на слабо не взяли? )))
Мозговой “штурм”, таки дал результат. 
После просмотра различных проектов, с использованием “заставок” на дисплее SSD1306, было установлено, что многие из них оформлены идентично. В авторском скетче, отсутствует недостающая строка , в конце блока, после delay(2000);
А именно.
display.clearDisplay();
Её дописывание, дало положительный результат.
Выглядит этот блок, теперь так.
void statup_text() {
display.setTextSize(1);
display.setCursor(13, 18);
display.print(F("Si5351 VFO/RF GEN"));
display.setCursor(6, 40);
display.print(F("JCR RADIO - Ver 2.0"));
display.display();
delay(2000);
display.clearDisplay();
}
После включения, отображается “заставка”, затем, где то через полторы, две секунды, высвечивается экран синтезатора и дальше всё работает, как надо. Таким образом, проблема решена. Выражаю благодарность всем, кто оказывал разностороннюю и посильную помощь в отладке скетча. 
Удачи вам! И до встреч на форуме 
Крайняя версия скетча, выглядит так.
/**********************************************************************************************************
10kHz to 225MHz VFO / RF Generator with Si5351 and Arduino Nano, with Intermediate
Frequency (IF) offset
(+ or -), RX/TX Selector for QRP Transceivers, Band Presets
and Bargraph S-Meter. See the schematics for
wiring and README.txt for details.
By J. CesarSound - ver 2.0 - Feb/2021.
***********************************************************************************************************/
//Libraries
#include <Wire.h> //IDE Standard
#include <Rotary.h> //Ben Buxton https://github.com/brianlow/Rotary
#include <si5351.h> //Etherkit https://github.com/etherkit/Si5351Arduino
#include <Adafruit_GFX.h> //Adafruit GFX https://github.com/adafruit/Adafruit-GFX-Library
#include <Adafruit_SSD1306.h> //Adafruit SSD1306 https://github.com/adafruit/Adafruit_SSD1306
//User preferences
//------------------------------------------------------------------------------------------------------------
#define IF 455 // 455 //Enter your IF frequency, ex: 455 = 455kHz, 10700 = 10.7MHz,
//0 = to direct convert receiver or RF generator, + will add and - will subtract IF //offfset.
#define BAND_INIT 1 //Enter your initial Band (1-21) at startup,
//ex: 1 = Freq Generator, 2 = 800kHz (MW), 7 = 7.2MHz (40m), 11 = 14.1MHz (20m).
#define XT_CAL_F 33000 //Si5351 calibration factor, adjust to get exatcly 10MHz. Increasing this value will decreases the frequency and vice versa.
#define S_GAIN 404 //Adjust the sensitivity of Signal Meter A/D input: 101 = 500mv; 202 = 1v; 303 = 1.5v; 404 = 2v; 505 = 2.5v; 1010 = 5v (max).
#define tunestep A0 //The pin used by tune step push button.
#define band A1 //The pin used //by band selector push button.
#define rx_tx A2 //The pin used by RX / TX selector switch, RX = switch open, TX = switch closed to GND. When in TX, the IF value is not considered.
#define adc A3 //The pin used by Signal Meter A/D input.
//------------------------------------------------------------------------------------------------------------
Rotary r = Rotary(2, 3);
Adafruit_SSD1306 display = Adafruit_SSD1306(128, 64, &Wire);
Si5351 si5351(0x60); //Si5351 I2C Address 0x60
unsigned long freq, freqold, fstep;
long interfreq = IF, interfreqold = 0;
long cal = XT_CAL_F;
unsigned int smval;
byte encoder = 1;
byte stp, n = 1;
byte count, x, xo;
bool sts = 0;
unsigned int period = 100;
unsigned long time_now = 0;
ISR(PCINT2_vect) {
char
result = r.process();
if (result == DIR_CW) set_frequency(1);
else if (result == DIR_CCW) set_frequency(-1);
}
void set_frequency(short dir)
{
if (encoder == 1) { //Up/Down frequency
if (dir == 1) freq = freq + fstep;
if (freq >= 225000000) freq = 225000000;
if (dir == -1) freq = freq - fstep;
if (fstep == 1000000 && freq <= 1000000) freq = 1000000;
else if (freq < 1) freq = 1;
}
if (encoder == 1) { //Up/Down graph tune pointer
if (dir == 1) n = n + 1;
if (n > 42) n = 1;
if (dir == -1) n = n - 1;
if (n < 1) n = 42;
}
}
void setup() {
Wire.begin();
display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
display.ssd1306_command(0xA0);
display.ssd1306_command(0xC0);
display.clearDisplay();
display.setTextColor(WHITE);
display.display();
pinMode(2, INPUT_PULLUP);
pinMode(3, INPUT_PULLUP);
pinMode(tunestep, INPUT_PULLUP);
pinMode(band, INPUT_PULLUP);
pinMode(rx_tx, INPUT_PULLUP);
statup_text(); //If you hang on startup, comment
si5351.init(SI5351_CRYSTAL_LOAD_8PF, 0, 0);
si5351.set_correction(cal, SI5351_PLL_INPUT_XO);
si5351.drive_strength(SI5351_CLK0, SI5351_DRIVE_8MA);
si5351.output_enable(SI5351_CLK0, 1); //1 - Enable / 0 - Disable CLK
si5351.output_enable(SI5351_CLK1, 0);
si5351.output_enable(SI5351_CLK2, 0);
PCICR |= (1 << PCIE2);
PCMSK2 |= (1 << PCINT18) | (1 << PCINT19);
sei();
count = BAND_INIT;
bandpresets();
stp = 1;
setstep();
}
void
loop() {
if (freqold != freq) {
time_now = millis();
tunegen();
freqold = freq;
}
if (interfreqold != interfreq) {
time_now = millis();
tunegen();
interfreqold = interfreq;
}
if
(xo != x) {
time_now = millis();
xo = x;
}
if (digitalRead(tunestep) == LOW) {
time_now = (millis() + 300);
setstep();
delay(300);
}
if (digitalRead(band) == LOW) {
time_now = (millis() + 300);
inc_preset();
delay(300);
}
if (digitalRead(rx_tx) == LOW)
{
time_now = (millis() + 300);
sts = 1;
} else sts = 0;
if ((time_now + period) > millis()) {
displayfreq();
layout();
}
sgnalread();
}
void tunegen() {
si5351.set_freq((freq +
(interfreq * 1000ULL)) * 100ULL, SI5351_CLK0);
}
void displayfreq() {
unsigned int m = freq / 1000000;
unsigned int k = (freq % 1000000) / 1000;
unsigned int h = (freq % 1000) / 1;
display.clearDisplay();
display.setTextSize(2);
char buffer[15] = "";
if (m < 1) {
display.setCursor(41, 1); sprintf(buffer, "%003d.%003d", k, h);
}
else if (m < 100) {
display.setCursor(5, 1);
sprintf(buffer, "%2d.%003d.%003d", m, k, h);
}
else if (m >= 100)
{
unsigned int h = (freq % 1000) / 10;
display.setCursor(5, 1); sprintf(buffer, "%2d.%003d.%02d", m, k, h);
}
display.print(buffer);
}
void
setstep() {
switch (stp) {
case 1: stp = 2; fstep = 1; break;
case 2: stp = 3; fstep = 10; break;
case 3: stp = 4; fstep = 100; break;
case 4: stp = 5; fstep = 1000; break;
case 5: stp = 6; fstep = 2500; break;
case 6: stp = 7; fstep = 5000; break;
case 7: stp = 8; fstep = 6250; break;
case 8: stp = 9; fstep = 8330; break;
case 9: stp = 10; fstep = 9000; break;
case 10: stp = 11; fstep = 10000; break;
case 11: stp = 12; fstep = 12500; break;
case 12: stp = 13; fstep = 25000; break;
case 13: stp = 14; fstep = 50000; break;
case 14: stp = 15; fstep = 100000; break;
case 15: stp = 16; fstep = 1000000; break;
case 16: stp = 1; fstep = 10000000; break;
}
}
void inc_preset()
{
count++;
if (count > 22) count = 1;
bandpresets();
delay(50);
}
void
bandpresets() {
switch (count) {
case 1: freq = 1; tunegen(); break;
case 2: freq = 153000; break;
case 3: freq = 522000; break;
case 4: freq = 1800000; break;
case 5: freq = 3650000; break;
case 6: freq = 4985000; break;
case 7: freq = 6180000; break;
case 8: freq = 7200000; break;
case 9: freq = 10000000; break;
case 10: freq = 11780000; break;
case 11: freq = 13630000; break;
case 12: freq = 14100000; break;
case 13: freq = 15000000; break;
case 14: freq = 17655000; break;
case 15: freq = 21525000; break;
case 16: freq = 27015000; break;
case 17: freq = 28400000; break;
case 18: freq = 50000000; break;
case 19: freq = 87500000; break;
case 20: freq = 118000000; break;
case 21: freq = 144000000; break;
case 22: freq = 220000000; break;
}
si5351.pll_reset(SI5351_PLLA);
stp = 2; setstep();
}
void layout() {
display.setTextColor(WHITE);
display.drawLine(0, 20, 127, 20, WHITE);
display.drawLine(0, 43, 127, 43, WHITE);
display.drawLine(105, 24, 105, 39, WHITE);
display.drawLine(87, 24, 87, 39, WHITE);
display.drawLine(87, 48, 87, 63, WHITE);
display.drawLine(15, 55, 82, 55, WHITE);
display.setTextSize(1);
display.setCursor(50, 23);
display.print(F("STEP"));
display.setCursor(46, 33);
if (stp == 2) display.print(F(" 1Hz"));
if (stp == 3) display.print(F(" 10Hz"));
if (stp == 4) display.print(F(" 100Hz"));
if (stp == 5) display.print(F(" 1kHz"));
if (stp == 6) display.print(F(" 2.5kHz"));
if (stp == 7) display.print(F(" 5kHz"));
if (stp == 8) display.print(F("6.25kHz"));
if (stp == 9) display.print(F("8.33kHz"));
if (stp == 10) display.print(F(" 9kHz"));
if (stp == 11) display.print(F(" 10kHz"));
if (stp == 12) display.print(F("12.5kHz"));
if (stp == 13) display.print(F(" 25kHz"));
if (stp == 14) display.print(F(" 50kHz"));
if (stp == 15) display.print(F(" 100kHz"));
if (stp == 16) display.print(F(" 1MHz"));
if (stp == 1) display.print(F(" 10MHz"));
display.setTextSize(1);
display.setCursor(92, 48);
display.print(F("IF:"));
display.setCursor(92, 57);
display.print(interfreq);
display.print(F("k"));
display.setTextSize(1);
display.setCursor(110, 23);
if (freq < 1000000) display.print(F("kHz"));
if (freq >= 1000000) display.print(F("MHz"));
display.setCursor(110, 33);
if (interfreq == 0)
display.print(F("VFO"));
if (interfreq != 0) display.print(F("L O"));
display.setCursor(91, 28);
if (!sts) display.print(F("RX")); if (!sts) interfreq = IF;
if (sts) display.print(F("TX")); if (sts) interfreq = 0;
bandlist(); drawbargraph();
display.display();
}
void bandlist() {
display.setTextSize(2);
display.setCursor(0, 25);
if (count == 1) display.print(F("GEN"));
if (count == 2) display.print(F("LW"));
if (count == 3) display.print(F("MW"));
if (count == 4) display.print(F("160m"));
if (count == 5) display.print(F("80m"));
if (count == 6) display.print(F("60m"));
if (count == 7) display.print(F("49m"));
if (count == 8) display.print(F("40m"));
if (count == 9) display.print(F("31m"));
if (count == 10) display.print(F("25m"));
if (count == 11) display.print(F("22m"));
if (count == 12) display.print(F("20m"));
if (count == 13) display.print(F("19m"));
if (count == 14) display.print(F("16m"));
if (count == 15) display.print(F("13m"));
if (count == 16) display.print(F("11m"));
if (count == 17) display.print(F("10m"));
if (count == 18) display.print(F("6m"));
if (count == 19) display.print(F("WFM"));
if (count == 20) display.print(F("AIR"));
if (count == 21) display.print(F("2m"));
if (count == 22) display.print(F("1m"));
if (count == 1) interfreq = 0;
else if (!sts) interfreq = IF;
}
void
sgnalread() {
smval = analogRead(adc); x = map(smval, 0, S_GAIN, 1, 14); if (x > 14) x = 14;
}
void drawbargraph() {
byte y = map(n, 1, 42, 1, 14);
display.setTextSize(1);
//Pointer
display.setCursor(0, 48);
display.print(F("TU"));
switch (y) {
case 1: display.fillRect(15, 48, 2, 6, WHITE); break;
case 2: display.fillRect(20, 48, 2, 6, WHITE); break;
case 3: display.fillRect(25, 48, 2, 6, WHITE); break;
case 4: display.fillRect(30, 48, 2, 6, WHITE); break;
case 5: display.fillRect(35, 48, 2, 6, WHITE); break;
case 6: display.fillRect(40, 48, 2, 6, WHITE); break;
case 7: display.fillRect(45, 48, 2, 6, WHITE); break;
case 8: display.fillRect(50, 48, 2, 6, WHITE); break;
case 9: display.fillRect(55, 48, 2, 6, WHITE); break;
case 10: display.fillRect(60, 48, 2, 6, WHITE); break;
case 11: display.fillRect(65, 48, 2, 6, WHITE); break;
case 12: display.fillRect(70, 48, 2, 6, WHITE); break;
case 13: display.fillRect(75, 48, 2, 6, WHITE); break;
case 14: display.fillRect(80, 48, 2, 6, WHITE); break;
}
//Bargraph
display.setCursor(0, 57);
display.print(F("SM"));
switch (x) {
case 14: display.fillRect(80, 58, 2, 6, WHITE);
case 13: display.fillRect(75, 58, 2, 6, WHITE);
case 12: display.fillRect(70, 58, 2, 6, WHITE);
case 11: display.fillRect(65, 58, 2, 6, WHITE);
case 10: display.fillRect(60, 58, 2, 6, WHITE);
case 9: display.fillRect(55, 58, 2, 6, WHITE);
case 8: display.fillRect(50, 58, 2, 6, WHITE);
case 7: display.fillRect(45, 58, 2, 6, WHITE);
case 6: display.fillRect(40, 58, 2, 6, WHITE);
case 5: display.fillRect(35, 58, 2, 6, WHITE);
case 4: display.fillRect(30, 58, 2, 6, WHITE);
case 3: display.fillRect(25, 58, 2, 6, WHITE);
case 2: display.fillRect(20, 58, 2, 6, WHITE);
case 1: display.fillRect(15, 58, 2, 6, WHITE);
}
}
void statup_text() {
display.setTextSize(1);
display.setCursor(13, 18);
display.print(F("Si5351 VFO/RF GEN"));
display.setCursor(6, 40);
display.print(F("JCR RADIO - Ver 2.0"));
display.display();
delay(2000);
display.clearDisplay();
}
Это какой-то маразм, потому что данная строка идет СРАЗУ после вызова функции вывода заставки.
Возможно, но работает.
А не лгбт328 ардуины нет? Проверить чтобы…
Есть, но она уже используется в другом проекте. Чуть позже, для чистоты эксперимента, запишу в неё этот же скетч. О результате отпишусь.
Только не последний, а предпоследний (мой) вариант интересен.
Хорошо.