WIP UI for setting a large number e.g. a frequency using only clockwise and CCW movements of a rotary encoder. ESP8266 with Arduino SDK.

a.h

struct TwoNibbles
{
  byte  v1:4;
  byte  v2:4;
};

union Nibble
{
  TwoNibbles nibble;
  byte Nibble[sizeof(TwoNibbles)];
};
Nibble wibble;

synth.ino

/*
Frequency synth with rotary encoder and OLED   
*/
#include <Arduino.h>
#include <U8g2lib.h>
#include <Wire.h>
#include <MD_REncoder.h>
#include <si5351.h>
#include "a.h"

Si5351 si5351;

// set up encoder object
MD_REncoder R = MD_REncoder(0, 2);

//set up display
U8G2_SSD1306_128X32_UNIVISION_F_HW_I2C u8g2(U8G2_R0, 0, 5, 4);

//GloboVars
uint32_t max_v = 1000000000; 
int32_t f = 1; // 1 Hz
uint32_t old_f = 0;
uint32_t v = 1;
int8_t y = 0;

  
void setup() 
{
      // Start serial and initialize the Si5351
  Serial.begin(57600);
  si5351.init(SI5351_CRYSTAL_LOAD_8PF, 0, 0);

  // Set CLK0 to output 14 MHz
  si5351.set_freq(1400000000ULL, SI5351_CLK0);

  // Set CLK1 to output 175 MHz
  si5351.set_ms_source(SI5351_CLK1, SI5351_PLLB);
  si5351.set_freq_manual(17500000000ULL, 70000000000ULL, SI5351_CLK1);

  // Query a status update and wait a bit to let the Si5351 populate the
  // status flags correctly.
  si5351.update_status();
  delay(500);

  // OLED
  u8g2.setI2CAddress(0x78);
  u8g2.begin();
  Serial.begin(9600);
  u8g2.setFont(u8g2_font_8x13_mr);
  u8g2.setDrawColor(1);
  u8g2.setFontMode(1);
  
  //Rotary
  R.begin();
  Serial.print("Up! Up! Away!\n");

  //MORE GloboVars??
  wibble.nibble.v1 = 0b0000;
  wibble.nibble.v2 = 0b0101;

}

void set_freq(uint32_t frq)
{
}

bool wibbler(uint8_t dir)
{
    // Wiggle-detector
    wibble.nibble.v1 = wibble.nibble.v1 << 1 ;
    if (dir == DIR_CW)
    {
      wibble.nibble.v1 = wibble.nibble.v1 | 0b0001;
    }
//    Serial.print(wibble.nibble.v1,2);
//    Serial.print("\n");
    if (wibble.nibble.v1 == wibble.nibble.v2)
    {        
      Serial.print("Wiggling!\n");
      return true;
    }
    return false;
}

void disp()
{
  uint8_t len_v = 0; 
  String cursor = String(v);
  len_v = cursor.length();
  len_v = len_v * 8;
  cursor = "_";

  uint8_t len_f = 0; 
  String freq = String(f);
  len_f = freq.length();
  len_f = len_f * 8;
  
  u8g2.firstPage();
  do {
    u8g2.setCursor(104 - len_f, 10);
    u8g2.print(f);
    u8g2.setCursor(104 - len_v, 11);
    u8g2.print(cursor);
    u8g2.setCursor(106, 10);
    u8g2.print("Hz");
    u8g2.setCursor(8, 32);
    u8g2.print(y);
    yield();
  } while ( u8g2.nextPage() );
}

void loop() 
{
  uint8_t x = R.read();
  int32_t test = 0;
  int32_t test2 = 0;
  old_f = f;
  if (x) 
  {
    //TO-DO: Make wiggle modal
    if (x == DIR_CW)
    {
      if (y == 9 && v < 1000000000)
      {
        v = v * 10;
        y = 0;
      }
      if (y < 10)
      {
        y += 1;
      }
    } 
    if (wibbler(x))
    {
      if (v >= 10)
      {
        v = v/10;
      } else
      {
        v = 1;
      }
      y = 1;
    }
    test = f+v;
    test2 = f-v;
    if ((test >= 0 && x == DIR_CW) || (test2 >= 0 && x != DIR_CW))
    {
      x == DIR_CW ? f=f+v : f=f-v;
    }
  }

  if (old_f != f)
  {
    set_freq(f);
    disp();
  }

}

What it does:
https://www.youtube.com/watch?v=KdfTfonmecc