Open Projects

After starting on my 7400 Calculator project, I thought about how many projects I have open at the moment. Like a lot of hobbyists, I seem to start more than I finish. Most of these are not abandoned, since I do plan to get back to them someday. I decided to take stock here of how far I am and what is left to do. There are two projects here I have not posted about: 6502 Optimizing Assembler and 6502 Calculator Emulator. There isn't really enough yet to write about, but I might as well mention them since I have been working on them now and then for a while. They would also be good for the github page I want to set up and list when I apply for jobs. My plan is to finish most if not all of these projects before I start on something else. Otherwise the list might keep growing without finishing any of them.

Tiny Calculator

80%

A few weeks ago, I put a lot of work into the interface code, which ended up taking more time and effort than I imagined. Like in my RPN Scientific Calculator, the interface code takes up a lot of space compared to the math routines. So far, I can enter numbers and do addition and subtraction, but I still need to fix the sign of the result. For the rest of the calculations, I need to write a lot of checking code for things like scaling angles before applying trig functions and avoiding inputting negative numbers in logarithm functions. Two weeks ago I made some major progress with the keypad, which I will post about soon. Posts related to Tiny Calculator

Pocket Calculator

20%

This project kind of stalled because I got busy with other stuff and also because I hit a few hardware problems that I did not solve. The LCD would work when the speed of the SPI clock was set very low and it would continue to work once I turned the speed up to what the LCD is rated for, but if I turned the calculator off, the LCD would not work again until I set the speed back to very slow. The LCD also did not work when I fully closed the case. Eventually, one of the very stiff ethernet wires I was using broke right off. I will need to rewire the LCD with different wire and start over on the emulator I was working on. Posts related to Pocket Calculator

Programmable RPN Calculator

95%

The last time I worked on this calculator in 2015, I considered it finished other than labels for the keypad. One thing I figured out in the meantime is that the key reading is not entirely reliable since two of the pins on the LPC1114FN28 are open drain, which I did not account for. It also seems that I did not write the key reading code correctly, so there is a chance of shorting pins when you press two keys at once. These probably won't take long to fix, then I will make labels for the keypad, which should be easy now that I have done it for my Tiny Calculator. Another thing that might be helpful is to review the source code and make sure all of my checking code is correct. Posts related to Programmable RPN Calculator

7400 Logic Calculator	30%
This is my latest project and I have been doing a lot of work on it, so I am still really excited about it. I might try to finish some of the projects that are close to done before I return to this to get them out of the way. Posts related to 7400 Logic Calculator

6502 Graphing Calculator

???

In summer 2015, I put this calculator together very quickly to try to get it ready for Makevention that year. Not suprisingly, it did not work at all since I was in such a hurry putting it together. I'm not sure it would be worth it to resurrect this project, but I would like to at least return to it and figure out why it didn't work. One possibility is that the LCD was damaged when I was trying to drive it with an MSP430 since the wires I used had metal connectors that kept shorting to each other. Another thing I noticed when I pulled the board out a few weeks ago was that the EEPROM is not a modern CMOS chip but something older that uses 140mA and has a slow 250ms access time. The latches to drive the LCD are 74LS874s, which I now realize consume way more power than HC chips. There could also be other problems like the CPLD. In any case, it will be fun to try to figure everything out. Posts related to 6502 Graphing Calculator

6502 Optimizing Assembler

25%

This is a project I started to practice my Python skills and have a concrete project to talk about in job interviews. My plan is to analyze assembly files (not compiled binaries) and make optimizations based on a few simple assumptions. This will should be able to shave a few cycles off of unneeded instructions, which is especially useful when you are using macros. It will also be able to manage zero page much better than just assigning fixed addresses to each function or sacrificing the X register for use as a slow pseudo stack. Posts related to 6502 Optimizing Assembler

6502 Calculator Emulator

40%

This is a 6502 emulator written in JavaScript that will let me test calculator firmware in the browser without having to upload it to a physical calculator. The emulation works at over 50MHz currently and passes Klaus' test suite. The input and screen output work, as well as memory paging like the real calculator will have. Now I have to find a way to speed up screen drawing, since I think this is the source of lag when typing. Then I will need to start writing the firmware for the calculator. Someday I will host it on a website so that other people can help me find bugs. Posts related to 6502 Calculator Emulator

Makevention 2015

Makevention went really well! I was able to show everything I have been working on, although I didn't have time to finish everything. I made a poster board explaining some concepts related to calculators like RPN, BCD, and CORDIC on the left side with information about my RPN Scientific Calculator on the right. I also had a print out of how my external RAM preprocessor works. A few of the people who came by my table were programmers and understood the solution.

On the table itself I had different calculators and projects laid out in roughly chronological order. The first was an MK-61 RPN calculator I added to my collection over 10 years ago. Of course I didn't make it but I wanted to show an example of an RPN calculator and what inspired me to first make calculators. When I first got the calculator out to test it, it didn't show anything on the screen and I was afraid it was broken. I took it completely apart and looked for burst capacitors. I also brushed off the battery contacts since they didn't look very clean. When I turned it on, it still didn't show anything but when I pressed a number it showed up! It seems the calculator just doesn't display anything when first turned on, instead of the zero I was expecting. Somehow I had forgotten that in the 10 years its been since I last used it.

The Casio Algebra FX 2.0 was included to show the RPN stack program I wrote for it in 2002 or so. That program was designed to make the calculator function more like the HP-48GX I had at the time which I admired so much. First, I had to buy a calculator to replace the Algebra FX 2.0+ I sold in college. Loading the program onto the calculator took quite a while to figure out! In 2002 I used a serial cable, which I think was an SB-87, to load programs without any trouble. Nowadays I don't have a serial port, so I bought an FA-124 USB cable. It turns out that this cable doesn't work on Windows 7. I also couldn't get it to work correctly under Windows XP running in an emulator. Next I tried a lot of different third-party software and finally got the program to load using FlashCOM 1.4v and an FTDI cable. For the link port I bought a 2.5mm plug that I could easily wire to the FTDI cable. After that, the program I loaded kept crashing and I thought the transfer was corrupt. After more fiddling I got Turbo C up and running in an emulator and compiled the program source myself, which produced a fairly stable program that was good enough for exhibiting, although it did crash a couple times during Makevention. Someday I would like to fix the program up and make it more stable.

The next thing I showed was a launchpad hooked up to a breadboard with a button and blinking LED to show how easy it is to get started with electronics. The next step was my binary LED calculator. I soldered a coin battery holder on to the board. After that I showed the RPN Scientific Calculator and explained that it was not too difficult to move to this stage after I managed to finish the binary LED calculator. The Programmable RPN Scientific Calculator was next. I printed temporary a colored keypad on poster paper which I taped to it, but it did not work very well. Unexpectedly, the calculator showed random keypresses when I put my hand near it. All the keys were in the same column on the keyboard, which makes me thing there is a bad connection in that row of the key matrix.

Next I showed the Improved 6502 Virtual Trainer up and running on my laptop, along with the first 6502 Virtual Trainer. I also showed what I had accomplished with the 6502 graphing calculator. So far, I have most of the hardware for it installed but I am having problems keeping the buffer chips for driving the LCD at the appropriate voltage level. Maybe the pins joining the sections of the calculator together aren't making a good connection. In any case, I showed the calculator taken apart into its sections. I also showed the Logic Tool I have been using to debug the graphing calculator and the EEPROM programmer I built. The last thing I showed was the TI-86 I bought to hook up to an ESP8266. My plan was to relay text from the calculator over WiFi to a computer running Matlab or similar software to allow the calculator to solve more complex equations. I still haven't gotten the ESP8266 voltage levels to work right, but I want to finish this project when I have time.

All in all it was a great day and I really enjoyed showing the public all the things I have been working
on.

RPN Scientific Calculator on Hackaday

In August my RPN Scientific Calculator was featured on Hackaday in Hacklet 70 - Calculator Projects. It was really great to see my project on Hackaday! There was also a small scientific calculator running a 6502 emulator, a KIM-1 replica with a calculator form factor, and a project called CalcHack that adds functionality to TI graphing calculators. CalcHack is an awesome idea and one thing it adds is a 2.4GHz radio. One of my ideas for Makevention was adding an ESP8266 to the inside of a TI-86, but this solution is much better! Hackaday also created a page for calculator projects where I added my RPN Scientific Calculator, Improved RPN Scientific Calculator, and 6502 Graphing Calculator.

New Project: Logic Tool

Since my plan to make a wireless breadboard did not work out, I decided to use the MCP23S17 shift registers I bought for it to make something similar. It is similar to a logic analyzer but shows information in real time rather than recording a short period of time at high resolution. It has female headers for 64 pins but so far I have only soldered 32 of them in. As time gets closer to Makevention, I'm trying to save time however I can to get my last few projects working.

With 64 pins I will have enough to monitor all of the lines of my 6502 project. Unlike a logic analyzer, I can

also switch any of the pins to be outputs. Now I am doing this with a simple GUI I made that shows or controls the lines with virtual LEDs. This saves me from hanging loads of LEDs on all the lines of my project. Later I would like to add other forms of input and output that I can drag and drop to the dark gray area in the middle of the form. This could include buttons, sliders or hex readouts like I had for my 6502 Virtual Project. At the moment, only the LEDs work and I am using them to drive the address and data lines of the 6502 socket for my graphing calculator project. This lets me check the control signals of the CPLD fairly quickly.

In the end I think this project will work out better than the plan I had for the Wireless Breadboard. When I tried the breadboard I was going to use with another project, it showed a big voltage drop across the rails. I did not know this could happen and it would have been unfortunate to build the whole project and then find out the board was not very good.

New Project: 6502 Graphing Calculator

One of the last things I want to make progress on before Makevention in a few weeks is a graphing calculator I have been planning for quite a while. It will use a 65C02 and is the main reason I started on my 6502 Virtual Trainer last year. Most models of Texas Instrument's graphing calculators use a Z80, which was a competitor to the 6502 in the past. Those models have a 6 or 15MHz clock. The Z80 needs roughly three times more cycles than a 6502 to accomplish the same task, so running this calculator at 5MHz should be as fast as one of those calculators at 15MHz. At the maximum speed of 14MHz, it should be nearly three times as fast. Interestingly, Hewlett Packard did produce several non-graphing calculators like the HP-35S, which used a 8502 chip based on the 6502. Later models of TI calculators used the Motorola 68000 and the latest use ARM processors. These will certainly outperform anything I could make with a 6502. My hope is to make something in power between the Z80 and 68000 series with a few added features.

So far I have finished most of the decoding logic in the ATF1508 CPLD I have been working on. It has 57

IO pins and I have managed to use them all without finishing all the functionality I need. My plan is to use a 512KB SRAM chip with the first 2KB of address space always mapped to the first 2KB of the SRAM. The next two 30KB chunks of address space will point to two windows in the SRAM controlled by the CPLD. This will let me copy large chunks of data between two parts of the SRAM without changing the window between copies. Hopefully 30KB is enough for any single program the calculator will need to run. The last 2KB will be the EEPROM. This is hopefully enough space for a small bootloader that will copy firmware from an SD card and into the SRAM banks. The peripherals like the keyboard and LCD are mapped into the last few bytes of the first 2KB of SRAM.

For the LCD I found a fairly cheap 3.2 inch 320x240 color LCD on eBay. It is the perfect size for a calculator like this. It was also easy to get working with an MSP430 and some shift registers. The only downside is that I did not get an SPI version and I do not have enough CPLD pins to drive the 16 bit data port. The good news is that the LCD seems to expect 3v signals, so I will need some shift registers for level shifting anyway, which frees up a lot of pins. I don't think I will be able to finish very much more of this project for Makevention but I would like to get it drawing to the LCD from a program stored on an EEPROM by then. If I do have extra time, I will work on getting it to read a keypad.