year old legacy red LED display calculator, perhaps even a first compared to the latest high-end graphic calculators from the big three manufacturers. In fairness to those manufacturers, the need to be approved for high school examinations probably rules out external radio communication.

Figure 16. The new Teenix board installed in my HP33.

Bluetooth radio was invented by LM Ericcson at their Research Lab in Lund, Sweden in 2000, using frequency hopping in the 2.4 GHz ISM band. They partnered with IBM; hence, the ThinkPad was one of the first laptops to get Bluetooth. It was named after Harald ‘Bluetooth’ Gormson — a 10th century Danish king who united some parts of Scandinavia, although he died in battle in AD 986 fighting a rebellion led by his son, Sweyn ‘Forkbeard.’

Consequently, the Bluetooth protocol was promoted as uniting short-range communications. Later, the BLE (Bluetooth Low Energy) variant was developed to reduce power and bandwidth even further for the Internet of Things.

Once I got the calculator reassembled, I explored the new features. Instead of a single 49 merged-step program lost at switch-off on my original HP33E, there are now 10 blocks of memory, each capable of holding 30 x 49 step programs permanently, even if the battery is removed (doing that would erase all so-called CMOS RAM ‘Continuous Memory’ on the original HP33C)

These programs can be transferred with Bluetooth back to the PC for viewing and printing or be stored in the new calculator EEPROM. Figure 17 shows a simple test program handled this way.

Figure 17. Program downloaded to the PC from HP33 with CalCom.

The Block Directory of programs can also be downloaded to see what’s there and edited as needed. Programs can simply be given a number or an alphanumeric name.

The settings of the HP33 can be downloaded and rewritten, including low/bright LEDs, off/soft/loud beeper, alphanumeric program instructions at the calculator display instead of key codes, etc.

Figure 18. HP33 settings downloaded to the PC via Bluetooth.

Thus far, it’s not possible to use the beeper or the clock features directly in a program. The clock itself allows for a very accurate time-of-day (and date information) to be called up on the calculator, as well as offering alarm and timer functions which can sound the beeper as a signal.

To sum this up, my favorite “Thing” (as in The Internet of Things) currently is the Sage Teenix HP33 red LED calculator from 1979, rejuvenated for the 21st century.

Figure 19. The final upgraded and reassembled HP33 calculator.

It has been a rewarding and fun journey to the Spice Islands; no shipwrecks thus far. The cosy glow of the red LED display in winter doesn’t eat up the battery either.