Saturday, August 23, 2014

6502 Virtual Trainer: Calculator Program

After I got my new Hakko FX-888, the first thing I did was solder the trainer to perfboard. It took some time to get used to the new iron but the board turned out well:

The FT232 cable connects to a few header pins soldered to a piece of perfboard connected perpendicularly to the main board by more pins that are bent at an angle. They make header pins at an angle but I used the last of mine on the board I made for programming the Z16F. There was also room to add some sockets and pins that can be used to program the MSP430, as well as power and status LEDs.

After using the board for about a week I had a strange error where the chip jumped to address 8080 on reset instead of 8000 which is stored at the reset vector. After some investigating, it seems there was a short between the Vcc pin of the IO expander and the pin controlling A7 of the 6502's address bus. On the back side of the board there was clearly no bridge which had me stumped but it worked fine after scraping away some of the flux between the joints. The problem reappeared after a week or two but was fixed again by cleaning the space between the joints. In frustration I ordered a W65C02 thinking the chip might have been damaged due to some acidental bus contention that had happened during testing. It seems that the pinout is a little different from the W65C816 I am using but it will still be useful because I plan to use it for an improved version of the trainer.

With the software I have made a little progress as well. The PC software skips sending any uninitialized memory and with a few other tweaks can program the whole 64kB memory in less than 3 seconds, down from 6 seconds. The microcontroller firmware is also a little better. Compiling with the -o3 flag in GCC brings average cycles per second up to about 16,000 from 12,000. However, setting the update rate to 100ms from 200ms to make typing more comfortable brings it back to around 14,000.

To learn 6502 assembly I decided to make a little test calculator program. It has turned out to be a really good way to get to know the chip, even though it has taken a lot longer than I expected. Below is a screenshot.

On the left is a text window for the 4-function RPN stack. It has 10 levels and each element is 8 bytes of packed BCD. This is convenient because the 6502 has a BCD mode. On the right is a window to enter formulas which can be graphed, like the first three, or repeated like the Celsius and Fahrenheit conversions. Formulas and graph colors can be selected with the dip switches. The toggle switch at the bottom selects between the two text windows and an LED above each window shows which one is selected. All of this could be done with a keyboard interface but I wanted to show off some of the peripherals that can be used. There is also a timer peripheral ticking every 500ms that the program reads to draw a flashing cursor in whichever of the two input text windows is active. The last peripheral is a multiplication ROM that works when single stepping the processor but not when running from SRAM yet.

There are still several improvements to be made to the trainer but I am done working on it for the moment. My plan is to port the code I have to an STM32 which will be much faster and also be able to store all 64kB of the 6502 address space in its internal RAM.

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