RPN scientific calculator firmware for $13 calculator kit

[jklsadf]

One last thing I just thought of...in order to fully turn off the calculator so that it can be turned on again to run the bootloader, it might be necessary to prevent the microcontroller from being parasitically powered through the serial port pins if you have the USB-to-serial adapter connected..

The easiest way to do that is to unplug the USB-to-serial adapter from your computer while turning off the calculator.

For a more permanent setup, you can add a diode/resistor as shown on page 12 in the datasheet (note the pinout of the microcontroller shown is different, but the basic idea is the same: the diode prevents the calculator from being parasitically powered, and I think the 300ohm resistor is just for isolation). The diode used is unimportant, just about any diode will work. The stcgal FAQ has other possible ideas for how to prevent parasitic powering, but the datasheet method works well.