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It is known that address of OLED SSD1306 is 3C. The frame format is as follows:
The figure implies that the binary pattern should be 01111000 for the first frame (considering write mode and the second last bit is a dont care).
So I was trying to send the first frame using Wire.h library. The following is a code
Wire.beginTransmission(0b01111001);
But i receive a nack (return value of 2)
However, If I send 00111100, i receive a ack(return value of 0). What is wrong in my interpretation?
You misinterpret how the Wire library is used. Though the first byte in the transmission is called the address byte, the actual address consists only of the 7 higher bits in that byte. The direction bit is not in the address.
So the Wire.beginTransmission() method wants the address as parameter, without the direction bit. That bit is set corresponding to the used function (0 for Wire.beginTransmission() and 1 for Wire.requestFrom()).
0b0111100 is the real address (when connnecting the address pin to ground)(note, that there are only 7 bits, since the address only has 7 bits). The write operation address byte will then be 0b01111000, though the library is hiding that way from you. You need to provide the 7 bit address to the function.
the second last bit is a dont care
As mentioned above, that is not correct. This bit (the least significant bit of the address) is set by the address pin SA0. It seems, that you have it tied to ground, which sets the corresponding bit in the address to zero. If you would tie it to Vcc, it would be 1.
Wire.startTransmission() (clears the bit to address the slave in write mode) and Wire.requestFrom() (sets the bit to address the slave in read mode). It's important to only use the 7-bit address, preferably in another format than binary, which can be a bit confusing
Wire.beginTransmission() and Wire.endTransmission()) it will use zero for the direction bit. If you use the read function Wire.requestFrom(), it will use 1 for the direction bit.
Wire.endTransmission(). Before that no data will be transmitted.
I2C has a 7-bit address space. The device's actual address is 0b0111100 (=0x3c). The sequences 0b01111001 and 0b01111000 address the same slave while the first one (least significant bit is 1) starts a read transmission, the second one starts a write transmission.
Those least significant bits are nothing you need to care about when using Wire.h: the mode of operation is automatically set when using Wire.startTransmission(addr) (returns 0 when slave ACK'ed write command) and Wire.requestFrom(addr) (returns 0 when slave ACK'ed read command).
0b01111000 is not what you are supposed to pass to Wire.beginTransmission(). It takes the 7-bit address. You better don't mess with binary format, simply do Wire.beginTransmission(0x3c)...