Another schematic error
While looking for more A/D pins for adding some more functions in the SlopBucket II I found that there is another error in the schematic. The board is correct, just the schematic. Pins 17 and 19 are swapped on U8, the big microcontroller. That reverses the SPI clock and data that feeds the display. So, if you are like me, trying to breadboard the thing, then it won’t work until you swap them. The input to the display clock should come from pin 19 and the data should connect to pin 17.
The other error that I refer to is on U3 where the outputs (pins 4 and 5) are swapped on the schematic. These are only relevant if you are trouble shooting.
Free A/D pins
Sometimes it’s nice to have some Analog-to-Digital pins available so that you can implement a battery voltmeter, an array of buttons, read the temperature of the finals, or maybe add an S meter. Pin 23 is an A/D pin that has a trace that is fairly easy to access. Currently it feeds pin 2 on the display and is the Chip Enable (or Chip Select) signal. Since the SB2 only has one display on the SPI bus then we can just cut that trace and hardwire the display pin to ground for permanent selection. The remaining trace to pin 23 can now be used as an A/D input if you re-configure the firmware.
Another A/D pin looks like it is available if you move that reset signal from pin 26 to pin 18 which is just a digital pin and is currently not doing anything.
A bit more cuttin’-n-tackin’ will free up yet another A/D pin if you combine the two buttons to one A/D pin by using them to bypass parts of a series resistance chain to create a couple of different voltages which are then read as analog voltages and interpreted. This makes it possible to add even more function switches if you have the hankering.
The firmware changes to implement these changes are really quite simple. So easy that I might be able to help if anyone has questions! My QRZ address is good. Although none of this is posted on ND6T.com, you can reach me through there also.
Want to experiment with firmware without pulling the chip and placing it in a Uno, re-flashing, and putting it back on the breadboard? You can interface the display by using four level shift circuits (one for each of the needed pins). You can do it with just a 2N7000 and a 10K resistor for each of those four pins. That way you can leave the Uno connected to your computer and reflash and test in just seconds. Without any chip swaps.
The drain pin goes to the Uno, the gate goes to the 3.3 volt output of the Uno and one end of the 10K resistor (any wattage), finally the source pin and other end of that resistor goes toward the display. You’ll need one for each of the Reset, Data/Command, Data, and Clock pins. Well worth the effort!