Microprocessor Notes

Unit 7

Memory Interface 2, I/O interface 1

7.1 Basic Input Interface

• A set of three state buffers.
• IN refers moving data from I/O devices to the microprocessor.
• The external TTL is connected to the input of the buffer.
• Output of the buffer is connected to the data bus.
• The kit allows the microprocessor to read the content of the eight switches that connect to the eight bit of the data bus when SEL is '0'.
• If there is logic '1' the kit disconnects the switches from the data bus.

7.2 Basic Output interface:

• It's a set of data latches.
• OUT refers moving data out of the microprocessor to I/O device.
• It receives data from the microprocessor.
• When OUT instruction executes, data from AL, AX, or EAX are transferred to the data bus.
• In 74ALS374 the inputs (D) are connected to the data bus, and capture the output data.
• And the outputs (Q) are attached to the LEDs.
• Now when the OUT instruction executes, SEL signal activates, and captures the data.
• This data held until next OUT instruction executes.
• That means, whenever there is an OUT signal, the data from AL register appear on the LEDs

7.3 Handshaking:

Connector DB25

Connector CENT36

Connector DB25

• Many I/O devices rate (speed) is slower than the microprocessor.
• Speed or rate means accepting or releasing the information.
• Handshaking synchronize the I/O device with the microprocessor.
• The speed rate problem of I/O device is solved.
• We take the example of a printer.
• Printer's capacity to print letters are few hundread characters per second (CPS).
• But microprocessor can send more than a few hundreds of CPS to the printer.
• There are two signal for the communication between microprocessor and printer.
• BUSY - indicates printer is busy.
• STB - a clock pulse used to send a data to the printer for printing.
• So when there is pulse on the STB connection, the data is send to the printer.
• As soon as the printer receives the data, it places logic '1' on the BUSY pin.
• This indicates that the printer is busy in printing the data.
• Then microprocessor polls whether the printer is busy.
• If printer is busy, microprocessor waits, otherwise sends the next ASCII characters.
• This process of interrogating the printer is called handshaking

7.4 Interface Circuitry.

7.4.1Input Devices.

 

[Fig.7.4.1]A single-pole, single-throw switch interfaced as a TTL device.

• Input devices are TTL compatible.
• TTL levels:-
• For logic '0' :- 0.0V - 0.8V
• For logic '1' :- 2.0V - 5.0V