How to print a Variable in TFT 3.5 in with the MCUFRIEND Library?

Other Hardware Displays
4

Here is a helper function. The Free Fonts always print transparently. So you need to draw the background to rub out the previous number. You could add width, prec as parameters to control the format.

void print_float_at(float val, int x, int y)
{
    char buf[10];
    int16_t x1, y1, w, h;
    tft.getTextBounds("000.0", x, y, &x1, &y1, &w, &h); 
    dtostrf(val, 5, 1, buf);   //e.g. 123.4
    tft.fillRect(x1, y1, w, h, YELLOW);
    tft.setTextColor(RED);
    tft.setCursor(x, y);
    tft.print(buf);
}

Use like this:

void Imprimir_Voltage_CFE() {
    print_float_at(vrms, 190, 160);
}

and your setup(), loop() could be:

void setup()
{
    uint16_t ID = tft.readID();
    tft.begin(ID);
    tft.setRotation(1);
    tft.fillScreen(WHITE);
    Parametros_CFE();    //print main screen once
}

void loop()
{
    Leer_Voltage();
    Imprimir_Voltage_CFE();
}

David.