Xin Key Hyperterminal 7 Serial ICbus Interface

As the ESP8266-01 has no analog inputs at all, adding an ADC to it is something I wanted to do.

Xin Key Hyperterminal 7 Serial ICbus Interface

The PCF8591 is such an ADC: it is a single-chip, singlesupply lowpower 8bit CMOS data acquisition device with four analog inputs, one analog output and a serial ICbus interface.

Three address pins A0, A1 and A2 are used for programming the hardware address, allowing the use of up to eight devices connected to the ICbus without additional hardware.

Although one can get the individual chip, I have chosen for a module that actually already has some sensors on it.

The 0x90 counts the LSB of the 8 bit address, which is the RW bit.

With the Write Bit Low (active) the full address is 100100000x90, but the rightshift 1 removes the LSB again, making it 0x48.

The module is hardwired to 0x48 as the three address lines are soldered to ground.

That module is daisy chainable with other I2C modules in the same range (there is for example a PCF8547 digital IO module with similar connections).

The lower nibble selects one of the analog input channels defined by the upper nibble.

If the auto-increment flag is set the channel number is incremented automatically after each AD conversion.

If the auto-increment mode is desired in applications where the internal oscillator is used, the analog output enable flag in the control byte (bit 6) should be set.

This allows the internal oscillator to run continuously, thereby preventing conversion errors resulting from oscillator start-up delay.

The analog output enable flag may be reset at other times to reduce quiescent power consumption.

For now it is enough to know that the PCF8591 can be read byte for byte, but it can also be read in burst mode, in which we read the 4 analog values all at once.

The program I present is burstmode with autoincrement of the address.

The reason we read 5 bytes instead of 4 is because the first byte contains old data.

As the datasheet states in paragraph 8.4: The first byte transmitted in a read cycle contains the conversion result code of the previous read cycle.

The voltage would be: 0.64V In order to test that, I hooked up Aout to the A0 of an ESP 8266-12 and found a value of 192.

To calculate that to a voltage that is (1921023)0.18V.

Remember, the ESP gives readings from 0-1023, whereas 1023 being 1 Volt.) So that is not particularly close.

Oddly though when I hooked up the Aout to AIN2 or AIN1 that didn seem to give reliable readings.

A value of 50 written to Aout should give a value of 50 on AIN, but oddly it didnt.

Suppose you want to measure the difference between AIN0 and AIN1.

There are other ways of expanding the ADC capabilities of the ESP8266.