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.
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.
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.
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.”
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