INTERFACING I/O PORTS ~ B Tech Guru

Tuesday, November 20, 2012

9:34 AM Unknown 1 comment

I/O ports are devices through which the microprocessor (µP) communicates with other devices or external data sources /destination

INPUT ACTIVITY: The activity that enables the MP to read data from external devices

Eg: keyboard, joystick, mouse

OUTPUT ACTIVITY: Transfers from the MP to the external devices.

Eg: CRT-display, 7-segment display, printers etc.

INPUT DEVICES: Feed data into a MP system

OUTPUT DEVICES: Accept data from a MP system

That is for MP system i/p activity is similar to read operation & o/p activity is similar to write operations

IORD: Operation is related to reading data from i/p device

IOWR: Writing data to an o/p device

In programmable devices, the control word & status word can be written & read using i/p & o/p devices.
After executing OUT operation, the data appears on the data bus at the same time a device signal is generated using address & control signals.
If the data is to be there, at the o/p of the device till the next change , it must be latched.
If the o/p port is to be source large currents the port lines must be buffered.
So the latch acts as a good o/p port Chip 74LS373 contains 8 buffered latches & can be used as an 8 bit o/p port.
While reading, one should take care that much current should not be sourced or sunk from the data lines to avoid loading.
To avoid this problem, can use a tristate buffer as an i/p device.
An i/p port may not be a latch as it reads the status of a signal at a particular instant.
The chip 74LS245 contains 8 buffers & may be used as an 8 bit i/p port.
74LS245: A bidirectional buffer. But using it as an i/p device, only one direction is useful. This direction is selected using DIR pin.
The OE & CS are the chip selects of 74LS373 & 74LS245 respectively.
DS & QS are corresponding latch i/ps & o/ps.
CLK is the clock i/p for D Flip Flop.
If DIR=1, then direction is from A(i/p) to B(o/p) otherwise the data direction is from B(i/p) to A(o/p)

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