Friday, 23 March 2012

8085 based micrcomputer system

Since 8085 is a microprocessor, it does not consist of on chip memory for program as well as data and also it doesn’t have I/O ports to interface external peripherals.  All these extra ICs need to be connected to processor.
                8085 has lower order address bus multiplexed with data bus as already explained. So to get separate address as well as data bus, they need to be de-multiplexed. To do this 8085 provides ALE (address latch enable) signal. 8085 Asserts ALE signal and issues address of memory location or peripheral during first T-state in any machine cycle. An external octal latch (8282 or 74373) can be used for doing this de-multiplexing. Latch has data input as multiplexed address bus and latch enable signal is connected to ALE signal. So when ALE is asserted by processor latch is enabled and holds the data input content into latch is the address asserted by processor. So output of latch is Address bus (lower order, and higher order address bus is provided by processor). Ad0-AD7 pins cab be directly used as data bus.
                If the data bus needs to drive multiple devices i.e. more peripherals then external bus drivers or bus buffers are required. They are optional depending on no. of peripherals and their driving current. So they are shown as optional by using dotted lines in the above figure.
                8085 issues RD’, WR’, M/IO’ control signals. But we require separate signal for reading and writing into memory or IO device. So some extra logic circuit is required for getting individual control signals for memory and IO devices (IOR, IOW, MEMR, MEMW).
                To store the program as well as data external memory is required. For storing program ROM or EPROM or EEPROM or flash is required. And storing data, RAM is required. For accessing them some address decoding circuitry, which can be implemented using NAND gates or 3 to 8 decoder like 74138 can be used. However this occupies large space on the board. So programmable logic devices like PAL can be used, which occupies very less board space.
Data bus, Address bus and control buses combined are called System bus. To access external IO devices IO ports are required, So Programmable peripheral interface chip is required. For serial communication a UART chip is required and for handling the devices on interrupt basis a programmable interrupt controller is required. So this requires around 10-12 chips along with the 8085 processor to completely build a microcomputer system using a processor like 8085. This results in a very large sized board.  So this type of design is made only when computing requirement is very high. For control dominated applications microcontroller based system can be used as their board size is very less and low cost.  Of course today’s microcontrollers have extremely high computing power. 

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