Wednesday, 18 April 2012

IO interfacing to 8085

There are two techniques through which devices can be interfaced to microprocessor.
  1. Memory mapped I/O
  2. Peripheral mapped I/O or I/O mapped I/O 
Memory mapped I/O:
I/O devices are interfaced using address from memory space. That means IO device address are part of addresses given to memory locations.8085 uses 16-bit address to memory interfacing. So any address between 0000H-FFFFH can be given to each peripheral. But the addresses given to peripheral can’t be used for memory.
                Memory control signals are used as read and write control signals for peripherals. And all the operations that can be performed on memory can also be performed on peripherals. No need of using IO instructions such as IN, OUT.
IO mapped IO:
In this method separate address space is given to IO devices. Each IO device is given a 8-bit address. Hence maximum 256 devices can be interfaced to the processor. The address range for the IO devices is 00H-FFH. IO control signals are used to perform read, write operations.
For reading data from IO device or writing data to IO device IN, OUT instructions needs to be used. Arithmetic and logical operations can’t be performed directly on IO devices as in memory mapped IO.    
IO devices can be interfaced, by using buffers for simple IO i.e. by using address decoding circuit to enable buffer.  For handshake IO or to interface more peripherals ICs like 8255 peripheral programmable interface (PPI) can be used.  

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