#define PCF8575_I2C_ADDR(ADx) (0x40 + 2*(ADx))
PCF8575_I2C_ADDR returns the 8-bit I2C address for the PCF8575 for the address configuration signals AD0, AD1, and AD2 presented in ADx. AD0 is the least significant bit and AD2 is the most significant bit, hence 0x6 represents AD2=1, AD1=1, AD0=0.
The address is calculated as 0x40 + 2*ADx.