This project adds three I2C devices to an ESP-01 or a Raspberry Pi.

The three devices are a Microchip MCP23016 digital I/O expander, NXP PCF8591 ADC/DAC and Microchip 24LC512 EEPROM.

These were chosen because they add useful interfacing and logging capabilities to the ESP-01 or RPi and all three are available in DIP format from CPC in the United Kingdom.

You may also consider adding an I2C RTC to this project and perhaps it shall be updated to include one at a later time.

The circuit is expected to be used with either an ESP-01 attached and with its UART connected to a computer to load in scripts, else without an ESP-01 and with the I2C lines connected to a Raspberry Pi.

It should be reasonably simple to construct on stripboard. Tripad board will be used to demonstrate this at a later time.

The I2C bus is simple affair with only two lines required for its operation. This is perfect for the ESP-01 module which only has two GPIOs exposed on its header pins. For the Raspberry Pi, there are two I2C buses, but only one is normally available.

These addresses are the defaults for all three devices with the address pins tied low. Multiple devices of the same type are easily accommodated on the I2C bus but this is not demonstrated here.

Probing the bus on the RPi with I2C Tools should reveal the following.

i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: 20 -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- 48 -- -- -- -- -- -- -- 
50: 50 -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- -- 

The MCP23016 is general purpose digital I/O expander. There are more modern and faster varieties of this device but as yet they are not available at CPC.

To use the MCP23016 first set the GPIO direction register. Microchip use 1 for input and 0 for output. After that you can either read the input register or write to the output register.

The NodeMcu firmware can be controlled via a serial console. This can be a manual operation else with a specially written tool for the job: eSPY.

First we need to fetch eSPY, and then build it. The examples for eSPY on this page come with the eSPY source.

hg clone http://hg.kewl.org/pub/espy
cd epsy
make

This example script blinks an LED attached to the MCP23016 GP0.0 pin.

#!espy /dev/ttyUSB0

-- pin 3 = GPIO0
sda=3

-- pin 4 = GPIO2
scl=4

-- init I2C
i2c.setup(0,sda,scl,i2c.SLOW)

-- write MCP
function write_mcp(control,value)
        i2c.start(0)
        i2c.address(0,0x20,i2c.TRANSMITTER)
        i2c.write(0,control)
        i2c.write(0,value)
        i2c.stop(0)
end

-- init GP0.0
write_mcp(2,0)
write_mcp(6,0xFE)

-- blink LED on GP0.0 3 times
for i = 1, 3, 1 do
        write_mcp(2,1)
        print ("on")
        tmr.delay(1000000)
        write_mcp(2,0)
        print ("off")
        tmr.delay(1000000)
end

The script comes with the eSPY application and can be demomstrated like this.

./mcp.esp
off
on
off
on
off

I2C Tools on the RPi are used to demonstrate the MCP23016 GPIO.

Here we clear the output latch, set GP0.0 to an output, then set the pin high on the RPi.

i2cset -y 1 0x20 2 0
i2cset -y 1 0x20 6 0xFE
i2cset -y 1 0x20 2 1

Now on the RPi we set GP1.0 as an input then read it twice. The first read is with GP1.0 taken low and the second read with GP1.0 taken high.

i2cset -y 1 0x20 7 1
i2cget -y 1 0x20 1
0xde
i2cget -y 1 0x20 1
0xdf

Please refer to the data-sheet for full information for this part.

The PCF8591 combines 4 analogue inputs and a single analogue output. The inputs can be more than simple input channels but this isn't looked at here.

To write to the DAC, we set the control to 64 and write a value between 0 and 255 which represents 0 to VREF volts on the output pin.

To read from an ADC, we first send a control representing the channel, then we perform two reads. The first read starts the conversion process and the second read fetches the result (this also starts another conversion).

If we are using the DAC, then we must also set the analog OE bit else the DAC will be disabled.

A simple ramp demo can be achieved like this with NodeMcu on the ESP-01.

#!espy /dev/ttyUSB0

-- pin 3 = GPIO0
sda=3

-- pin 4 = GPIO2
scl=4

-- init I2C
i2c.setup(0,sda,scl,i2c.SLOW)

-- write DAC
function write_dac(control,value)
        i2c.start(0)
        i2c.address(0,0x48,i2c.TRANSMITTER)
        i2c.write(0,control)
        i2c.write(0,value)
        i2c.stop(0)
end

-- DAC output
for i = 0, 255, 10 do
        write_dac(64,i)
        print (i)
        tmr.delay(100000)
end

When the script is run with eSPY we can see the DAC output ramp up with a running status.

0
10
20
30
...
230
240
250

Using I2C Tools on the RPi we reset the output to 0V, then raise it to 1.6V and finally 3.3V.

i2cset -y 1 0x48 64 0
i2cset -y 1 0x48 64 128
i2cset -y 1 0x48 64 255

With a potentiometer connected to AIN0 a reading was made on the RPi.

i2cset -y 1 0x48 64 ; i2cget -y 1 0x48 ; i2cget -y 1 0x48
0x1b
0x8d

The second value above is the new reading, the first is the previous reading.

This is simplified view of the operation of this device. Please read the data-sheet for the full details.

I2C EEPROMs have varied addressing schemes. The simplest is a 8-bit address offset, another one where a block selection is made with an I2C address and a byte selection is made with an 8-bit address offset. Yet another, where the I2C address is fixed but it now has a 16-bit address offset and lastly one with both of the last methods combined.

The 24LC512 looked at here uses a fixed I2C address and 16-bit addressing. The addressing byte order is high byte, low byte.

On the ESP-01 we can use this NodeMcu LUA script to read the EEPROM.

#!espy /dev/ttyUSB0

-- pin 3 = GPIO0
sda=3

-- pin 4 = GPIO2
scl=4

-- init I2C
i2c.setup(0,sda,scl,i2c.SLOW)

-- read EEPROM
function read_eeprom(addrhigh,addrlow)
        i2c.start(0)
        i2c.address(0,0x50,i2c.TRANSMITTER)
        i2c.write(0,addrhigh)
        i2c.write(0,addrlow)
        i2c.stop(0)

        i2c.start(0)
        i2c.address(0,0x50,i2c.RECEIVER)
        c=i2c.read(0,1)
        i2c.stop(0)
        return c
end

-- read byte
reg = read_eeprom(0x00,0x00)

-- output byte
print(string.byte(reg))

Here the script is executed using eSPY.

./eeprom.esp
238

On the RPi we can reset the internal address pointer of the chip to 0 like this.

i2cset -y 1 0x50 0 0

To read, we first write an address to start at then begin reading, if we keep reading the address is auto-incremented.

i2cset -y 1 0x50 0 0
i2cget -y 1 0x50
0x55
i2cget -y 1 0x50
0xaa

If we want to write to a location we send the address followed by the byte of data.

i2cset -y 1 0x50 0 0 0xEE i
i2cset -y 1 0x50 0 0 
i2cget -y 1 0x50
0xee

This overview gives enough information to use the device at a byte level, read the data-sheet for the page writing mode of operation.

MCP23016 data-sheet
PCF8591 data-sheet
24LC512 data-sheet