Let s assume the same microprocessor with 10 address lines (1KB memory)Ĭomparison of full and partial address decodingĮg: To interface 2k x 8 RAM to 8085 microprocessorįor 2k locations, no. Physical memory must be placed on the upper half of the memory map However, this time we wish to implement only 512 bytes of memory Let s assume a microprocessor with 10 address lines (1KB memory) Since not all the address space is implemented, only a subset of theĪddress lines are needed to point to the physical memory locations If only a portion of the addressable space is going to be implemented thereĪre two basic address decoding strategiesĪll the address lines are used to specify a memory location2. The previous example specified that all addressable memory space was to We will need 3 address lines to select each one of the 8 chipsĮach chip will need 7 address lines to address its internal memory cells Lets assume we wish to implement all its memory space and we use 128x8 Lets assume a very simple microprocessor with 10 address lines (1K memory) The M least significant signals are passed to the devices as addresses to the The address bus lines are split into two sectionsġ.The N most significant bits are used to generate the CS* signals for the differentĭevices.2. The address bus for each device in the system This signal is complemented using inverter IC 74LS04 and ANDed with RD and WR signals togenerate IOR(io read) and IOW (io write) control signalsĪddress decoding is the process of generating chip select (CS*) signals from When IO/M goes high, it indicates io operation When both the inputs are low the output of the gate is low and generate MEMR signal(memory read) This signal is ANDed with RD and WR signals by using 74LS32 The signal IO/M goes low for the memory operation The above figure shows 4 different control signals are generated by combining the signals RD, WRand IO/M Similarly two different write signals are generated Since this signal is used for both reading memory and an input device, it is necessary for generatingtwo different read signals: one for memory and other for input device Generation of control signals (MEMR, MEMW, Now the o/p of the latch represents the lower order address bus A7 A0. When ALE goes low the address is latched until the next ALE O/p of the latch is the lower order address. When ALE is high, the output of the latch changes according to input, hence the The ALE signal is connected to the enable(G) pin of the latch and the o/p control The bus AD7 - AD0 is connected as input to the latch 74LS373. In the above figure a latch and an ALE signal is used to demultiplex the bus. This address needs to be latched and used for identifying the memory address However lower order address is lost after the first clock period. The address on the higher order bus remains on the bus for three clock periods. Demultiplexing of address and data bus by ALE signalĭemultiplexing of address and data bus by ALE
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