Parallel Interface of microprocessor with I/O and memory


Modes of data transfer:


There are four modes of data transfer which are described below:-
1.      Simple Input and Output:-
It is the most primitive form of data transfer. To get the data input from a simple switch such as thermostat into a microprocessor, we have to connect the switch to an input port line and read the switch. Similarly, to output data to a simple display device such as LED, we have to connect the LED buffer or an output port pin and output the logic level required to turn on the light.
 Simple data transfer

2.      Simple strobe I/O:-
When a valid data is present on an external device only at certain time, it should be read at appropriate time. For example, input from a keyboard. When a key is pressed, it sends the ASCII code for the pressed key data lines and also sends out a strobe signal on another line to indicate that a valid data is present on the data lines.
 Strobe I/O

3.      Single Handshake I/O:-
In this mode, two handshaking lines are used. The peripheral sends parallel data and also issues a strobe (STB’) signal to the microprocessor. After detecting STB’ signal, the microprocessor reads the data and sends acknowledge (ACK) signal to the peripheral to indicate that the data has been read. Then, the peripheral can send next byte of data. From microprocessor side, this operation is referred to as a handshake or strobe input. In this mode, the sending device is designed not to send the next data until the receiving device indicates with an ACK signal that it is ready to accept the next byte.
 Handshake data transfer

4.      Double Handshake data transfer:-
When more coordination is required between the sending and receiving system, double handshake is used. The connection lines are same with that of single handshake system but in this mode, each edge of handshake signals has a meaning. The sending device makes STB’ line low to ask, “Are you ready to accept the data?” The receiving device raises the ACK line high to reply, “Yes, I am ready.” Then, the peripheral sends the byte of data and raise its STB’ line high to inform, “Here is a valid data for you.” After receiving the data, the receiver drops its ACK line low to say, “I have accepted the data.”
 Double Handshake data transfer



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