USING HX711 ( WITH LOADCELL) WITH PCF8574 IN NODEMCU

Question

I have used hx711 and loadcell with nodemcu (esp8266) in its D1 and D2 pins ( here I have used Logic level shifter and a 5v supply to power hx711).It worked perfectly fine.

Due to the lack of enough pins to accommodate other sensors in Nodemcu, I'm using PCF8574. I have connected hx711 to pins P0 and P1 of PCF8574.I'm not able to get any output in serial monitor while running like this.

The PCF8574 was connected using resource -

https://www.instructables.com/id/PCF8574-GPIO-Extender-With-Arduino-and-NodeMCU/

The Hx711 was connected using this code.It is compiling but not getting any output in serial monitor.

#include "Arduino.h"
#include "PCF8574.h"

// Set i2c HEX address
PCF8574 pcf8574(0x20); // A0,A1,A2 all connected to GND
unsigned long timeElapsed;


include <HX711.h>
HX711 scale;

void setup() {
  Serial.begin(9600);
  Serial.println("initializing");
  pcf8574.pinMode(P0,INPUT);
  pcf8574.pinMode(P1,OUTPUT);
  pcf8574.begin();

  Serial.println("initializing");

  scale.begin(P0,P1); //dt , sck
  scale.get_value(5);  // print the avg of 5 readings from ADC minus tare weight 
  scale.set_scale(-450.f); // callibration value
  scale.tare();// reset scale to zero
}

void loop() {


  int weight = scale.get_units(3);
  Serial.println(weight);
  delay(1000)

}

Or how about this code without a library? Can I do some changes to it and use pcf8574 ?

#define DT D1
#define SCK D2


long sample=0;
float val=0;
long count=0;

unsigned long readCount(void)
{
  unsigned long Count;
  unsigned char i;
  pinMode(DT, OUTPUT);
  digitalWrite(DT,HIGH);
  digitalWrite(SCK,LOW);
  Count=0;
  pinMode(DT, INPUT);
  while(digitalRead(DT));
  for (i=0;i<24;i++)
  {
    digitalWrite(SCK,HIGH);
    Count=Count<<1;
    digitalWrite(SCK,LOW);
    if(digitalRead(DT)) 
    Count++;
  }
  digitalWrite(SCK,HIGH);
  Count=Count^0x800000;
  digitalWrite(SCK,LOW);
  return(Count);
}

void setup()
{
  Serial.begin(9600);
  pinMode(SCK, OUTPUT);
  Serial.println("Weight");
  Serial.println(" Measurement ");
  delay(1000); 
  calibrate();
}

void loop()
{
  count= readCount();
  int w=(((count-sample)/val)-2*((count-sample)/val));
  Serial.print("weight:");
  Serial.print(w);
  Serial.println("g");


 delay(100);
   // val=0;
   // sample=0;
    w=0;
    count=0;
   // calibrate();

}

void calibrate()
{

  //Serial.println("Calibrating...");
  //Serial.println("Please Wait...");
  for(int i=0;i<100;i++)  
  {
    yield(); 
    count=readCount();
    sample+=count;
   // Serial.println(count);
  }
  sample/=100;
  //Serial.print("Avg:");
  //Serial.println(sample);
  count=0;
  while(count<1000)
  {
    yield(); 
    count=readCount();
    count=sample-count;
    //Serial.println(count);
  }

  //Serial.println("Please Wait....");
  delay(100);
  for(int i=0;i<100;i++)  
  {
    yield(); 
    count=readCount();
    val+=sample-count;
   //Serial.println(sample-count);
  }
  val=val/100.0;
  val=val/100.0;       
  //Serial.println(val);
}

Answer 1

PCF8574 takes around 256us to update pins (at standard 100khz i2c). This too high compared to hx711 sck timing constraint which is 50us max. So, it's not possible to communicate with hx711 using PCF8574. wasted two days in finding out this.

Answer 2

According to library, hx711 sending pulses to mcu and we are scaling with take this pulses. I think you must connect DOUT and SCK pins directly to the MCU or you must counting pulses via I2C after all create a scaling value according the library code and send it to mcu.

Answer 3

I didnt try this method but; when i check the data sheet, i found this code on last page:

    sbit ADDO = P1^5;
    sbit ADSK = P0^0;
    unsigned long ReadCount(void){
     unsigned long Count;
     unsigned char i;
     ADDO=1;
     ADSK=0;
     Count=0;
     while(ADDO);
     for (i=0;i<24;i++){
     ADSK=1;
     Count=Count<<1;
     ADSK=0;
     if(ADDO) Count++;
     }
     ADSK=1;
     Count=Count^0x800000;
     ADSK=0;
     return(Count); 
}

Here is the pulse counting method i think. So if we are using I2C IC, we must get pulse datas over it to our MCU and count them. But before this scaling method; we must calibrate loadcells and tare them absolutely. And i found a link that pointing i2c with hx711. They used sparkfuns IO expander(communicating via I2C) to fix low io problem and connect loadcells to this expander and how read scalings completely. Here is the link: if you can check, maybe find a solution for your problem. Sparkfun's I2c Expander with Hx711