{"id":1922,"date":"2017-11-22T11:21:18","date_gmt":"2017-11-22T19:21:18","guid":{"rendered":"http:\/\/internetofhomethings.com\/homethings\/?p=1922"},"modified":"2017-11-22T11:21:18","modified_gmt":"2017-11-22T19:21:18","slug":"expanding-esp8266-adc-1-vdc-range","status":"publish","type":"post","link":"https:\/\/internetofhomethings.com\/homethings\/?p=1922","title":{"rendered":"Expanding ESP8266 ADC 1 VDC range"},"content":{"rendered":"

\"\"<\/a><\/p>\n

An obvious weakness of the ESP8266 is the limited capability of it’s single ADC input. At best, the voltage range is 0-1 Vdc. While coupling an Arduino module<\/a> to the ESP8266 solves this problem, is does introduce significant data acquisition delays for the Arduino-to-ESP8266 communication. A preferred solution uses the ESP8266 without requiring an additional processor.<\/p>\n

The method presented here adds 4 ADC inputs, each supporting voltages from 0-5V. In addition, one true DAC output is available to replace the PWM feature used for that purpose.<\/p>\n

And no additional processor is required.<\/p>\n

This is a very inexpensive addition, requiring hardware that acquired for approximately 1 USD.<\/p>\n

ESP8266 ADC Range Improvement<\/strong><\/h2>\n

The enhancement is achieved by adding an external i2C bus ADC device to the ESP8266 system. This module,\u00a0PCF8591, was added to my project recently, purchased from Ali-Express<\/a> for 93 cents. It sports 4 ADC channels, one DAC channel, a potentiometer, thermister, and a photo-sensor. But for my needs, I have disabled the additional components and use it strictly as a 4-channel ADC.<\/p>\n

The device uses an i2C bus, which requires 2 digital pins (any two) to operate.<\/p>\n

There are two limitations I discovered while testing this module, neither of which adversely impact it’s functionality in my application.<\/p>\n

    \n
  1. The ADCs and DAC are 8-bit devices. This results in a somewhat course resolution of 19mV-per-bit when using a 5V reference.<\/li>\n
  2. The DAC range is linear 0-4Vdc, but compresses beyond that and tops out at 4.25Vdc.<\/li>\n<\/ol>\n

    Even with these constraints, I was still pleased to have the capability of measuring voltages 0-5V. This was a huge improvement over the ESP8266 native 0-1Vdc ADC.<\/p>\n

    The ADCs can be used single-ended (4 channels) or differential (2 channels), an added capability which allows you to measure signals with a common-mode voltage. Changing the ADC mode is a simple matter of changing the control byte when setting up the ADC for a measurement. As you will see in the sketch below, single-ended measurements are made using the control bytes:<\/p>\n

    #define PCF8591_ADC_CH0             0x40 \/\/Measure Ain0 single-ended at CH0\r\n#define PCF8591_ADC_CH1             0x41 \/\/Measure Ain1 single-ended at CH1\r\n#define PCF8591_ADC_CH2             0x42 \/\/Measure Ain2 single-ended at CH2\r\n#define PCF8591_ADC_CH3             0x43 \/\/Measure Ain3 single-ended at CH3<\/pre>\n
    Other options, if desired, would expand the set of control bytes:<\/p>\n
    #define PCF8591_ADC_CH0             0x40   \/\/Measure Ain0 single-ended at CH0\r\n#define PCF8591_ADC_CH1             0x41   \/\/Measure Ain1 single-ended at CH1\r\n#define PCF8591_ADC_CH2             0x42   \/\/Measure Ain2 single-ended at CH2\r\n#define PCF8591_ADC_CH3             0x43   \/\/Measure Ain3 single-ended at CH3\r\n\r\n#define PCF8591_ADC_DIFF_CH0CH3_CH0 0x50   \/\/Measure Ain0-Ain3 differentially at CH0\r\n#define PCF8591_ADC_DIFF_CH1CH3_CH1 0x51   \/\/Measure Ain1-Ain3 differentially at CH1\r\n#define PCF8591_ADC_DIFF_CH2CH3_CH2 0x52   \/\/Measure Ain2-Ain3 differentially at CH2\r\n\r\n#define PCF8591_ADC_MIXED_CH0       0x60   \/\/Measure Ain0 single=ended at CH0\r\n#define PCF8591_ADC_MIXED_CH1       0x61   \/\/Measure Ain1 single=ended at CH1\r\n#define PCF8591_ADC_MIXED_CH2_CH3   0x62   \/\/Measure Ain2-Ain3 differentially at CH2\r\n\r\n#define PCF8591_ADC_DIFF_CH0CH1_CH0 0x70   \/\/Measure Ain0-Ain1 differentially at CH0\r\n#define PCF8591_ADC_DIFF_CH2CH3_CH1 0x71   \/\/Measure Ain2-Ain3 differentially at CH1<\/pre>\n
     <\/p>\n
    ADC Module Interfaces<\/strong><\/h2>\n
    \"\"<\/a><\/p>\n
    The ESP8266 interface requires 4 signals:<\/p>\n
      \n
    1. Vcc (5 Vdc, before the 3.3 Vdc regulator)<\/li>\n
    2. Gnd (Ground)<\/li>\n
    3. SDA (Any ESP8266 Digital pin – GPIO4 in this example)<\/li>\n
    4. SCL (Any EAP8266 Digital pin – GPIO2 in this example)<\/li>\n<\/ol>\n
      The jumper pins (P4-P6) are removed for 4-channels ADC configuration.<\/p>\n
      Here is the minimal Arduino sketch I used to test this ADC. As you can see, the ADC read code can easily be included in a more complex sketch.<\/p>\n
      #include \"Wire.h\"\r\n#define PCF8591 (0x90 >> 1)\r\n#define PCF8591_DAC_ENABLE 0x40\r\n#define PCF8591_ADC_CH0 0x40   \/\/Measure Ain0 single-ended at CH0\r\n#define PCF8591_ADC_CH1 0x41   \/\/Measure Ain1 single-ended at CH1\r\n#define PCF8591_ADC_CH2 0x42   \/\/Measure Ain2 single-ended at CH2\r\n#define PCF8591_ADC_CH3 0x43   \/\/Measure Ain3 single-ended at CH3\r\n\r\n#define SDA 4\r\n#define SCL 2\r\nbyte adcvalue0, adcvalue1, adcvalue2, adcvalue3;\r\nbyte dac_value=0;\r\nbyte adc_value;\r\n\r\nvoid putDAC(byte dac_value)\r\n{\r\n  Wire.beginTransmission(PCF8591);  \/\/Calls the 8591 to attention.\r\n  Wire.write(PCF8591_DAC_ENABLE);   \/\/Send a DAC enable word.\r\n  Wire.write(dac_value);            \/\/Send the desired DAC value (0-255)\r\n  Wire.endTransmission();\r\n}\r\n\r\nbyte getADC(byte config)\r\n{\r\n  Wire.beginTransmission(PCF8591);\r\n  Wire.write(config);\r\n  Wire.endTransmission();\r\n  Wire.requestFrom((int) PCF8591,2);\r\n  while (Wire.available()) \r\n  {\r\n    adc_value = Wire.read(); \/\/This needs two reads to get the value.\r\n    adc_value = Wire.read();\r\n  }\r\n  return adc_value;\r\n}\r\n \r\nvoid setup()\r\n{\r\n   Wire.begin(SDA,SCL);\r\n   Serial.begin(115200);\r\n   putDAC(0);\r\n   Serial.println(\"dac,ain0,ain1,ain2,ain3\");\r\n}\r\n \r\nvoid loop()\r\n{\r\n   adcvalue0 =  getADC(PCF8591_ADC_CH0);\r\n   adcvalue1 =  getADC(PCF8591_ADC_CH1);\r\n   adcvalue2 =  getADC(PCF8591_ADC_CH2);\r\n   adcvalue3 =  getADC(PCF8591_ADC_CH3);\r\n\r\n   Serial.print(dac_value++);\r\n   Serial.print(\",\");\r\n   Serial.print(adcvalue0);\r\n   Serial.print(\",\");\r\n   Serial.print(adcvalue1);\r\n   Serial.print(\",\");\r\n   Serial.print(adcvalue2);\r\n   Serial.print(\",\");\r\n   Serial.print(adcvalue3);\r\n   Serial.println();\r\n   \r\n   putDAC(dac_value);\r\n    \r\n   delay(100);\r\n}<\/pre>\n
      This simple sketch configures the I2C library to attach to ESP8266 GPIO2 and GPIO4 and loop through a sequence of setting the DAC and reading all 4 ADC channels. For this setup, the DAC output is connected to all 4 ADC inputs and jumpers P4-P6 are removed. As one would expect, for each DAC setting, identical ADC values are measured for all 4 ADCs. But there are some deviations from the DAC setting:<\/p>\n
      \"\"<\/a><\/p>\n
      DAC Limitations<\/strong><\/h2>\n
      The ADC measured values deviate from the DAC setting by no more than 1 bit (~ 20mV) from 0-2 Vdc. This deviation increases to 4 bits around 4 Vdc, with the DAC output topping out at about 4.2 Vdc.<\/p>\n
      \"\"<\/a><\/p>\n
      From this we can conclude that the DAC has a functional range from 0-4 Vdc with an error no greater than 4 bits (80mV). Not super accurate, but it is adequate for many applications.<\/p>\n
      I measured the ADC output with a DVM (digital voltmeter) to confirm the DAC output never exceeded 4.2 Vdc.<\/p>\n
      ADC Voltage Range<\/strong><\/h2>\n
      And to confirm the ADC has a full range of 0-5VDC (0-255 byte value), I applied 5 Vdc to one of the Ain inputs and verified the measure value as 255 (5 Vdc). I also used the built-in potentiometer to swing the ADC measurements between 0 and 255 (0-5 Vdc). With a slight modification to the sketch to display the ADC readings from the potentiometer (Ain3) in Vdc, we observe good correlation between the applied voltage and the ADC measurement.<\/p>\n