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+---
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+-- @description Expands the ESP8266's GPIO to 8 more I/O pins via I2C with the MCP23008 I/O Expander
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+-- MCP23008 Datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/21919e.pdf
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+-- Tested on NodeMCU 0.9.5 build 20150213.
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+-- @date March 02, 2015
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+-- @author Miguel
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+-- GitHub: https://github.com/AllAboutEE
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+-- YouTube: https://www.youtube.com/user/AllAboutEE
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+-- Website: http://AllAboutEE.com
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+--------------------------------------------------------------------------------
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+
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+local moduleName = ...
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+local M = {}
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+_G[moduleName] = M
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+
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+-- Constant device address.
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+local MCP23008_ADDRESS = 0x20
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+
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+-- Registers' address as defined in the MCP23008's datashseet
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+local MCP23008_IODIR = 0x00
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+local MCP23008_IPOL = 0x01
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+local MCP23008_GPINTEN = 0x02
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+local MCP23008_DEFVAL = 0x03
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+local MCP23008_INTCON = 0x04
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+local MCP23008_IOCON = 0x05
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+local MCP23008_GPPU = 0x06
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+local MCP23008_INTF = 0x07
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+local MCP23008_INTCAP = 0x08
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+local MCP23008_GPIO = 0x09
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+local MCP23008_OLAT = 0x0A
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+
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+-- Default value for i2c communication
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+local id = 0
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+
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+-- pin modes for I/O direction
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+M.INPUT = 1
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+M.OUTPUT = 0
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+
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+-- pin states for I/O i.e. on/off
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+M.HIGH = 1
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+M.LOW = 0
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+
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+-- Weak pull-up resistor state
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+M.ENABLE = 1
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+M.DISABLE = 0
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+
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+
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+---
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+-- @name write
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+-- @description Writes one byte to a register
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+-- @param registerAddress The register where we'll write data
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+-- @param data The data to write to the register
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+-- @return void
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+----------------------------------------------------------
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+local function write(registerAddress, data)
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+ i2c.start(id)
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+ i2c.address(id,MCP23008_ADDRESS,i2c.TRANSMITTER) -- send MCP's address and write bit
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+ i2c.write(id,registerAddress)
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+ i2c.write(id,data)
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+ i2c.stop(id)
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+end
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+
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+---
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+-- @name read
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+-- @description Reads the value of a regsiter
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+-- @param registerAddress The reigster address from which to read
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+-- @return The byte stored in the register
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+----------------------------------------------------------
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+local function read(registerAddress)
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+ -- Tell the MCP which register you want to read from
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+ i2c.start(id)
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+ i2c.address(id,MCP23008_ADDRESS,i2c.TRANSMITTER) -- send MCP's address and write bit
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+ i2c.write(id,registerAddress)
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+ i2c.stop(id)
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+ i2c.start(id)
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+ -- Read the data form the register
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+ i2c.address(id,MCP23008_ADDRESS,i2c.RECEIVER) -- send the MCP's address and read bit
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+ local data = 0x00
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+ data = i2c.read(id,1) -- we expect only one byte of data
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+ i2c.stop(id)
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+
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+ return string.byte(data) -- i2c.read returns a string so we convert to it's int value
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+end
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+
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+---
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+--! @name begin
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+--! @description Sets the MCP23008 device address's last three bits.
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+-- Note: The address is defined as binary 0100[A2][A1][A0] where
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+-- A2, A1, and A0 are defined by the connection of the pins,
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+-- e.g. if the pins are connected all to GND then the paramter address
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+-- will need to be 0x0.
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+-- @param address The 3 least significant bits (LSB) of the address
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+-- @param pinSDA The pin to use for SDA
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+-- @param pinSCL The pin to use for SCL
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+-- @param speed The speed of the I2C signal
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+-- @return void
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+---------------------------------------------------------------------------
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+function M.begin(address,pinSDA,pinSCL,speed)
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+ MCP23008_ADDRESS = bit.bor(MCP23008_ADDRESS,address)
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+ i2c.setup(id,pinSDA,pinSCL,speed)
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+end
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+
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+---
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+-- @name writeGPIO
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+-- @description Writes a byte of data to the GPIO register
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+-- @param dataByte The byte of data to write
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+-- @return void
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+----------------------------------------------------------
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+function M.writeGPIO(dataByte)
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+ write(MCP23008_GPIO,dataByte)
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+end
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+
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+---
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+-- @name readGPIO
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+-- @description reads a byte of data from the GPIO register
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+-- @return One byte of data
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+----------------------------------------------------------
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+function M.readGPIO()
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+ return read(MCP23008_GPIO)
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+end
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+
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+---
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+-- @name writeIODIR
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+-- @description Writes one byte of data to the IODIR register
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+-- @param dataByte The byte of data to write
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+-- @return void
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+----------------------------------------------------------
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+function M.writeIODIR(dataByte)
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+ write(MCP23008_IODIR,dataByte)
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+end
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+
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+---
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+-- @name readIODIR
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+-- @description Reads a byte from the IODIR register
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+-- @return The byte of data in IODIR
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+----------------------------------------------------------
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+function M.readIODIR()
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+ return read(MCP23008_IODIR)
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+end
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+
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+---
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+-- @name writeGPPU The byte to write to the GPPU
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+-- @description Writes a byte of data to the
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+-- PULL-UP RESISTOR CONFIGURATION (GPPU)REGISTER
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+-- @param databyte the value to write to the GPPU register.
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+-- each bit in this byte is assigned to an individual GPIO pin
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+-- @return void
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+----------------------------------------------------------------
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+function M.writeGPPU(dataByte)
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+ write(MCP23008_GPPU,dataByte)
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+end
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+
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+---
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+-- @name readGPPU
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+-- @description Reads the GPPU (Pull-UP resistors register) byte
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+-- @return The GPPU byte i.e. state of all internal pull-up resistors
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+-------------------------------------------------------------------
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+function M.readGPPU()
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+ return read(MCP23008_GPPU)
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+end
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+
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+return M
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