BASIC INPUT AND OUTPUT INTERFACING

 8085A communicate with outside world using the I/O devices.  Since memory and I/O devices share the system bus, therefore, only one device can active or communicate with 8085A in one time.  To active one device in one time, the decoding circuit is required.  The input of the decoding circuit are address bus and IO/M signal.  This topic will discuss about the technique used to interface the I/O devices with 8085A and how to design the decoding circuit.

 Upon completing this topic, you should be able to: Explain the differences between peripheral- mapped and memory-mapped I/O techniques Design a decoder circuit for I/O devices using peripheral-mapped I/O techniques

 Unlike modern microprocessors, the 8085A microprocessor has two address spaces.  A 16-bit address space, nominally for memory devices (memory map)  An 8-bit address space, nominally for input/output devices (I/O map)  The two address spaces are distinguished by the processor by the state of the IO/M status line. Memory MapI/O Map

 Memory and I/O map shows all the possible address in a microprocessor system and what they are assigned to.

 For interfacing I/O devices, a latch is used for an output port and a tri-state buffer is used for an input port

 Another option is to use PPI 8255A – a support chip that sets up multiple parallel I/O ports that are programmable

 The I/O device can be interfaced with 8085 microprocessor using two approach: i. Memory-mapped  Use Memory Map to define the I/O address  Assume I/O devices is a part of memory system or I/O map is a part of the memory map ii. Peripheral-mapped  Connecting I/O devices using difference address range  I/O Map is separated from memory map  The IO/M signals used to distinguish between these two methods

 The I/O device is identified by 16-bits address (A 15 – A 8 and AD 7 – AD 0 )  Defining them same as memory location  Use memory instructions to read and write the I/O devices  Execution of these instructions will force the IO/M signal goes low (“0”) to indicate that microprocessor is accessing the memory

 The device is identified using 8-bits address (A 15 – A 8 or AD 7 – AD 0 )  Used IN and OUT instructions for data transfer IN  input data from the input device OUT  output data to output device  Execution of these instructions will force the IO/M signal goes high (“1”) to indicate that microprocessor is accessing the I/O device

 Advantage It does not use any of the memory address range  Disadvantage Only I/O instructions can be used when addressing I/O devices

 Either the high-order bus (A 15 – A 8 ) or the demultiplexed low-order bus (AD 7 – AD 0 ) can be decoded to generate the pulse corresponding to the address of the bus.  The 8085A places the device address (port number) on the demultiplexed low-order and high-order address bus when executing an IO instruction (recall IO Read and IO Write machine cycle).  The device address pulse is ANDed with the appropriate control signal to select the IO port

1. Draw I/O Map 2. Draw an address decoding table 3. Draw a decoding circuit  Using combinational circuit (suitable for a small I/O devices)  Using address decoder circuit

 Design a decoding circuit to interface the I/O devices using the following I/O map.

2. Draw an address decoding table DeviceAddA7A7 A6A6 A5A5 A4A4 A3A3 A2A2 A1A1 A0A0 040H H H CH H H H CH

3. Draw a decoding circuit  Using address decoder circuit

 Design a decoding circuit to interface a PPI 8255A to 8085A microprocessor. The base address for the 8255A is 80H.

 Expend your design by adding two more PPI 8255A. The base address for the new 8255A are A0H and C0H respectively.