Hello! I'm relatively new to programming and I'm working on a project for which I'm having trouble coming up with logic for a part of the code.
Here's what I want to achieve:
an odometer and speedometer for my cycle.
A hall sensor detects when my bike wheel makes one revolution. a variable NUMREV is incremented with each revolution (and then multiplied with circumference to give distance). the distance is then displayed on a oled display.
now, when a button is pressed, the screen is changed to show speed, instead of showing distance.
the distance part, I achieved. the speed part I was not able to achieve and I need your expert help.
here's the problematic part of the code ( the entire code, should you need it, is at the end.)
when the button is pressed, SCRSTATE value changes from 0 to 1 and the programmer starts to calculate speed and displays it.
else if (SCRSTATE == 1) //button is pressed, screen is switched to display speed
{
PREVNUMREV = NUMREV; //store the current number of revolutions
PREVMILLIS = millis();
if(millis() - PREVMILLIS == 1000); //one second later
{
NUMREVDELTA = NUMREV - PREVNUMREV; //difference in number of revolutions
DISTDELTA = (WHEELCIRC*NUMREVDELTA); //distance that was covered in one second
SPEED = DISTDELTA; //speed in meters per second
display.clearDisplay();
display.setCursor(20,20);
display.setTextSize(3);
display.setTextColor(SSD1306_WHITE);
display.print(SPEED); //print speed on display
display.display();
}
now I know what the problem is. the if statement is evaluating to false. so the Arduino skips this step.
but I don't know a solution
when the button is pressed, I need the Arduino to store current number of revolutions, then continue counting the number of revolutions (do nothing else) , then one second later, subtract the number of revolutions for previous number of revolutions.
I'm very sorry if this is confusing/a pain in the ass. but I could really use some help and I'd appreciate it.
thanks in advance!! 
FULL CODE;
#include <SPI.h>
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
#define SCREEN_WIDTH 128 // OLED display width, in pixels
#define SCREEN_HEIGHT 64 // OLED display height, in pixels
#define HALLPIN 10
#define OLED_RESET 4 // Reset pin #
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, OLED_RESET);
float WHEELCIRC = 2.00; // wheel circumference in meters
int NUMREV = 0;
int PREVNUMREV,NUMREVDELTA;
float DIST = 0.00 , DISTDELTA = 0.00, SPEED = 0.00;
byte HALLSTATE;
byte State1 = 0;
bool State2 = false;
byte SCRSTATE = 0;
byte SCRSTATE_MAX = 1; //always remember to subtract 1 from number of screens required as SCRSTATE starts from 0
unsigned long PREVMILLIS;
void setup() {
Serial.begin(9600);
// SSD1306_SWITCHCAPVCC = generate display voltage from 3.3V internally
if(!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) { // Address 0x3D for 128x64
Serial.println(F("SSD1306 allocation failed"));
for(;;); // Don't proceed, loop forever
}
// Show initial display buffer contents on the screen --
// the library initializes this with an Adafruit splash screen.
display.display();
delay(2000); // Pause for 2 seconds
// Clear the buffer
display.clearDisplay();
}
void loop() {
if(digitalRead(9) == 1 && State2 == false)
{
if(SCRSTATE < SCRSTATE_MAX)
{
SCRSTATE++;
}
else if (SCRSTATE == SCRSTATE_MAX)
{
SCRSTATE = 0;
}
State2 = true;
}
else if (digitalRead(9) == 0)
{
State2 = false;
}
HALLSTATE = digitalRead(HALLPIN);
if(HALLSTATE == 1 && State1 == 0)
{
NUMREV++;
State1 = 1;
}
else if(HALLSTATE == 0)
{
State1 = 0;
}
DIST = NUMREV * WHEELCIRC;
Serial.println(NUMREV);
if(SCRSTATE == 0)
{
display.clearDisplay();
display.setCursor(40,20);
display.setTextSize(3);
display.setTextColor(SSD1306_WHITE);
display.print(DIST);
display.display();
}
else if (SCRSTATE == 1)
{
PREVNUMREV = NUMREV;
PREVMILLIS = millis();
if(millis() - PREVMILLIS == 1000);
{
NUMREVDELTA = NUMREV - PREVNUMREV;
DISTDELTA = (WHEELCIRC*NUMREVDELTA);
SPEED = DISTDELTA; //speed in meters per second
display.clearDisplay();
display.setCursor(20,20);
display.setTextSize(3);
display.setTextColor(SSD1306_WHITE);
display.print(SPEED);
display.display();
}
}
}