When the μBITX was manufactured as a “semi-kit” the Raduino control module was designed to simply plug into the main transceiver circuit board. This made wiring easier and also permitted close and controlled connection for the high-frequency synthesizer outputs. Much more reliable than extra cabling and shorter runs, too. The downside is that more of the input/output pins from the Nano micro controller are dedicated to native functions of the radio, and only two are available for additional modifications by the user.
One of those pins is already used as the key input for the CW mode. That left just one pin, an analog capable one, to be used for such purposes as monitoring supply voltage, an S meter, an RF power meter, or VSWR indicator.
The two available “spare” pins are the only two that do not have an internal “pull-up voltage” option available on the Nano processor. These pins are designated A6, and A7. In the initial design the A6 pin is used as a CW key input and so requires an external resistor to the +5 volt supply in order to detect key closures. That detection requires an Analog to Digital Conversion (A/D conversion) and consequent decoding and processing of the resultant measurement in order to determine each key closure.
It worked, but was highly prone to error, especially if key, cable, and connectors had anything but excellent low resistant contacts and connections. I found myself regularly burnishing key contacts and having to solder across every press-fit junction in both key plug and jack. Add to that the added shielding and bypassing of the key line and there still remained an annoying glitch or two at high speed. Ah! Opportunity!
Digital pins D0 and D1 are used for serial communication. D0 is labeled “RX0” on the board, D1 is labeled “TX1”. If you were to use an RS-232 arrangement, these would be the pins that you would have to use. When you use the serial monitor (like my pocket generator uses) then these are the two pins used. Even when you program the Nano, the on-board USB controller circuit uses those pins. However…
When you are not using a serial communication function, these two pins are “up for grabs”!
I opted to use D1 for a key input. As long as I don’t have a key plugged in and don’t have it pressed, the program loads nicely into the Nano and no one is the wiser. Since this pin has pull-up voltage capability, I don’t need an external resistor. Without A/D conversion the key is sampled in microsecond intervals. Since it uses TTL logic levels (0 and +5 volts) it is not prone to poor resistance connections, bypass, or shielding problems. Works like a charm.
Additionally, these two pins have their own little LED indicator lights on board the Nano. When one of the pins is taken to a low state (close to ground potential) the corresponding LED lights. Those nifty little indicators are usually hidden since the Nano is placed between its circuit board mounting and the display, but if you position yourself just right then you can see it light up. Nice little trouble-shooting extra!
Use the attached photo to locate the two pins on the back side of the board. The end pin is D1 and the next one is D2. Yes, they are out of order with the remainder of the pin numbers in that row. Cut a two pin section of right angle .1” spaced header block. Use alpha cyanoacrylate (Krazy Glue®) to mount the body of the plastic block to the printed circuit board, with the pins snugged up next to those two on the circuit board (D0 and D1). Then solder the new connector to those pins. That makes it convenient and easy to connect to your key jack or whatever else you want to sense or control.
The remaining digital pin is a good one to use for an added Function switch. I use it as a convenient access to a menu for changing sideband modes, switching VFOs, activating RIT, or adjusting my IF band pass.
That leaves two analog inputs. Hmmm!