7/7/2015 Microcontroller Memory Expansion

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Microcontroller Memory ExpansionSome recent microcontrollers, such as the Microchip PIC range, have been designed to beconfigured to access external memory and I/O.

The above diagram shows a microcontroller with two ports, A and B. If these two ports are alsoused for external data and address bus,  the microcontroller can be configured for microprocessormode.

Port A is now used as part of the address bus, with port B used for both the remaining

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address bus signals and the data bus. So port B pins form part of the address bus, while, alittle later, they are the data bus. This is a common arrangement, called a multiplexedaddress/data bus configuration. It uses less pins than would be needed for separate addressand data bus connections.

In order to obtain the multiplexed address and data signals from the port B pins, an externaldata latch is required. This is used to hold the address bits, when the port B pins change tobecome the data bus. The purpose of the address strobe is to latch the address bits into thedata latch at the right moment, before the port B pins change to become the data bus. 

The above diagram illustrates the basic arrangement of a multiplexed Address/Data bus. It issimilar to that used for the intel 80X86 processor. The basic address/data multiplexed timing isillustrated below.

7/7/2015 Microcontroller Memory Expansion

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Note that the address remains fixed until the next address strobe, also that the memory read(when the read line is asserted) and the memory write (when the write line is asserted) takeplace towards the end of the data waveform.