This is a replacement firmware for the [diyleyuan calculator kit](http://www.diyleyuan.com/jc/L8Q.html). The calculator kit is available for purchase for less than $13 shipped from eBay by searching for "diy calculator kit". The calculator uses an STC IAP15W413AS microcontroller (an 8051 instruction set-compatible microcontroller) with a built in serial bootloader. This project uses SDCC to compile the C code and [stcgal](https://github.com/grigorig/stcgal) to load the new firmware.
The replacement firmware supports floating-point calculations, with a 4-level RPN stack. (The original firmware supported only fixed-point calculations in chain mode.) The new firmware currently completely fills the flash of the microcontroller (more on this below). I have not added in the resistor value calculator or decimal/hexadecimal converter on the original firmware.
Note that once you change the firmware on the calculator, it isn't possible to go back to the original firmware (it isn't posted for download anywhere). It also isn't possible to read back the flash from an existing STC microcontroller. This is a deliberate "feature" of the STC bootloader to prevent readbacks.
Note that once you change the firmware on the calculator, it isn't possible to go back to the original firmware. The STC microcontroller used has a bootloader permanently stored in ROM that allows downloading new firmware over a serial port (but not reading the existing firmware). You can re-program it using a USB-to-logic-level-serial (5V) dongle, and the stcgal program. WARNING: a lot of USB-to-logic-level-serial dongles are for 3.3V logic levels. The diyleyuan calculator runs at 5V to make it easier to power/drive the LCD display. You have a couple of options:
1. Buy a USB to logic level serial dongle that supports 5V operation (these dongles may have a jumper you need to set to switch between 3.3V and 5V). This is the best option.
- Here is one that works: https://www.amazon.com/gp/product/B00N4MCS1A/
1. If you have an adjustable power supply, power the diyleyuan calculator at 3.3-3.5V for programming, instead of 5V, and use a 3.3V USB to logic level serial dongle.
You must "cross over" TX/RX going between the microcontroller and the USB dongle (i.e. RX on the microcontroller goes to TX on the USB dongle, and TX on the microcontroller goes to RX on the USB dongle). You must also connect ground to the dongle. A good point to use is either pin 1 of U1, or the header P1.
Be careful when working on the calculator. The 7550 voltage regulator used has no short circuit protection. It does have a very low quiescent current and low dropout voltage though, and you must match the low dropout voltage if replacing the regulator. If you do end up damaging the regulator (like I did), a good replacement is the Microchip MCP1700-5002E/TO. In the picture, I have removed the 7550, shorted pins 2 and 3, and added a capacitor between pin 1 and pins 2/3. I am powering the calculator externally, instead of with batteries.
Here is the schematic from the diyleyuan website. Note that the schematic symbol for the microcontroller mistakenly labels P5.4 as P0.0, and mistakenly labels P5.5 as P0.1. The net name labels are correct.
The soft-latching power switch works as follows: Initially the calculator is off. Both Q1 and Q2 are off. Pressing the On key (S4) turns on Q1 through R1 and D2. Q1 then supplies 5V to the system. Once the microcontroller has power and starts running, it turns on Q2, which keeps Q1 on through R1. To turn off, the microcontroller turns off Q2, which in turn will turn off Q1.
The switches used are a knockoff of the Omron B3F series. A good replacement is the B3F-5050 (the switches included take quite a bit of force to depress). The LCD used is a fairly standard LCD based on a HD44780-compatible controller. The hole spacing for the screw holes on the LCD is 31mm x 75mm. There are many replacements available, including ones that don't need the backlight on to be readable. I recommend a positive FSTN type, although the one included is definitely usable.
See below for the stcgal output. Replace `stc_rpncalc/main.hex` with the actual path to the main.hex you built. In this example, I'm programming at a relatively high line rate of 230,400 bits/s. You may want to try at a slower speed 1st to get things working (omit the `-b 230400` option).
The = key is used for Enter. There is automatic stack lift so that `9`, `Enter`, `*` is equivalent to 9^2. The stack is a classic 4-level RPN stack, where the T register duplicates.
1. The calculator does not properly check for underflow or overflow sometimes.
1. Calculations with numbers with exponents near the max/min of +/-63 may give improper results, especially for division where intermediate results may go out of range, even if the result is in range.
1. After division by 0 or over/underflow, it's possible to negate `Error` to get an invalid number. It's also possible to subtract `Error`.
The original firmware that came with this calculator used a fixed point format, which significantly limited the range of numbers usable. Also, the implementation was slightly buggy (e.g. `3,162.277*3,162.28` gave `10,000,010` instead of the correct `10,000,005.31156`).
This calculator firmware uses decimal floating point, using base-100 to store numbers and do calculations. Base-100 allows for efficient storage into 8-bit bytes, and is easier to work with than packed-BCD. Unlike straight binary representations, base-100 is still fairly easy to display as decimal. Also unlike binary representations, there is no conversion error from binary/decimal (e.g. numbers like `0.1` can be represented exactly).
Each `uint8_t` stores a base-100 "`digit100`", referred to as an "`lsu`", for least significant unit (the terminology is borrowed from the decNumber library: I originally considered using the decNumber library similar to the WP-34S calculator, but just the library itself takes several times more flash than is available on this calculator). The number format is as follows:
There is an implicit decimal point between the 1 and 3 in `lsu[0]`, so the number is 1.350 * 10^1, which is equivalent to 13.5. Similarly, the number 1.35 would be stored the exact same way, except now the exponent is 0.
## Arithmetic
- Addition is done the same way as it's done by hand, although in base-100 instead of decimal.
- Subtraction is similarly done similar to how it's done by hand, although with carries instead of borrows using the "equal additions" algorithm, and also in base-100.
- Multiplication is done the same way it's done by hand, although in base-100. Partial sums are added up and shifted after each digit, instead of waiting until the very end to sum up all partial sums (as is common when doing long multiplication by hand).
- Division is done using a fixed number of Newton-Raphson iterations.
The keyboard matrix is scanned once every 5ms. The keyboard debouncing is based on the quick draw/integrator hybrid algorithm described [here](https://summivox.wordpress.com/2016/06/03/keyboard-matrix-scanning-and-debouncing/). This algorithm combines the advantages of both methods:
1. It signals a key press immediately, the very first instant a keyboard matrix scan detects a key is pressed (similar to the "quick-draw" method).
1. It has an "integrator" to determine both when a key is fully pressed and when a key is fully released. This prevents the mechanically bouncy keys from registering multiple times when pressed.
In practice, the keyboard debouncing works much better than the original firmware (which would occasionally miss keystrokes).
This was my 1st time using an 8051 microcontroller. The architecture is a bit limiting for programming in C compared to more modern architectures -- even compared to other 8-bit microcontrollers such as the AVR. Most significantly, there is no stack-pointer-relative addressing, which makes C functions takes up a lot of code space, since they must emulate stack-pointer-relative addressing. Unfortunately, the microcontroller used only has 13K of code space. It's almost completely full just implementing a basic 4-function, decimal-floating-point calculator.
I've avoided relying on the functions being able to be re-entrant, so that they do not depend on having a stack. Some "large" local variables are declared as static in functions to save on the code space needed to emulate a stack.
Another weird thing about the 8051 is that not all of the memory is addressed the same way. On this microcontroller, there are 512 bytes of ram total, of which:
Thus, there are special compiler directives to tell it where to place things in memory. (Even for a simple calculator, there isn't enough directly addressable memory (128 bytes) to store everything.)
Currently all of the code space is full, although there are ways to free some of it up. This would involve changing the code and build to not use `--stack-auto`. After doing so, it might be possible to add back in square root (using Newton-Raphson iterations), maybe logarithms/exponents (and thus arbitrary powers including square roots), and maybe if there's still space left, the original resistor value calculator and the decimal/hex converter.